HISTORICAL METEOROLOGICAL ANALYSIS IN 

SUPPORT OF THE 2003 SAN JOAQUIN VALLEY PM10

STATE IMPLEMENTATION PLAN 

FINAL REPORT 

Prepared By: 

Shawn R. Ferreria 

Air Quality Meteorologist / Atmospheric Scientist 

and 

Evan M. Shipp 

Supervising Air Quality Meteorologist 

San Joaquin Valley Air Pollution Control District 

January 24, 2005 
ii

ACKNOWLEDGEMENTS 

We wish to acknowledge the contributions of Air Resources Board Staff, Ms. Karen 

Magliano, Ms. Kasia Turkiewicz, and Mr. Rich Hackney who provided valuable 

information in the compiling and reviewing of this document. We wish to also 

acknowledge the contribution of the members of the PM10 Modeling group who provided 

extremely useful discussions during the planning, intermediate stages, and 

development of this document.
iii

 TABLE OF CONTENTS 

Section Page

ACKNOWLEDGEMENTS..................................................................... ii 

LIST OF FIGURES..............................................................................v 

LIST OF TABLES............................................................................... vi 

EXECUTIVE SUMMARY..................................................................... vii 

1.0 NOVEMBER 6, 1997 EPISODE SYNOPTIC METEOROLOGICAL 

DISCUSSION............................................................................1 

2.0 DECEMBER 31, 1998 AND JANUARY 12, 1999 EPISODE 

SYNOPTIC METEOROLOGICAL DISCUSSION...............................4 

3.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE JANUARY 12, 1999 EPISODE.........10 

4.0 OCTOBER 21, 1999 EPISODE SYNOPTIC METEOROLOGICAL 

DISCUSSION...........................................................................15 

5.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE OCTOBER 21, 1999 EPISODE .......18 

6.0 NOVEMBER 14, 1999 EPISODE SYNOPTIC METEOROLOGICAL 

DISCUSSION.......................................................................... 22 

7.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE NOVEMBER 14, 1999 EPISODE......26 

8.0 DECEMBER 17 AND 23, 1999 EPISODE SYNOPTIC 

METEOROLOGICAL DISCUSSION.................................................29 

9.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE DECEMBER 1999 EPISODE............35 

10.0 DECEMBER 1999 AND JANUARY 2000 PM2.5 & PM10 

 METEOROLOGICAL DISCUSSION..............................................40 

11.0 DECEMBER 2000 AND JANUARY 2001 EPISODE SYNOPTIC 

METEOROLOGICAL DISCUSSION.................................................51 

12.0 DISPERSION METEOROLOGY FOR THE FALL 2000 AND WINTER 

 2001 CRPAQS..........................................................................58 
iv

13.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE JANUARY 2001 EPISODE...............69 

14.0 NOVEMBER 9, 2001 EPISODE SYNOPTIC METEOROLOGICAL 

DISCUSSION...........................................................................77 

15.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE NOVEMBER 9, 2001 EPISODE.......81 

16.0 MAY 19 AND 20, 2002 EPISODE SYNOPTIC METEOROLOGICAL 

DISCUSSION...........................................................................84 

17.0 REFERENCES.........................................................................90 
v

LIST OF FIGURES 

1 Corcoran PM10, Fresno Precipitation, and T850 Stability Fall 1997.............1 

2 Atmospheric Temperature Profile at Fresno on November 06, 1997...........2 

3 T850 Stability and Precipitation vs. PM10 at Bakersfield (12/98-01/99).........4 

4 Atmospheric Temperature Profile at Bakersfield on 12/31/1998.................6 

5 Atmospheric Temperature Profile at Bakersfield on 01/12/1999.................8 

6 Bakersfield-Golden(Coarse, PM10 &2.5) 12/1998 & 01/1999.....................11 

7 Visalia and Oildale PM10 December 1998 & January 1999......................11 

8 Fresno-1st PM10 &2.5, Precipitation, T850 Stability and Model PM10 10/99...16 

9 Atmospheric Temperature Profile at Fresno on October 21, 1999.............17 

10 October 1999 Solar Intensity.............................................................17 

11 Corcoran (Coarse, PM10 &2.5) and Fresno-1st (PM2.5) 10/21/1999..............18 

12 Turlock-Minaret and Fresno-Drum. PM10 and Fresno-1st (PM2.5) 10/21/99...19 

13 T850 Stability & Precipitation vs. PM10 &2.5 at Bakersfield-Gold F/W99....22 

14 Atmospheric Temperature Profile at Bakersfield on 11/14/99...................24 

15 Bakersfield Golden (PM2.5 &Coarse) and Fresno-1st (PM2.5) 11/14/99........26 

16 Bakersfield Calif. (Coarse, PM2.5 &10) and Fresno-1st (PM2.5) 11/14/99.......27 

17 T850 Stability & Precipitation at Hanford vs. PM10 &2.5 at Corcoran 12/99....29 

18 Atmospheric Temperature Profile at Fresno on December 17, 1999...........31 

19 T850 Stability and Precipitation vs. PM10 &2.5 at Hanford, Dec. 1999...........33 

20 Atmospheric Temperature Profile at Fresno on December 23, 1999...........34 

21 Corcoran (Coarse, PM2.5 &10) and Fresno-1st (PM2.5) 12/17/99..................35 

22 Fresno Drummond (PM10) and Fresno 1st (PM2.5) 12/17/99.......................36 

23 Hanford (PM10) and Fresno 1st (PM2.5) 12/23/99.....................................38 

24 T850 Stability vs. PM10 and PM2.5 at Corcoran for 12/99 and 01/00............40 

25 PM2.5 at Fresno 1st and Bakersfield California for 12/99 and 01/00.............42 

26 Atmospheric Temperature Profile at Fresno on Dec. 17, 1999..................43 

27 T850 Stability and Precipitation vs. PM10 at Hanford 12/99.......................46 

28 Atmospheric Temperature Profile at Fresno on Dec. 23, 1999..................46 

29 Wind Rose at Clovis Air Monitoring for 12/17 & 12/23/99, & 01/07/2000......49 

30 Fresno-1st Stability and Pollutant Trends for Fall 2000 & Winter 2001.........52 

31 Fresno-1st Fall 2000 and Winter 2001 Episode.......................................59 

32 Fresno-1st (Coarse) & (PM2.5) January 2001 Episode..............................65 

33 Bakersfield California (Coarse) &(PM2.5) January 2001 Episode.................65 

34 Bakersfield Golden (Coarse, PM10 &2.5) &Baker. Cal. (PM2.5) ’01 Episode.....65 

35 Hanford, Oildale, & Modesto (PM10) & Fresno-1st &Baker. Cal. (PM2.5).........66 

36 Fresno-1st (Coarse) and (PM2.5) January 2001 Episode............................70 

37 Bakersfield California (Coarse) and (PM2.5) January 2001 Episode..............72 

38 Bakersfield Golden (Coarse, PM10 &2.5) &Baker. Cal. (PM2.5) ’01 Episode.....72 

39 Hanford, Oildale, & Modesto (PM10) & Fresno-1st &Baker. Cal. (PM2.5) 

January 2001 Episode........................................................................72 

40 T850 Stability & Precipitation vs. PM10 at Hanford, Fall & Winter 2001..........77 

41 Atmospheric Temperature Profile at Fresno on November 9, 2001..............79 

42 Hanford (PM10) and Fresno 1st (PM2.5), November 9, 2001.........................81 

43 Corcoran (PM2.5&Coarse) and Fresno-1st (PM2.5), November 9, 2001...........82 
vi

LIST OF FIGURES (continued) 

44 Visalia Lower Air Profiler Image for May 19 – May 20, 2002....................86 

45 Lost Hills Lower Air Profiler Image for May 19 – May 20, 2002.................86 

46 Hourly Ave. Wind Speeds at Bakersfield Golden on 5/19 – 5/20, 2002......89 

LIST OF TABLES 

1 Federal Reference Method (FRM) Daily Ave. PM Measurements, 11/6/97.... 1 

2 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS, & CIMIS sites for 11/6/97..... 3 

3 FRM Daily Ave. PM Measurements for sites across SJV for 12/31/1998....... 5 

4 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS, & CIMIS sites for 12/31/98... 6 

5 FRM Daily Ave. PM Measurements for sites across SJV for 01/12/1999....... 7 

6 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS, &CIMIS sites for 1/12/99......8 

7 FRM Daily Ave. PM Measurements for sites across the SJV for 11/14/99....23 

8 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS, & CIMIS sites for 11/14/99...25 

9 FRM Daily Ave. PM Measurements for sites across the SJV for 12/17/99.....30 

10 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS, & CIMIS sites for 12/17/99...31 

11 FRM Daily Ave. PM Measurements for sites across the SJV for 12/23/99.....32 

12 CRPAQS Daily Ave. PM Meas. for Fresno-Dru. & Hanford for 12/23/99.......32 

13 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS, & CIMIS sites for 12/23/99...34 

14 FRM Daily Ave. PM Measurements for sites across the SJV for 12/17/99.....42 

15 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS,& CIMIS sites for 12/17/99....44 

16 FRM Daily Ave. PM Measurements for sites across the SJV for 12/23/99.....45. 

17 CRPAQS Daily Ave. PM Meas. for Fresno-Dru. & Hanford for 12/23/99.......45 

18 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS, & CIMIS sites for 12/23/99...47 

19 CRPAQS Episode Event Summary Table at Fresno................................53 

20 PM10 Chemical Composition Data for Jan. 1, 4, &7 2001 at selected sites.....64 

21 PM10 Chemical Composition Data for Jan. 1, 4, & 7 2001 at selected sites....71 

22 FRM Daily Ave. PM Measurements for sites across the SJV for 11/9/2001....78 

23 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS, &CIMIS sites for 11/9/2001....79 

24 Peak & Daily Ave. PM Measurements for sites across the SJV for 5/19/2002..84 

25 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS, & CIMIS sites for 5/19/2002...85 

26 Peak & Daily Ave. PM Measurements for sites across the SJV for 5/20/2002..87 

27 24 hr. Ave. Wind Speeds at SJVAPCD, ASOS, & CIMIS sites for 5/20/2002...88 
vii

EXECUTIVE SUMMARY 

Study Purpose 

This document characterizes meteorology and air quality experienced during the 

Particulate Matter (PM) season from November 1997 through May 2002. The evaluation 

was focused on understanding the causes of particulate matter concentrations that 

exceeded the National Ambient Air Quality Standards (NAAQS) in the San Joaquin 

Valley by exploring the meteorology and chemistry of each episode. Utilizing daily 

PM2.5 and PM10 concentrations, chemical composition, and meteorological data, each 

PM10 episode was identified. An episode was defined as the complete period, from the 

beginning through the end, where PM10 concentrations increased to a peak that 

exceeded the Federal PM10 Standard of 150 μg/m3 and then decreased dramatically as 

atmospheric dispersion conditions improved. This analysis was a collaborative effort 

with contributions from the Air Resources Board staff, including Ms. Karen Magliano, 

Ms. Kasia Turkiewicz, and Mr. Rich Hackney and also members of the PM10 modeling 

group. The general objectives of the characterization analyses are to provide: 

1. Support for the 2003 San Joaquin Valley PM State Implementation Plan (SIP) and 

other District programs 

2. Development and improvement of the conceptual model 

3. An understanding of San Joaquin Valley meteorology and how it influences 

particulate formation and accumulation. 

 

Summary of Findings 

During each episode, cooler air at the surface and warm air above the mixing layer 

trapped pollutants under a strong temperature inversion. Horizontal movement of air 

was minimal and disorganized reducing dispersion and transport of pollutants. These 

conditions caused particulates to increase throughout the San Joaquin Valley. Under 

the poor mixing conditions, coarse and fine particulates accumulated leading to high 

particulate concentrations. 

A majority of the episodes were characterized by a prolonged period (two to three 

weeks) of limited mixing and light wind flow. The November 14, 1999 and November 9, 

2001 were an exception, when strong stability and local emissions drove concentrations 

at the PM10 monitoring sites over the NAAQS in less than a week and a half. Winter 

exceedances (December and January) were characterized by an increase in fine 

particles to a level that dominated filter samples. Fall exceedances (October and 

November) were dominated by coarse particles. To a lesser extent in the fall and a 

greater extent in the winter, cool damp mornings and restricted vertical air movement 

contributed to the formation of nitrates and sulfates. Total carbon concentrations from 

combustion sources remained proportionally the same during the fall and winter 

exceedances. Due to stagnant weather conditions, the elevated PM10 and PM2.5 

measurements that resulted in an exceedance of the NAAQS were caused primarily by 
viii

local emission sources, rather than background or long-range transport of material in 

most of the episodes. An analysis of a wind blow dust event that occurred on May 19 

and 20, 2002 was also done. 

As stated above during most episodes winds and transport were minimal. However, 

during the CRPAQS 2001 episode given the length of the episode and the large 

contributions from secondary components, there was an underlying regional component 

to this episode as it progressed. Local carbon and geologic contributions added to this 

regional component and influenced site to site concentration variations. As the 

CRPAQS episode continued, PM and precursors became more homogeneous across 

the region. This resulted in PM2.5 concentrations at rural sites lagging those of urban 

sites, and rural concentrations continued to build throughout the episode. 

The October 1999 particulate episode was unique and did not follow the general 

meteorological and chemical pattern observed in other episodes. Concentrations during 

this event were dominated by geological particles (PM10), with significant contributions 

from fine particulates of ammonium nitrate and sulfate and total carbon. The abundance 

of fine particulates in the samples may have been due to abnormalities in atmospheric 

chemistry reactions. Due to several wildfires to the north and a major tire fire at Westley 

earlier in October, particulate loading aloft may have decreased solar radiation intensity 

measurements across the Valley Floor. With reduced solar radiation, the atmospheric 

chemistry reactions may have changed from the ozone forming regime of mid-October 

to the secondary particulate regime of late November. Although geological particulates 

dominated the samples, fine fractions of PM were elevated. Limited afternoon heating 

and stagnant weather conditions, resulted in local sources driving PM10 concentrations 

to exceed the NAAQS. 

With these similarities and differences among the episodes, each PM10 episode is 

discussed in detail in the following sections. PM10 and PM2.5 concentrations, chemical 

composition, and meteorological data around the exceedance dates are evaluated and 

analyzed to identify the characteristics and uniqueness of the exceedances at the FRM 

and CRPAQS PM10 and PM2.5 monitoring sites.
1

1.0 NOVEMBER 06, 1997 EPISODE SYNOPTIC METEOROLOGICAL 

DISCUSSION 

October 11 – November 7, 1997

After a cold front and trough passage on October 11th, high pressure from the eastern 

Pacific built into the region, with increasing stability and poor dispersion conditions 

through the exceedance date on November 6th, Figure 1. At Corcoran, Patterson Ave., 

a 24-hour PM10 (Particulate Matter) concentration of 199 μg/m3 was measured. Table 1

outlines federal reference method (FRM) Daily Average Particulate Matter 

Measurements for sites across the San Joaquin Valley (SJV). In order to understand 

the variability of these measurements, an in depth examination of the synoptic pattern 

and surface winds and observations, and aircraft soundings leading to the episode were 

analyzed. 

Figure 1: Corcoran PM10, Fresno Precipitation, and T850 Stability

Fall 1997

0

25

50

75

100

125

150

175

200

10/1/199710/3/199710/5/199710/7/199710/9/199710/11/199710/13/199710/15/199710/17/199710/19/199710/21/199710/23/199710/25/199710/27/199710/29/199710/31/199711/2/199711/4/199711/6/199711/8/199711/10/199711/12/199711/14/199711/16/199711/18/199711/20/199711/22/199711/24/199711/26/199711/28/199711/30/1997

PM10 (micrograms/meter^3) and Precipitation/100 

(inches)

-20

-15

-10

-5

0

5

10

15

T850 Stability Parameter (C)

Corcoran PM10

Fresno PPT/100

FAT T850 Stability

TABLE 1: Federal Reference Method (FRM) Daily Average Particulate Matter 

Measurements for sites across the SJV for November 06, 1997.

 FRM FRM FRM 

Site Name 24-Avg. Site Name 24-Avg. Site Name 24-Avg. 

Corcoran* 199 Visalia 83 Bakersfield-CA 95

Corcoran# 154 Turlock 56 Bakersfield-Gold 124

Clovis 83 Modesto 44 Hanford 143

Fresno-1st 92 Taft 42 Oildale 125

*-Patterson #-Van Dorsten 

units in μg/m3

 
2

The meteorological synoptic analysis showed after an extended period of strong high 

pressure and surface and upper air stability from October 11th through November 5th, 

leading to building PM conditions, a trough developed across the eastern Pacific, the 

morning of the 6th. The trough and accompanying weak cold front slowly moved 

southeastward across California during the late afternoon hours. The morning surface 

charts depicted a surface high pressure ridge draped across Central California from the 

Four Corners Region to San Francisco. The 12Z surface pressure gradient was –0.6 

millibars from San Francisco (SFO) to Las Vegas (LAS). A –0.6 millibars pressure 

gradient means light offshore flow, which results in weak disorganized wind flow across 

the San Joaquin Valley. 

The morning temperature sounding over Fresno showed a strong inversion (stable 

layer) of 19 degrees Fahrenheit from the surface up to 1,500 feet as is evident in Figure 

2. At Bakersfield the temperature sounding showed a similar strong inversion of 17 

degrees Fahrenheit from the surface up to 1,500 feet. The temperature soundings on 

the 6th, are conducive of elevated PM levels due to low mixing depths, which keep 

pollutants trapped near the surface.

Figure 2: Atmospheric Temperature Profile at Fresno on November 06, 1997

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

50 55 60 65 70 75 80

Temperature (deg F)

Feet (AGL)

Upper level charts indicated a moderate low near Vancouver Island, with a weak trough 

curving south-southeastward across the eastern Pacific and along the West Coast. A 

moderate temperature gradient at 850 MB and an upper level jet were evident over 

central California. Weak pressure gradients across the San Joaquin Valley 

strengthened slightly during the afternoon hours, with the approach of the cold front. 

Table 2 shows the 24 hour daily average wind speeds at SJVAPCD air monitoring, 

ASOS, and CIMIS sites for November 06, 1997. 
3

Table 2: 24 hour average wind speeds at SJVAPCD air monitoring, ASOS, and CIMIS 

sites for November 06, 1997. 

SJVAPCD Air Monitor ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

Clovis 4.8 FAT 6.8 Shafter/USDA 3.8 Famoso 3.6

Fresno Sierra Sky Park (SSP) 3.3 Firebaugh/Telles 4.6 Westlands 4.5

Corcoran 6.8 Stratford 5 Panoche 4

 Kettleman 3.8 Arvin-Edison 3.2

 Visalia/Americas 3.1 Lindcove 2.1

 Parlier 4.3 Kesterson 3.1

 Blackwells Corner 3.7 Lodi West 2.9

 Los Banos 4.5 Modesto 3.6

 Manteca 2.8 Fresno State 5.1

Due to strong stability lasting for over 28 days, PM10 steadily increase region-wide until 

November 6th. Light disorganized wind flow, a strong morning inversion and pre-frontal 

stability; lead to limited dispersion conditions on the 6th causing local PM emissions in 

Corcoran to rapidly increase past the Federal 24-hour PM10 standard to 199 μg/m3

. 

Since other FRM Monitoring sites did not register above the Federal 24-hour PM10

standard on the 6th, the Corcoran PM10 measurement signifies local emission 

characteristics. Crop calendar activity shows that cotton/safflower and alfalfa were 

being prepared and harvested during early November. A weak frontal boundary moved 

through the region on the 7th, bringing better dispersion conditions and lowering 

Corcoran’s PM measurement to well below the standard. 
4

2.0 DECEMBER 31, 1998 AND JANUARY 12, 1999 EPISODE 

SYNOPTIC METEOROLOGICAL DISCUSSION 

December 20, 1998 – January 16, 1999

The period from December 20, 1998 to January 16, 1999 was marked by 28 days of 

strong stability and poor atmospheric dispersion conditions. Strong high pressure over 

the Intermountain Region and eastern Pacific dominated the period, leading to limited 

dispersion. The period began with the passage of a cold, arctic front and trough on the 

20th. Between the 20th and the 31st, increasing stability and poor dispersion conditions 

resulted in a PM10 (Particulate Matter) exceedance at Bakersfield Golden and Visalia 

Church on the 31st, Figure 3. 

Figure 3: T850 Stability and Precipitation vrs. PM10 

at Bakersfield Golden 

December 1998 and Mid January 1999

0

20

40

60

80

100

120

140

160

12/1/199812/3/199812/5/199812/7/199812/9/199812/11/199812/13/199812/15/199812/17/199812/19/199812/21/199812/23/199812/25/199812/27/199812/29/199812/31/19981/2/19991/4/19991/6/19991/8/19991/10/19991/12/19991/14/19991/16/19991/18/1999

PM10 (micrograms/meter^3) and 

Precipitation/100 (inches)

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0T850 Stability Parameter(C)

Precipitation/100(inches)

PM10

T850 Stability(OAK)

T850 Stability(VBG)

At Bakersfield Golden and at Visalia Church, a 24-hour PM10 concentration of 159 and 

160 μg/m3

 were measured, respectively. Table 3 outlines federal reference method 

(FRM) Daily Average Particulate Matter Measurements for sites across the San Joaquin 

Valley (SJV). In order to understand the variability of these measurements, an in depth 

examination of the synoptic pattern and surface winds and observations, and aircraft 

soundings leading to the episode were analyzed. 
5

TABLE 3: Federal Reference Method (FRM) Daily Average Particulate Matter 

Measurements for sites across the SJV for December 31, 1998.

 FRM FRM FRM 

Site Name 24-Avg. Site Name 24-Avg. Site Name 24-Avg. 

Bakersfield-Gold 159 Turlock 87 Hanford 109

Bakersfield-CA 148 Modesto 80 Stockton# 99

Visalia 160 Taft 73 Stockton** 95

Fresno-1st 104 Fresno-Drum 117 Corcoran* 116

*-Patterson #-Wagner, **- Hazleton 

units in μg/m3

 

The meteorological synoptic analysis showed after a period of strong atmospheric 

stability from December 20th through the 31st, a weak disturbance from the eastern 

Pacific approached the region. Ahead of the weak trough, stability lingered over the 

southern parts of the San Joaquin Valley trapping particulates within the Valley 

boundary layer. The afternoon surface charts on the 31st depicted a surface ridge 

extending southeastward across southern California from a strong high 200 NM west of 

Eureka. A weak cold front was draped northwestward along the Sierra Nevada 

Mountain range from near Bishop to Reno. The 0Z surface pressure gradient was 

+13.0 millibars from San Francisco (SFO) to Las Vegas (LAS), with isobars(constant 

surface pressure) orientated northeast to southwest. With the alignment of the isobars 

and the +13.0 millibars pressure gradient, this represents light to moderate northnortheasterly flow across the San Joaquin Valley. Visibilities throughout the day 

across the San Joaquin Valley were reporting haze. 

The morning temperature aircraft sounding over Bakersfield showed multiple inversions, 

with a moderately strong inversion (stable layer) of 10 degrees Fahrenheit from the 

surface up to 2,000 feet, with a secondary inversion of 4 degrees Fahrenheit from 2,500 

to 4,500 feet as is evident in Figure 4. The temperature sounding on the 31st, is 

conducive of elevated PM levels due to low mixing depths and multiple inversions, 

which keep pollutants trapped near the surface. At surface observations across the San 

Joaquin Valley, minimum temperature at Fresno was 38 degrees Fahrenheit and 

Bakersfield was 37 degrees Fahrenheit. These cold and poor dispersion conditions 

generally result in elevated concentrations of ammonia nitrate. Also on New Year’s Eve 

(December 31), there is a great amount of residential wood combustion in the area. 
6

Figure 4: Atmospheric Temperature Profile at Bakersfield on December 31, 1998

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

40 45 50 55 60

Temperature (deg F)

Feet (AGL)

Upper level charts indicated a strong high 600 NM west of Santa Barbara, with a ridge 

building northward into the eastern Gulf of Alaska. A weak trough over the 

Intermountain region extended southwestward across northern California. Weak 

pressure gradients across the San Joaquin Valley strengthened during the afternoon 

hours, with the approach of the cold front. 

Table 4 shows the 24 hour daily average wind speeds at SJVAPCD monitoring sites, 

ASOS, and CIMIS sites for December 31, 1998. 

SJVAPCD Monitoring Sites ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

Clovis 5.5Fresno 6.5Shafter/USDA 3.0 Famoso 3.2

Fresno SSP 3.0Bakersfield 4.0Firebaugh/Telles 5.4 Westlands 6.5

Corcoran 4.7Hanford 4.7Stratford 3.8 Panoche 5.6

Edison 4.3 Kettleman 3.6 Arvin-Edison 3.0

Parlier 5.3 Visalia/Americas 2.8 Lindcove 2.1

Arvin 3.6 Parlier 4.2 Kesterson 5.0

Visalia 1.8 Blackwells Corner 3.3 Lodi West 3.9

 Los Banos 6.3 Modesto 5.6

 Manteca 4.9 Fresno State 5.2

Due to the strong stability lasting for over 11 days, PM10 steadily increase region-wide 

until December 31st. With the approach of the weak upper level trough from northern 

California, moderately strong stability lingered over the southern parts of the San 

Joaquin Valley. This resulted in local PM10 concentrations at Bakersfield-Golden and 

Visalia to increase above the Federal 24-hour PM10 standard to 159 and 160 μg/m3

, 

respectively. Due to the tightening pressure gradient in the northern part of the San 

Joaquin Valley and weak upper level trough passage, widespread San Joaquin Valley 

PM10 exceedances did not occur on this date. The weak upper level trough traversed 

the entire region late on the 31st bringing slightly better dispersion conditions to the 

entire San Joaquin Valley on January 1st, resulting in lower PM Valley-wide. 
7

After the weak upper level trough passage on the 31st, strong high pressure rebuilt into 

the region from the Great Basin and eastern Pacific, with increasing stability and poor 

dispersion conditions through the next exceedance date on January 12th at Oildale, 

Figure 3. At Oildale, a 24-hour PM10 (Particulate Matter) concentration of 156 μg/m3

was measured. 

Table 5 outlines federal reference method (FRM) Daily Average Particulate Matter 

Measurements for sites across the San Joaquin Valley (SJV). In order to understand 

the variability of these measurements, an in depth examination of the synoptic pattern 

and surface winds and observations, and aircraft soundings leading to the episode were 

analyzed. 

TABLE 5: Federal Reference Method (FRM) Daily Average Particulate Matter 

measurements for sites across the SJV for January 12, 1999. 

 FRM FRM FRM 

Site Name 24-Avg. Site Name 24-Avg. Site Name 24-Avg. 

Bakersfield-Gold 109 Turlock 53 Oildale 156

Bakersfield-CA 92 Modesto 48 Stockton# 39

Visalia 82 Taft 70 Stockton** 48

Fresno-1st 85 Fresno-Drum 76 Corcoran* 51

*-Patterson #-Wagner, **- Hazleton 

units in μg/m3

 

The meteorological synoptic analysis showed after a period of strong atmospheric 

stability from December 31st through the 12th, a weak upper level disturbance from the 

eastern Pacific approached the region. Ahead of the weak trough, mid-tropospheric 

stability lingered over the southern parts of the San Joaquin Valley trapping particulates 

within the Valley boundary layer. The morning surface charts of January 12th depicted 

a surface ridge extending southeastward across the central San Joaquin Valley from a 

strong high just west of San Francisco. The 12Z surface pressure gradient was +9.0 

millibars from San Francisco (SFO) to Las Vegas (LAS), with isobars (constant surface 

pressure) orientated northeast to southwest. With the alignment of the isobars and the 

+9.0 millibars pressure gradient, this represents light to moderate north-northeasterly 

flow across the San Joaquin Valley. Visibilities across the San Joaquin Valley in the 

morning reported fog turning to haze by the afternoon. 

The morning temperature aircraft sounding over Bakersfield showed multiple inversions, 

with a moderately strong inversion (stable layer) of 12 degrees Fahrenheit from the 

surface up to 1,000 feet, with a secondary inversion of 4 degrees Fahrenheit from 1,500 

to 3,000 feet as is evident in Figure 5. The temperature sounding on the January 12th, 

is conducive of elevated PM levels due to low mixing depths and multiple inversions, 

which trap pollutants near the surface. At surface observations across the San Joaquin 
8

Valley, minimum temperature at Fresno was 30 degrees Fahrenheit and Bakersfield 

was 29 degrees Fahrenheit. 

Figure 5: Atmospheric Temperature Profile at Bakersfield on January 12, 1999

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

30 35 40 45 50

Temperature (deg F)

Feet (AGL)

Upper level charts indicated a moderate high well off of Baja, with a ridge draped 

northeastward across central California. A weak trough bends southward along the 

Pacific Northwest Coast from a low over the eastern Gulf of Alaska. Pressure gradients 

weakened slightly during the afternoon hours further decreasing boundary layer flow in 

the San Joaquin Valley. 

Table 6 shows the 24 hour daily average wind speeds at SJVAPCD monitoring sites, 

ASOS, and CIMIS sites for January 12, 1999. 

SJVAPCD Monitoring Sites ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

Clovis 2.6Fresno 1.8Shafter/USDA 2.6 Famoso 2.7

Fresno SSP 1.5Bakersfield 4.8Firebaugh/Telles 2.5 Westlands 3.1

Corcoran 3.0Hanford 1.3Stratford 2.9 Panoche 3.1

Edison 4.0 Kettleman 3.2 Arvin-Edison 2.9

Parlier 2.9 Visalia/Americas 2.4 Lindcove 2.0

Arvin 3.3 Parlier 2.5 Kesterson 2.4

Visalia 1.4 Blackwells Corner 2.5 Lodi West 2.7

 Los Banos 2.9 Modesto 4.5

 Manteca 4.0 Fresno State 2.7

Due to the strong stability lasting for over 24 days, PM10 steadily increase region wide 

until January 12th. With the approach of the weak trough from northern California, 

moderately strong stability lingered over the southern parts of the San Joaquin Valley 

leading to local PM concentrations at Oildale increasing above the Federal 24-hour 

PM10 standard at 156 μg/m3. Due to the increasing dispersion aloft from the weak 

upper level trough passage, widespread San Joaquin Valley PM10 exceedances did not 

occur on this date; thus the exceedance at Oildale was representative of local 

emissions driving the particulate measurement. Weak upper level disturbances 
9

continued to traverse the region through January 16th, bringing better dispersion 

conditions and an end to the PM episode that recorded exceedances at Bakersfield and 

Visalia on December 31st, 1998 and Oildale on January 12th, 1999. 
10

3.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE JANUARY 12, 1999 EPISODE

The January 1999 particulate episode was characterized by a prolonged period (four 

weeks) of strong stability and light wind flow, which resulted in poor atmospheric 

dispersion conditions across the San Joaquin Valley. Coarse and fine particulates 

accumulated during the period, leading to two separate exceedance days on Thursday, 

December 31, 1998 and Tuesday, January 12, 1999. A weak upper level disturbance 

moved over the region on December 31, 1998, dividing the episode into two separate 

periods. Concentrations were dominated by the fine particulates (PM2.5) of ammonia 

nitrate and sulfate and were most prevalent in the central and southern San Joaquin 

Valley. The first period was marked by strong stability and light wind flow, which led to 

exceedances at Bakersfield-Golden with a PM10 concentration of 159 μg/m3

 and VisaliaChurch with a PM10 concentration of 160 μg/m3

 on December 31, 1998 (Figure 6 & 7). 

PM10 concentrations at other central and southern Valley locations were much lower, 

below 115 μg/m3

, with the exception of nearby Bakersfield-California, which measured a 

PM10 concentration of 148 μg/m3

. However, the December 1998 exceedances are 

included in the discussion to provide context to the episode. After the weak trough 

passage on December 31, 1998 both primary and secondary pollutants accumulated 

once again leading to an exceedance of the 24-hour standard at Oildale-Manor, with a 

PM10 concentration of 156 μg/m3

 on January 12, 1999 (Figure 7). The next highest 

PM10 concentration measured was at Bakersfield-Golden with 109 μg/m3. PM10

concentrations at other central and southern Valley locations were below 90 μg/m3

. 

During both periods, strong high pressure at the surface and aloft resulted in limited 

afternoon mixing and light offshore wind flow. Cool damp mornings and strong stability 

contributed to the formation of nitrates and sulfates during the episode. Chemical 

composition and meteorological data around December 31, 1998 and January 12, 1999, 

were evaluated and analyzed to identify the characteristics and uniqueness of the 

exceedances at Bakersfield-Golden, Visalia-Church, and Oildale-Manor. 

The December 31, 1998 exceedances are not considered for compliance with the 

current Particulate Matter State Implementation Plan (SIP) because the exceedances 

fall outside of the three year required window. Although PM10 sampling is not 

conducted daily, information from changes in the meteorological conditions suggest that 

the highest PM10 concentration was not captured on the sampled day (December 31, 

1998). Daily meteorological conditions suggested that the highest PM10 concentrations 

may have occurred on December 29th, when stability was the strongest. With the lack 

of chemical data for Bakersfield-Golden, similarities in trends from other sites were 

analyzed in order to conclude which chemical components were responsible for the 

exceedance. The analysis shows that the exceedance was driven by high 

concentrations of ammonium nitrate and sulfate that comprised 74% of the PM10 mass 

at nearby Bakersfield-California. At Visalia, the nitrate and sulfate chemical component 

of the PM10 sample was slightly higher compared to Bakersfield-California. These have 

accounted for the slight differences in the PM10 mass between the two sites. The other 

major chemical components of the sample were 21% total carbon and 13% geological 

of the PM10 mass. The size fraction data across the Valley indicated that most of the 
11

PM10 was in the PM2.5 fraction, with an average PM2.5/PM10 mass ratio of greater than 

0.8. 

Figure 6: Bakersfield-Golden (Coarse, PM10, PM2.5) 

 December 1998 & January 1999 Episode

0

20

40

60

80

100

120

140

160

12/20/1998

12/22/1998

12/24/1998

12/26/1998

12/28/1998

12/30/1998

1/1/1999

1/3/1999

1/5/1999

1/7/1999

1/9/1999

1/11/1999

1/13/1999

1/15/1999

Particulate Matter (micrograms/cubic meter)

BFL-Gold PM10

BFL-Gold Coarse

BFL-Gold PM2.5

Figure 7: Visalia and Oildale PM10 

December 1998 & January 1999 Episode

0

20

40

60

80

100

120

140

160

12/20/1998

12/21/1998

12/22/1998

12/23/1998

12/24/1998

12/25/1998

12/26/1998

12/27/1998

12/28/1998

12/29/1998

12/30/1998

12/31/1998

1/1/1999

1/2/1999

1/3/1999

1/4/1999

1/5/1999

1/6/1999

1/7/1999

1/8/1999

1/9/1999

1/10/1999

1/11/1999

1/12/1999

1/13/1999

1/14/1999

1/15/1999

1/16/1999

Particulate Matter (micrograms/cubic meter)

Vis PM10

Oildale PM10

This was further supported by the high PM2.5 concentrations that reached 122 μg/m3 at 

nearby Bakersfield-California accounting for 81 percent of the PM10 mass and at Visalia 

of 139 μg/m3

 accounting for 83 percent of the PM10 mass. 

The concentration of coarse material may have been higher at the Bakersfield-Golden 

as is indicated by nearby Bakersfield-California and Visalia having the highest 

geological component of 13 μg/m3

 compared to other Valley locations. Urban 

geological may have influenced the sample at Bakersfield-Golden and Visalia. Light 

afternoon mixing and wind flow may have allowed some transported contribution from 

agricultural burning in Merced, Stanislaus, and San Joaquin Counties on December 

31st. However a no burn day across the rest of the Valley, indicates that stagnant 
12

conditions favored a larger influence from local emissions. Burn variances and 

noncompliant agricultural burns may have contributed to a small portion of the samples. 

Overnight minimum temperatures in the mid to upper 30’s suggest that residential wood 

burning may have been a significant source of PM10. The potential of increased 

residential wood burning on New Year’s Eve, may have led to increased concentrations 

of total carbon in the samples. Analysis of PM10 and PM2.5 concentrations showed that 

meteorological conditions led to the pollution buildup. Ammonium nitrate and sulfates, 

carbon, and geological material increased prior to the exceedances on December 31, 

1998 and then later on January 12, 1999. 

Meteorologically, dispersion worsened and particulate formation conditions 

strengthened from December 20th to the 29th. However, on December 31st, the 

exceedance day, dispersion conditions began to improve under decreasing stability. 

After the passage of a cold front on December 20th, which brought 0.10 inches of rainfall 

to Bakersfield, moisture was available for atmospheric chemistry reactions. Humidity 

measurements of 85 – 100 % in the morning across the Valley Floor showed a moist 

atmosphere with light fog and haze being reported. These cool damp mornings and 

strong stability favored the formation of nitrates and sulfates. Synoptically, the eastern 

Pacific high remained strong through December 29th and gradually weakened through 

December 31st. The high rebuilt over the region on January 1st, keeping a lid over the 

San Joaquin Valley trapping pollutants within the Valley boundary layer through January 

16, 1999. 

 

With a strong lid in place and maximum high temperatures on December 31, 1998 in the 

mid to upper 50’s, the afternoon hours were marked by limited mixing, resulting in 

increasing particulate conditions. Mixing heights at Fresno remained below 500 feet 

under a strong inversion for 14 hours on December 31, 1998 breaking out by 10:00 AM 

and reforming shortly after 7:00 P.M. During the afternoon hours, there were higher 

mixing depths (maximum mixing depth of 4,500 feet), but the bulk of the day had limited 

to marginal mixing. Maximum temperatures for the December 1998 episode were 

slightly above normal, further illustrating the intensity of the high pressure system that 

was controlling the region’s weather. Satellite imagery depicted extensive moisture 

(high and mid-level cloudiness) across the region on the 31st, resulting in slightly lower 

levels of solar radiation intensity. Along with the lower solar radiation intensities due to 

the low sun angle and limited daylight hours, the atmospheric chemistry reactions may 

have favored the secondary particulate forming regime. 

Chemical composition and meteorological data showed with limited mixing depths and 

light and disorganized wind flow, these conditions resulted in minimal transport and 

dispersion of pollutants. Both primary and secondary pollutants from local emissions 

around Bakersfield-Golden and Visalia accumulated resulting in the exceedance of the 

24-hour PM10 standard on December 31, 1998. 

Meteorological stability between December 31, 1998 and January 10, 1999 remained 

strong leading to moderate to elevated particulate conditions continuing across the San 

Joaquin Valley. Weak disturbances were moving through northern California prior to 
13

the exceedance day, inhibiting widespread exceedance levels. The strong ridge that 

dominated the region’s weather finally began to breakdown on January 10, 1999, with 

decreasing stability. However, both primary and secondary pollutants from local 

emissions around Oildale-Manor accumulated resulting in an exceedance of the 24-

hour PM10 standard on January 12, 1999 (Figure 7); with nearby Bakersfield Golden 

recording a peak PM10 concentration on the exceedance day of 109 μg/m3. 

Additionally, later on January 14, 1999 PM2.5 monitoring at nearby Bakersfield California 

showed peak concentrations of 109 μg/m3

. Concentrations were dominated by the fine 

particulates (PM2.5) of ammonia nitrate and sulfate and were most prevalent in the 

central and southern San Joaquin Valley. During the period, strong high pressure at the 

surface and aloft resulted in limited afternoon mixing and light offshore flow. Moisture 

from recent rainfall was available and strong morning stability contributed to the 

formation of nitrates and sulfates during the episode. These meteorological conditions 

led to elevated particulate concentrations. Chemical composition and meteorological 

data around January 12, 1999, was evaluated and analyzed to identify the 

characteristics and uniqueness of the exceedance at Oildale-Manor. 

Although PM10 sampling is not conducted daily, information from more frequent PM2.5

sampling combined with an assessment of changes in the meteorological conditions 

suggest that the highest PM10 concentration was not captured on the sampled day. 

Analysis of daily meteorological conditions suggested that the highest PM10

concentrations may have occurred on January 10th, when stability was the strongest. 

Chemical data from Oildale-Manor showed that the exceedance was driven by high 

concentrations of ammonium nitrate and sulfate comprising about 57 % of the PM10

mass. The size fraction data across the Valley indicated that most of the PM10 was in 

the PM2.5 fraction, with an average PM2.5/PM10 mass ratio of greater than 0.8. This was 

further supported by the high PM2.5 concentrations that reached 84 μg/m3 at nearby 

Bakersfield-California. 

The concentration of coarse material was minimal on the exceedance day, indicating 

that the majority of the sample was comprised of fine particulates. Agricultural burning 

may not have contributed as a potential source of PM on the exceedance day, because 

January 8 through January 12, 1999 were declared No Burn Day’s District-wide. 

However, burn variances and noncompliant agricultural burns may have contributed to a 

small portion of the samples. Overnight minimum temperatures in the upper 20’s to low 

30’s suggested that residential wood burning may have been a significant source of 

PM10. Residential wood burning may have been a potential source of carbon. 

Concentrations of total carbon were measured at three sites in the Valley and ranged 

from 12 μg/m3

 at Modesto-14th Street to 27 μg/m3at Fresno-1st. PM10 and PM2.5

concentrations showed that meteorological conditions led to the exceedance at OildaleManor. Local sources of ammonia nitrate and sulfate coupled with moderately strong 

atmospheric stability resulted in the high PM10 recorded at the exceedance site. 

Meteorologically, dispersion conditions began to improve slightly a few days prior to the 

January 12, 1999 exceedance. After the passage of a weak cold front on December 

20th, which brought 0.10 inches of rainfall to Bakersfield, some moisture was available 
14

for atmospheric chemistry reactions. Humidity measurements of 85-95% in the morning 

across the Valley Floor showed a moist atmosphere, with light to dense fog and haze 

being reported. These cool damp mornings and strong stability favored the formation of 

nitrates and sulfates. Synoptically, the eastern Pacific high began to break down on the 

10th, but stability remained strong across the region until the exceedance day. The high 

kept a strong lid in place over the Valley, trapping pollutants within the San Joaquin 

Valley boundary layer. 

With a strong lid in place and maximum high temperatures in the low to mid 50’s, the 

afternoon hours were marked by limited mixing, resulting in elevated particulate 

conditions. Mixing heights at Fresno remained below 500 feet for 21 hours on January 

12th, breaking out by 3 P.M. and reforming shortly after 5:00 P.M. During the afternoon 

hours, there were higher mixing depths (maximum mixing depth of 1,000 feet), but the 

bulk of the day had limited mixing. Light to dense fog was reported at the Valley surface 

during the morning hours of January 12th, resulting in lower solar radiation intensities. 

Along with lower solar radiation intensities due to the low sun angle and decreasing 

daylight hours, the atmospheric chemistry reactions may have favored the secondary 

particulate forming regime. 

Chemical composition and meteorological data showed limited mixing depths and light 

and disorganized wind flow resulted in minimal transport and dispersion of pollutants. A 

major cause of the increase in PM10 at Oildale prior to January 12 appears to be from 

nitrates, which was more than double that concentration measured at Bakersfield 

California. This would suggest that something very localized or unique was occurring 

around Oildale that went beyond what was happening in other parts of the Valley. Both 

primary and secondary pollutants from local emissions around Oildale-Manor 

accumulated resulting in the exceedance of the 24-hour PM10 standard on January 12, 

1999. 
15

4.0 OCTOBER 21, 1999 EPISODE SYNOPTIC METEOROLOGICAL 

DISCUSSION

October 07-October 28, 1999

Throughout the San Joaquin Valley on October 21, 1999, several sites exceeded the 

Federal 24-hour PM10 Standard. These sites included: Fresno-Drummond Street at 

162 μg/m3

, Corcoran-Patterson Avenue at 174 μg/m3, and Turlock – S Minaret Street at 

157 μg/m3

. In order to better understand the region wide occurrence of Particulate 

Material (PM) during this event, the synoptic meteorology, stability parameters, satellite 

imagery, atmospheric temperature profiles, and solar intensity measurements were 

investigated to determine the cause for the exceedances, which occurred on October 

21, 1999. 

Strong high pressure aloft and at the surface dominated the region’s weather 

throughout the month of October 1999. This month was characterized by above 

normal temperatures (+3.4 0F) and below normal rainfall (-0.53 inches). The October 

21, 1999 episode was preceded by deteriorating stability conditions after the passage of 

a weak trough into the Pacific Northwest on the 5th. Figure 8 shows precipitation, 

PM2.5, PM10, Model PM, and stability parameters for the October 1999 episode at the 

Fresno-1st site. 

Figure 8 also shows that the October 21st, 1999, episode was marked by a long period 

(21 days) of stable weather, with an average 500 MB height of 5,820 meters and 850 

MB temperature of +16.40C. According to Hackney et. al., both the 500 MB height and 

850 MB temperature were highly suggestive of a PM10 episode occurring. The San 

Joaquin Valley Air District uses regression equations to forecast PM10. As is evident in 

Figure 8, Modeled PM10 tracked closely with both the measured PM10 and 

meteorological parameters. PM10/PM2.5 ratios showed an increasing amount of fine 

particulates within the sample, representing a change from a more typical coarse 

dominate fall regime to an increasingly dominated fine particulate regime of late 

November as is illustrated in Figure 8. 
16

Figure 8: Fresno- 1st PM10 and 2.5, 

Precipitation, T850 Stability, and Model PM10

October 1999

0

10

20

30

40

50

60

70

80

90

100

110

120

130

10/1/199910/3/199910/5/199910/7/199910/9/199910/11/199910/13/199910/15/199910/17/199910/19/199910/21/199910/23/199910/25/199910/27/199910/29/199910/31/1999

PM10 and PM2.5 (micrograms /meter^3) and 

Precipitation /100

-10

-8

-6

-4

-2

0

2

4

6

8

10

T850 Stability Parameter(C)

PM10

Precipitation /100 (inches)

T850 Stability

PM2.5

Model PM

Next, winds speeds were analyzed during the exceedance date across the Valley. For 

example, at the Clovis monitoring site the 24 hour average wind speed was less than 

2.2 knots (2.4 miles per hour). Since there was a strong high positioned across the 

region, the winds were calm in the morning becoming light thermally driven during the 

afternoon, resulting in minimal transport and dispersion. These weak winds are 

indicative of historical PM10 episodes. 

These strong stable conditions continued through the 24th until the ridge began to 

breakdown and move eastward on the 25th. A cool, dry trough moved into the Pacific 

Northwest on the 28th, bringing good dispersion conditions, moderate to high boundary 

layer mixing heights and an end to the October 1999 episode. 

The morning temperature profile from several sites were also investigated to determine 

the inversion strength and potential mixing depth. As is evident in figure 9, the 

morning temperature profile measured at 4:00 a.m. from Fresno depicts a very strong 

inversion of 21 degrees Fahrenheit from the surface up to 1,500 feet, turning slightly 

isothermal (constant temperature) up to 3,000 feet. This type of strong trapping 

inversion was also evident in Sacramento (22 degrees Fahrenheit from the surface up 

to 1,500 feet) and Bakersfield (12 degrees Fahrenheit from the surface up to 2,000 

feet). The strong morning inversions present across the region coupled with very light 

wind flow, trapped most pollutants near the surface in a very low mix layer throughout 

most of the day resulting in the elevated PM10 measurements. 
17

Figure 9: Atmospheric Temperature Profile at Fresno on October 21, 1999

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

50 55 60 65 70 75 80 85 90

Temperature (deg F)

Feet (AGL)

Another meteorological parameter investigated was solar intensity. Solar intensity for 

the month of October was also analyzed to determine if particulate material aloft or at 

the surface was lowering the solar radiation input, which drives both photochemical 

reactions and particulate matter formation. As is evident in figure 10, three sites were 

analyzed; Madera, Parlier, and Arvin. It is evident especially with the solar intensity plot 

for Madera, that particulate matter was present over Madera, thus reducing the solar 

intensity measurement to below normal levels. This in effect, would change the 

atmospheric chemistry reactions from a more photochemistry-ozone forming regime to 

a more particulate regime that occurs during late November. 

Figure 10: October 1999 Solar Intensity

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Date

Value

Parlier 1200

Madera 1200

Arvin 1200

Madera Normal

Poly. (Madera Normal)

Satellite imagery analysis showed a weak low well off of Baja, drawing upper level 

moisture across the region. Varying amounts of high cloudiness were evident 

preceding the peak episode day and occurring through the 22nd. These clouds coupled 

with particulate aloft, drove the photochemistry reactions in the atmosphere into a fine 

particulate forming regime rather than an ozone forming regime. 

Upon investigating the synoptic meteorology, stability parameters, atmospheric 

temperature profiles, and satellite imagery, these all are indicative of a widespread PM10

event, occurring in the San Joaquin Valley on October 21, 1999. 
18

5.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE OCTOBER 21, 1999 EPISODE 

The October 21, 1999 particulate episode was characterized by a prolonged period 

(three weeks) of strong stability and light wind flow, which resulted in poor atmospheric 

dispersion conditions across the San Joaquin Valley. Coarse and fine particulates 

accumulated during the period, leading to an exceedance of the 24-hour PM10 standard 

at three monitoring sites on October 21, 1999. Concentrations were dominated by 

geological particulates (PM10), with significant contributions from fine particulates of 

ammonium nitrate and sulfate and total carbon and were most prevalent in the northern 

and central portions of the Valley and slightly lower in the southern portion (Kern 

County). The highest PM10 concentration of 174 μg/m3

 was measured at CorcoranPatterson (Figure 11). The second and third highest PM10 concentration of 162 and 

157 μg/m3

 were measured at Fresno-Drummond and Turlock, respectively (Figure 12). 

During the period, strong high pressure at the surface and aloft resulted in limited 

afternoon mixing and light offshore wind flow. Cool damp mornings and warm dry 

afternoons contributed to the geologic contribution, as well as nitrate and sulfate 

formation during the episode. A combination of these particulate species and poor 

dispersion conditions elevated particulate concentrations. Chemical composition and 

meteorological data around October 21, 1999, was evaluated and analyzed to identify 

the characteristics and uniqueness of the exceedances at Corcoran-Patterson, FresnoDrummond, and Turlock- S Minaret. 

Figure 11: Corcoran (Coarse, PM10 & PM2.5)

and Fresno-1st (PM2.5)

 October 21, 1999 Episode

0

20

40

60

80

100

120

140

160

180

10/7/1999

10/8/1999

10/9/1999

10/10/1999

10/11/1999

10/12/1999

10/13/1999

10/14/1999

10/15/1999

10/16/1999

10/17/1999

10/18/1999

10/19/1999

10/20/1999

10/21/1999

10/22/1999

10/23/1999

10/24/1999

10/25/1999

10/26/1999

10/27/1999

10/28/1999

Particulate Matter (micrograms/cubic meter)

COR PM10

COR Coarse

COR PM2.5

FAT PM2.5

Although PM10 sampling is not conducted daily, information from more frequent PM2.5

sampling combined with an assessment of changes in the meteorological conditions 

suggest that the highest PM10 concentration was not captured on the sampled day. 

Daily PM2.5 sampling and meteorological conditions suggested that the highest PM10

concentrations may have occurred on October 20th, when Fresno 1st measured a peak 

PM2.5 concentration of 68 μg/m3

. Chemical composition data indicated that geological 

material was the single largest component of the PM10 during this episode. 
19

Figure 12: Turlock-S Minaret and Fresno-Drummond (PM10)

and Fresno-1st (PM2.5)

October 21, 1999 Episode

0

20

40

60

80

100

120

140

160

180

10/7/1999

10/8/1999

10/9/1999

10/10/1999

10/11/1999

10/12/1999

10/13/1999

10/14/1999

10/15/1999

10/16/1999

10/17/1999

10/18/1999

10/19/1999

10/20/1999

10/21/1999

10/22/1999

10/23/1999

10/24/1999

10/25/1999

10/26/1999

10/27/1999

10/28/1999

Particulate Matter (micrograms/cubic meter)

FAT-Dru PM10

Turlock-Min PM10

FAT PM2.5

While geological dominated the PM10 mass, increases in other components (carbon, 

and ammonium nitrate and sulfate) were what drove the increase in concentration to 

over the PM10 Standard. It appears that the exceedance was driven by high 

concentrations of geologic comprising about 50 to 60 % of the PM10 mass. The other 

components of the samples were composed of 15 to 25 % ammonium nitrate and 

sulfate and 10 to 20 % total carbon of the PM10 mass. The size fraction data across the 

Valley indicated that most of the PM10 was in the PM10 fraction, with an average 

PM2.5/PM10 mass ratio greater than 0.35. This was further supported by the high coarse 

concentrations that reached 100 μg/m3

 at Corcoran-Patterson, accounting for 57 

percent of the PM10 mass (Figure 11).

The concentration of coarse material may have been elevated due to the lack of 

precipitation since June 2, 1999 contributing to the high geologic fraction. Dry 

conditions and low soil moisture content, resulted in higher dust emission activity from 

agricultural land preparing and harvesting and urban geological activities. Agricultural 

burning probably did not contribute to PM on the exceedance day, because October 17 

through October 21, 1999 were declared No Burn Day’s District-wide. However, burn 

variances and noncompliant agricultural burns may have contributed to a small portion 

of the samples. Overnight temperatures in the mid 50’s suggested that residential wood 

burning was not likely a potential source of PM10. PM10 and PM2.5 concentrations 

showed that meteorological conditions led to the pollution buildup. 

Meteorologically, dispersion worsened and particulate formation conditions 

strengthened from October 7th to the 20th. However, on October 21st, the day of the 

exceedance, dispersion conditions began to slightly improve under weakening stability. 

The lack of precipitation since June 2nd, 1999 may have contributed to the geologic 

fraction. Dry conditions and low soil moisture content, resulted in higher dust emission 

activity. Humidity measurements of 60-80% in the morning across the Valley Floor 
20

showed a dry to moist atmosphere, with no fog present. The cool damp mornings and 

strong stability favored some formation of nitrate and sulfate particulates. The eastern 

Pacific high built over the San Joaquin Valley on October 7th and dominated the region’s 

weather through the 20th. The high began to slowly break down on the 21st, but 

remained strong, trapping pollutants within the San Joaquin Valley boundary layer. 

With a strong lid in place aloft and maximum high temperatures in the mid-80’s, the 

afternoon hours were marked by limited mixing, resulting in elevated particulate 

conditions. Mixing heights at Fresno remained below 500 feet under a strong inversion 

for 16 hours on the 21st, breaking out by 12:00 P.M. and reforming by 5:00 P.M. During 

the afternoon hours, there were higher mixing depths (maximum mixing depth near 

5,000 feet), but the bulk of the day had limited mixing. Maximum and minimum 

temperatures for the October 1999 were above normal, further illustrating the intensity 

of the high pressure system that was controlling the region’s weather. Partly cloudy 

skies and hazy conditions reported at the Valley surface on October 21st, resulted in 

lower levels of solar radiation intensities. Along with several wildfires to the north and 

along the California Coast and a major tire fire at Westley earlier in the month, 

particulate loading aloft may have further decreased solar radiation intensity 

measurements across the Valley Floor. The reduction of solar radiation intensity may 

have led to lower mixing depths and increased atmospheric chemistry reactions forming 

particulates. Ambient air samples around the Westley tire fire site on the 21st confirmed 

minimal hydrocarbon and carbon monoxide concentrations, showing little if any impact 

around the tire fire. However, visual observations of the tire fire plume behavior 

indicated transport and mixing well down-wind of the site, which may have fumigated 

and impacted other parts of the San Joaquin Valley. 

With reduced solar radiation, the atmospheric chemistry reactions may have changed 

from the ozone-forming regime of mid-October to the secondary particulate regime that 

occurs during late November. The eight-hour average ozone measurements on 

October 20th showed one exceedance of the Federal standard at Clovis of 87 ppb. The 

following day a significant change in the photochemical reactions occurred in the 

atmosphere, with the eight-hour average ozone measurements dropping to well below 

the standard at Clovis to 73 ppb. Atmospheric stability remained rather strong and 

persistent on the 21st, however, solar radiation measurements Valley-wide showed 

reduced intensities. The change in the ozone measurements across the San Joaquin 

Valley may be illustrative of the atmospheric chemistry reactions shifting from an ozone 

to a particulate-forming regime. The change in the atmospheric chemistry reactions 

may have led to an increase in nitrate and sulfate in the PM samples that were 

measured on October 21st compared to the 15th. Increasing accumulations of carbon 

can be contributed to the deteriorating dispersion conditions experienced during the 

days preceding October 21st. It does not appear that any one source of carbon was a 

dominant contributor. Under the strong lid, wind flow was light and disorganized, 

leading to minimal transport and dispersion. As a result, local emissions may have 

contributed to the exceedances experienced on the 21st. 

 
21

Chemical composition and meteorological data showed limited mixing depths and light 

and disorganized wind flow resulted in minimal transport and dispersion of pollutants. 

Solar radiation measurements showed lower levels of intensities on the 21st compared 

to the 20th, resulting in an anomalous shift of the atmospheric chemistry reactions from 

an ozone to a particulate-forming regime. Both primary and secondary pollutants from 

local emissions around Corcoran-Patterson, Fresno-Drummond, and Turlock- S Minaret 

accumulated, resulting in the exceedance of the 24-hour PM10 standard at three 

monitoring sites on October 21, 1999. 

 
22

6.0 NOVEMBER 14, 1999 EPISODE SYNOPTIC METEOROLOGICAL 

DISCUSSION 

November 8 – November 17, 1999

After the trough passage on November 8th, which brought 0.31 inches of rainfall at 

Bakersfield, strong surface high pressure from the Intermountain Region and a 500 MB 

ridge from northwestern Mexico built into the region. These conditions lead to 

increasing stability and poor dispersion conditions through the exceedance date on 

November 14th, Figure 13. 

Figure 13: T850 Stability and Precipitation vrs. PM10 and PM2.5 

at Bakersfield Golden 

Fall/Winter 1999

0

20

40

60

80

100

120

140

160

180

200

10/1/199910/8/199910/15/199910/22/199910/29/199911/5/199911/12/199911/19/199911/26/199912/3/199912/10/199912/17/199912/24/199912/31/19991/7/20001/14/2000

PM10 and PM2.5 (micrograms/meter^3) and Precipitation 

(inches)

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

T850 Stability Parameter (C)

PM10

PPT

PM2.5

Model PM

T850 Stability

 

At Bakersfield, Golden, a 24-hour PM10 (Particulate Matter) concentration of 183 μg/m3

was measured on the 14th. Table 7 outlines federal reference method (FRM) Daily 

Average Particulate Matter Measurements for sites across the San Joaquin Valley 

(SJV). In order to understand the variability of these measurements, an in depth 

examination of the synoptic pattern and surface winds and observations, and aircraft 

soundings leading to the episode were analyzed. 
23

TABLE 7: Federal Reference Method (FRM) Daily Average Particulate Matter 

Measurements for sites across the SJV for November 14, 1999.

 FRM FRM FRM 

Site Name 24-Avg. Site Name 24-Avg. Site Name 24-Avg. 

10 2.5 10 2.5 10 2.5 

Bakersfield-Gold 183 134 Modesto 67 70 Hanford 138

Bakersfield-CA 138 115 Merced-M St. 75 67 Stockton** 60 55

Visalia 137 121 Taft 70 Clovis 108 98

Fresno-1st 124 115 Fresno-Drum 130 Corcoran* 142 114

*-Patterson **- Hazleton 

units in μg/m3 

The meteorological synoptic analysis showed after a period of moderately strong 

atmospheric stability from November 8th through the 17th, an upper level closed low from 

the eastern Pacific approached the region. Ahead of the closed low, strong stability 

occurred over the San Joaquin Valley trapping particulates within the Valley boundary 

layer. The morning surface charts on the 14th depicted a surface high pressure ridge 

draped across Central California from the Great Basin to San Francisco. The 12Z 

surface pressure gradient was –3.6 millibars from San Francisco to Las Vegas (SFOLAS), with isobars (constant surface pressure) orientated northwest to southeast. With 

the alignment of the isobars and the –3.6 millibars pressure gradient, light offshore flow 

resulted in weak disorganized wind flow across the San Joaquin Valley. Visibilities 

throughout the day across the San Joaquin Valley were reporting haze. 

The morning temperature aircraft sounding over Bakersfield showed a strong inversion 

(stable layer) of 17 degrees Fahrenheit from the surface up to 2,000 feet as is evident in 

Figure 14. At Fresno the temperature sounding showed a similar strong inversion of 24 

degrees Fahrenheit from the surface up to 2,000 feet. The temperature soundings on 

the 14th are conducive of elevated PM levels due to low mixing depths, which keep 

pollutants trapped near the surface. At surface observations across the San Joaquin 

Valley, minimum temperature at Fresno was 48 degrees Fahrenheit and Bakersfield 

was 49 degrees Fahrenheit. 
24

Figure 14: Atmospheric Temperature Profile at Bakersfield on November 14, 1999

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

50 55 60 65 70 75 80

Temperature (deg F)

Feet (AGL)

Upper level charts indicated a strong high over the Four Corners Region, with a ridge 

extending westward across California. At 850 MB a temperature ridge built 

northwestward across the San Joaquin Valley from southern California. A closed low 

was positioned 600 NM west of Santa Barbara. The pressure gradient across the San 

Joaquin Valley remained weak during the day leading to light and disorganized wind 

flow across the region. Table 8 shows the 24 hour daily average wind speeds at 

SJVAPCD air monitoring, ASOS, and CIMIS sites for November 14, 1999. The 24 hour 

daily average wind speeds across the Valley floor were conducive to elevated PM 

readings over the central and southern parts of the San Joaquin Valley. 
25

Table 8 shows the 24 hour daily average wind speeds at SJVAPCD monitoring, ASOS, 

and CIMIS sites for November 14, 1999. 

SJVAPCD Monitoring Sites ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

Clovis 3.9Fresno 4.1Shafter/USDA 3.5 Famoso 3.1

Fresno SSP 2.0Bakersfield 6.1Firebaugh/Telles 3.5 Westlands 3.9

Corcoran 3.0Hanford 2.9Stratford 3.0 Panoche 4.5

Edison 2.3 Kettleman 4.1 Arvin-Edison 3.2

Parlier 3.6 Visalia/Americas 2.4 Lindcove 1.9

Arvin 2.1 Parlier 3.0 Kesterson 3.2

Visalia 1.4 Blackwells Corner 5.4 Fresno State 3.4

 Los Banos 4.2 Modesto 3.1

 Manteca 2.9 

Due to the strong stability lasting for over 9 days, PM10 steadily increased region-wide 

until November 14th, 1999. Light disorganized wind flow, a strong morning inversion, 

limited mixing depths, and strong stability aloft; lead to limited dispersion conditions on 

the 14th causing local PM emissions in Bakersfield Golden to increase past the Federal 

24-hour PM10 standard to 183 μg/m3

. Since the other FRM Monitoring sites did not 

register above the Federal 24-hour PM10 standard on the 14th, the Bakersfield Golden 

PM10 measurement may signify impacts from local emission sources. 

Other FRM Monitoring sites across the central and southern San Joaquin Valley 

(Visalia, Fresno, Hanford, Clovis, and Corcoran) measured 24-hour PM10 above 100 

μg/m3, but did not exceed the Federal 24-hour PM10 standard. A weak upper level 

disturbance moved through the region late on the 14th and 15th, bringing better 

dispersion conditions and lowering Bakersfield’s PM measurements to less the National 

Ambient Air Quality Standard. 
26

7.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE NOVEMBER 1999 EPISODE 

The November 14, 1999 particulate episode was characterized by a short period (9 

days) of strong stability and light wind flow, which resulted in poor atmospheric 

dispersion conditions across the San Joaquin Valley. Coarse and fine particulates 

accumulated during the period, leading to an exceedance of the 24 - hour standard at 

one monitoring site on November 14, 1999. Concentrations were dominated by the fine 

particulates (PM2.5) of ammonia nitrate and sulfate and were most prevalent in the 

central and southern San Joaquin Valley. The highest PM10 measured was 183 μg/m3 

(Figure 15) at Bakersfield-Golden; whereas, PM10 concentrations at other central and 

southern Valley locations were much lower between 130 to 140 μg/m3

 (Figure 16). 

During the period, strong high pressure at the surface and aloft resulted in limited 

afternoon mixing and light offshore wind flow. Cool damp mornings and strong stability 

contributed to the formation of nitrates and sulfates during the episode. These 

meteorological conditions led to elevated particulate concentrations. Chemical 

composition and meteorological data around November 14, 1999, was evaluated and 

analyzed to identify the characteristics and uniqueness of the exceedance at 

Bakersfield-Golden. 

Figure 15: Bakersfield Golden (PM2.5 & Coarse)

and Fresno 1st (PM2.5) 

November 14, 1999 Episode

0

20

40

60

80

100

120

140

160

180

200

11/6/1999

11/7/1999

11/8/1999

11/9/1999

11/10/1999

11/11/1999

11/12/1999

11/13/1999

11/14/1999

11/15/1999

11/16/1999

11/17/1999

Particulate Matter (micrograms / cubic meter)

BAK-GOLD Coarse

BAK-GOLD PM2.5

FAT-1st PM2.5

Although PM10 sampling is not conducted daily, information from more frequent PM2.5

sampling combined with an assessment of changes in the meteorological conditions 

suggest that the highest PM10 concentration was captured on the sampled day. With no 

chemical composition data available at the exceedance site, similarities in trends from 

other sites were analyzed in order to conclude which chemical components were 

responsible for the exceedance. It appears that the exceedance was driven by high 

concentrations of ammonium nitrate and sulfate comprising about 50 to 60 % of the 

PM10 mass. The other chemical components of the samples were composed of 10 to 

15 % total carbon and 10 to 15% geological of the PM10 mass. The size fraction data 

across the Valley indicated that most of the PM10 was in the PM2.5 fraction, with an 
27

Figure 16: Bakersfield California (PM2.5, PM10, & Coarse)

and Fresno 1st (PM2.5) 

November 14, 1999 Episode

0

20

40

60

80

100

120

140

160

11/6/1999

11/7/199911/8/1999

11/9/1999

11/10/1999

11/11/199911/12/1999

11/13/1999

11/14/1999

11/15/199911/16/1999

11/17/1999

Particulate Matter (micrograms/cubic meter)

BAK-CA PM10

BAK-CA Coarse

BAK-CA PM2.5

FAT-1st PM2.5

average PM2.5/PM10 mass ratio of greater than 0.8. This was further supported by the 

high PM2.5 concentrations that reached 134 μg/m3 at the exceedance site, accounting 

for 73 percent of the PM10 mass, (Figure 15). All monitoring sites exceeded the federal 

PM2.5 standard on the exceedance day. 

The concentration of coarse material may have been higher at the exceedance site 

relative to other sites in the Valley, as indicated by the higher coarse fraction at 

Bakersfield Golden as compared to other sites in the Valley. The coarse mass at 

Bakersfield-Golden, at 49 μg/m3, was 26 μg/m3 higher compared to nearby BakersfieldCalifornia, indicating that a larger concentration of urban geological material at 

Bakersfield-Golden may be responsible for the large disparity in the PM10

concentrations. Agriculture burning probably did not contribute to PM on the 

exceedance day, because November 13 and 14, 1999 were declared No Burn Day’s 

District-wide. However, burn variances and noncompliant agricultural burns may have 

contributed to a small portion of the samples. Although minimum temperatures were 

mild compared to later in the middle of winter, overnight temperatures in the mid to 

upper 40’s suggest that residential wood burning may have been a potential source of 

PM10. PM10 and PM2.5 concentrations showed that meteorological conditions led to the 

short pollution buildup. During the buildup time leading to the exceedance on 

November 14, 1999, carbon concentrations decreased or remained stable, whereas 

concentrations of geologic, ammonia nitrate, and sulfate increased. 

Meteorologically, dispersion worsened and particulate formation conditions 

strengthened from November 8th to the 14th. After the passage of a cold front on 

November 8th, which brought 0.31 inches of rainfall to Bakersfield, moisture was 

available for atmospheric chemistry reactions. Humidity measurements of 85-100% in 

the morning across the Valley Floor showed a very moist atmosphere, with light to 

dense fog and haze being reported. The cool damp mornings and strong stability 

favored the formation of nitrates and sulfates. The eastern Pacific high built over the 

San Joaquin Valley on November 8th and dominated the region’s weather through the 
28

14th. The high strengthened and intensified a few days prior to the 14th, further 

tightening the lid and trapping pollutants within the San Joaquin Valley boundary layer. 

With a strong lid in place and maximum high temperatures in the mid to upper 70’s, the 

afternoon hours were marked by limited mixing, resulting in increasing particulate 

conditions. Mixing heights at Fresno remained below 500 feet under a strong inversion 

for 16 hours on November 14, 1999 breaking out by 11:00 AM and reforming shortly 

after 5:00 P.M. During the afternoon hours, there were higher mixing depths (maximum 

mixing depth of 2,000 feet), but the bulk of the day had limited mixing. Maximum and 

minimum temperatures for the November 1999 episode were above normal, further 

illustrating the intensity of the high pressure system that was controlling the region’s 

weather. Satellite imagery depicted scattered upper level moisture (high cloudiness) 

over the region and light fog being reported at the Valley surface on the November 14, 

resulting in slightly lower levels of solar radiation intensity. Along with lower solar 

radiation intensities due to the low sun angle and decreasing daylight hours, the 

atmospheric chemistry reactions may have favored the secondary particulate forming 

regime. 

Chemical composition and meteorological data showed limited mixing depths and light 

and disorganized wind flow resulted in minimal transport and dispersion of pollutants. 

Both primary and secondary pollutants from local emissions around Bakersfield-Golden 

accumulated, resulting in the exceedance of the 24-hour PM10 standard on November 

14, 1999. 
29

8.0 DECEMBER 17 AND 23, 1999 EPISODE SYNOPTIC 

METEOROLOGICAL DISCUSSION 

December 10 – December 31, 1999

The period from December 10, 1999 through December 31st, 1999 was marked by 21 

days of strong stability and poor atmospheric dispersion conditions. Strong high 

pressure over the Intermountain Region dominated the period, leading to light and 

disorganized wind flow and limited dispersion. The period began with the passage of a 

weak cold front and trough on the 9th. Between the 10th and the 17th, increasing stability 

and poor dispersion conditions resulted in a PM10 exceedance at Corcoran on the 17th, 

Figure 17. 

Figure 17: T850 Stability and Precipitation at Hanford vrs. PM10 and PM2.5 

at Corcoran

December 1999

39

81

58 59

49

174

115

135

145 139

0

20

40

60

80

100

120

140

160

180

200

12/1/199912/2/199912/3/199912/4/199912/5/199912/6/199912/7/199912/8/199912/9/1999

12/10/199912/11/199912/12/199912/13/199912/14/199912/15/199912/16/199912/17/199912/18/199912/19/199912/20/199912/21/199912/22/199912/23/199912/24/199912/25/199912/26/199912/27/199912/28/199912/29/199912/30/199912/31/1999

PM10 and PM2.5 (micrograms/meter^3) and 

Precipitation (inches)/100

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

T850 Stability Parameter (C)

PPT

PM10

Model PM

PM2.5

T850 Stability

 

At Corcoran, a 24-hour PM10 (Particulate Matter) concentration of 174 μg/m3

 was 

measured. Table 9 outlines federal reference method (FRM) Daily Average Particulate 

Matter measurements for sites across the San Joaquin Valley (SJV). In order to 

understand the variability of these measurements, an in depth examination of the 

synoptic pattern and surface winds and observations, and aircraft soundings leading to 

the episode were analyzed. 
30

TABLE 9: Federal Reference Method (FRM) Daily Average Particulate Matter 

measurements for sites across the SJV for December 17, 1999.

 FRM FRM FRM 

Site Name 24-Avg. Site Name 24-Avg. Site Name 24-Avg. 

10 2.5 10 2.5 10 2.5 

Bakersfield-Gold 92 Modesto 99 93 Corcoran* 174

Bakersfield-CA 111 90 Merced-M St. 75 Stockton** 78

Visalia 114 Clovis 18 Fresno-1st 107

*-Patterson **- Hazleton 

units in μg/m3

 

The meteorological synoptic analysis showed a period of moderately strong 

atmospheric stability from December 10th through the 17th. Ahead of a developing 

trough over the eastern Pacific, a strong mid and upper level ridge developed over the 

region on the 17th. This ridge strengthened the inversion over the San Joaquin Valley 

trapping particulates within the Valley boundary layer. The morning surface charts of 

the 17th depicted a strong surface high over the Great Basin and Intermountain Region. 

The 12Z (4 a.m.) surface pressure gradient was +1.8 millibars from San Francisco 

(SFO) to Las Vegas (LAS), with isobars (constant surface pressure) orientated west to 

east. The alignment of the isobars and the +1.8 millibars pressure gradient, represents 

light and disorganized wind flow across the San Joaquin Valley. Visibilities throughout 

the day across the San Joaquin Valley were reporting haze. 

The morning temperature aircraft sounding over Fresno on the 17th showed multiple 

inversions, with a strong inversion (stable layer) of 13 degrees Fahrenheit from the 

surface up to 1,000 feet, with a secondary strong inversion of 11 degrees Fahrenheit 

from 2,500 to 5,000 feet as is evident in Figure 18. The morning temperature sounding 

over Bakersfield also showed multiple inversions, with a strong inversion of 15 degrees 

Fahrenheit from the surface up to 3,000 feet, with a secondary strong inversion of 11 

degrees Fahrenheit from 4,500 feet to 5,000 feet. The temperature sounding on the 

17th, is conducive of elevated PM levels due to low mixing depths and multiple 

inversions, which keep pollutants trapped near the surface. During the early morning 

surface observations across the San Joaquin Valley were cold. The minimum 

temperatures recorded at Fresno and Bakersfield were 33 degrees Fahrenheit. The 

maximum high temperatures recorded at Fresno and Bakersfield were 59 degrees 

Fahrenheit. Fresno hourly temperature data shows very limited mixing conditions below 

500 feet for over 18 hours of the day, increasing with minor afternoon heating to a 
31

maximum mixing depth of 2,000 feet on the 17th. 

Figure 18: Atmospheric Temperature Profile at Fresno on December 17, 1999

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

35 40 45 50 55 60 65

Temperature (deg F)

Feet (AGL)

Upper level charts indicated a strong high just west of Santa Barbara, with a ridge 

building northeastward into the Great Basin. A weak trough over the extreme Pacific 

Northwest extended southward to near Eureka. Weak pressure gradients across the 

San Joaquin Valley remained rather flat through the day, leading to light and 

disorganized wind flow across the San Joaquin Valley. 

Table 10 shows the 24 hour daily average wind speeds at SJVAPCD monitoring, 

ASOS, and CIMIS sites for December 17, 1999. 

SJVAPCD Monitoring Sites ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

Clovis 2.2Fresno 1.7Shafter/USDA 2.1 Famoso 2.4

Fresno SSP 1.4Bakersfield 3.4Firebaugh/Telles 2.2 Westlands 2.6

Corcoran 2.8Hanford 1.0Stratford 2.4 Panoche 2.6

Edison 3.3 Kettleman 2.6 Arvin-Edison 2.3

Parlier 3.2 Visalia/Americas 2.1 Lindcove 1.8

Arvin 2.2 Parlier 2.2 Kesterson 2.4

Visalia 1.6 Blackwells Corner 2.0 Lodi West 1.8

 Los Banos 2.8 Modesto 2.9

 Manteca 2.4 Fresno State 2.2

Due to the strong stability lasting for over 7 days, PM10 steadily increase region-wide 

until the sampling day on December 17th. With the mid and upper level stability aloft, 

surface based inversion, and light and disorganized wind flow, this weather pattern was 

conducive of an elevated region-wide PM10 measurements. At Visalia and Fresno 1st, 

fine particulates were above 100 μg/m3

, further suggesting a widespread PM event 

across the San Joaquin Valley. 
32

After a weak upper level trough passage on the 18th, strong high pressure rebuilt into 

the region from the eastern Pacific, with increasing stability and poor dispersion 

conditions through the next exceedance date on December 23rd at Fresno-Drummond 

Street and Hanford-Irwin Street. At Fresno-Drummond, a 24-hour PM10 concentration 

of 168 μg/m3

 and at Hanford a 24-hour PM10 concentration of 156 μg/m3 was measured. 

 

Table 11 Federal Reference Method (FRM) Daily Average Particulate Matter and Table 

12 California Regional Particulate Air Quality Study (CRPAQS) Measurements shows 

for sites across the San Joaquin Valley (SJV) for December 23. In order to understand 

the variability of these measurements, an in depth examination of the synoptic pattern 

and surface winds and observations, and aircraft soundings leading to the episode were 

analyzed. 

TABLE 11: Federal Reference Method (FRM) Daily Average Particulate Matter 

Measurements for sites across the SJV for December 23, 1999. 

 FRM FRM FRM 

Site Name 24-Avg. Site Name 24-Avg. Site Name 24-Avg. 

10 2.5 10 2.5 10 2.5 

Bakersfield-Gold 74 Modesto 119 95 Corcoran* 135 91

Bakersfield-CA 109 72 Merced-M St. 83 Stockton** 79

Visalia 85 Clovis 22 Fresno-1st 119

* -Patterson #-Wagner, **- Hazleton 

TABLE 12: California Regional Particulate Air Quality Study (CRPAQS) Daily Average 

Particulate Matter Measurements for Fresno-Drummond and Hanford-Irwin for 

December 23, 1999. 

 CRPAQS 

Site Name 24-Avg. 

10 

Fresno-Drummond 168 

Hanford-Irwin 156 

units in μg/m3

 for Table 11 and 12 

The meteorological synoptic analysis showed after a period of strong atmospheric 

stability from December 18th through the 23rd, increasing stability and poor dispersion 

conditions lead to two exceedances measured using CRPAQS monitors at FresnoDrummond and Hanford, Figure 19. The morning surface charts on the 23rd depicted a 

surface ridge extending southwestward across the central San Joaquin Valley from 

strong highs anchored over the Intermountain Region. The morning surface pressure 

gradient was –1.6 millibars from San Francisco to Las Vegas (SFO-LAS), with isobars

(constant surface pressure) orientated southeast to northwest. With the alignment of 

the isobars and –1.6 millibars pressure gradient, this represents light southeasterly wind 
33

flow across the San Joaquin Valley. Visibilities across the San Joaquin Valley 

throughout the day reported hazy conditions. 

Figure 19: T850 Stability and Precipitation vrs. PM10 and PM2.5 

at Hanford

December 1999

0

20

40

60

80

100

120

140

160

180

12/1/199912/2/199912/3/199912/4/199912/5/199912/6/199912/7/199912/8/199912/9/199912/10/199912/11/199912/12/199912/13/199912/14/199912/15/199912/16/199912/17/199912/18/199912/19/199912/20/199912/21/199912/22/199912/23/199912/24/199912/25/199912/26/199912/27/199912/28/199912/29/199912/30/199912/31/1999

PM10 and PM2.5 (micrograms/meter^3) and Precipitation

(inches)/100

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

T850 Stability Parameter(C)

PPT

FRM PM10

CRPAQS PM10

Model PM

T850 Stability

The morning temperature aircraft sounding over Fresno depicted a strong inversion 

(stable layer) of 19 degrees Fahrenheit from the surface up to 1,000 feet as is evident in 

Figure 20. Bakersfield showed a strong inversion of 12 degrees Fahrenheit from the 

surface up to 1,000 feet turning isothermal up to 3,000 feet. The temperature 

soundings on the 23rd are indicative of elevated PM levels due to low mixing depths and 

strong inversions, which trap pollutants near the surface. During the early morning 

surface observations across the San Joaquin Valley were cold. The minimum 

temperatures recorded at Fresno and Bakersfield were 34 and 36 degrees respectively. 

The maximum high temperatures recorded at Fresno and Bakersfield were 62 and 64 

degrees Fahrenheit respectively. Fresno hourly temperature data shows very limited 

mixing conditions below 500 feet for over 16 hours of the day, increasing with minor 

afternoon heating to a maximum mixing depth of 2,000 feet on the 23rd
34

Figure 20: Atmospheric Temperature Profile at Fresno on December 23, 1999

0

1000

2000

3000

4000

5000

35 40 45 50 55 60

Temperature (deg F)

Feet (AGL)

Upper level charts indicated a strong high 600 NM west of Santa Barbara, with a ridge 

building northward along the West Coast into the eastern Gulf of Alaska. Pressure 

gradients remained weak throughout the day leading to poor dispersion conditions 

across the San Joaquin Valley. 

Table 13 shows the 24 hour daily average wind speeds at SJVAPCD monitoring, 

ASOS, and CIMIS sites for December 23, 1999. 

SJVAPCD Monitoring Site ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

Clovis 2.5Fresno 1.5Shafter/USDA 2.5 Famoso 2.5

Fresno SSP 2.1Bakersfield 3.1Firebaugh/Telles 2.5 Westlands 3.0

Corcoran 3.3Hanford 1.6Stratford 3.3 Panoche 2.7

Edison 3.7 Kettleman 3.3 Arvin-Edison 2.6

Parlier 3.9 Visalia/Americas 2.5 Lindcove 1.8

Arvin 2.4 Parlier 2.7 Kesterson 2.1

Visalia 1.7 Blackwells Corner 2.1 Lodi West 1.7

 Los Banos 2.8 Modesto 2.4

 Manteca 2.4 Fresno State 2.3

The period of December 10, 1999 through December 31st, 1999, marked one of the 

longest periods of strong stability in recent history. This pattern was similar to the one 

experienced during the California Regional Particulate Air Quality Study. The main 

difference between the two episodes was the CRPAQS event began a week later and 

continued into the early part of January. During the exceedances of 1999, they were 

preceded by an exceptionally dry fall. After a weak frontal passage on the 10th, which 

brought a cold air mass to the Valley, this air remained trapped within the Valley 

boundary layer resulting in strong surface based inversions. Coupled with the synoptic 

and surface pattern, which was indicative of light and disorganized wind flow, PM 

became a region-wide problem until a weak trough scoured out the area on New Year’s 

Eve. As is evident in the PM data, local emissions contributed to the exceedance of the 

National Ambient Air Quality Standards at Corcoran on the 17th and Fresno-Drummond, 

and Hanford on the 23rd. If monitoring were done on a daily basis, more exceedances 

would have been recorded during this time frame at other San Joaquin Valley locations. 
35

9.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE DECEMBER 1999 EPISODE 

The December 1999 particulate episode was characterized by a prolonged period (three 

weeks) of strong stability and light wind flow, which resulted in poor atmospheric 

dispersion conditions across the San Joaquin Valley. Coarse and fine particulates 

accumulated during the period, leading to two separate exceedance days on December 

17 and 23, 1999. Concentrations were dominated by fine particulates (PM2.5) of 

ammonia nitrate and sulfate and were most prevalent in the central and southern San 

Joaquin Valley. The highest PM10 measured on December 17, 1999 was 174 μg/m3 

(Figure 21) at Corcoran, a federal reference monitoring (FRM) site. PM10 at other 

central and southern Valley locations were elevated but not over the standard. For 

example, Fresno-Drummond recorded 153 μg/m3 on December 17, 1999 (Figure 22). 

Stagnation conditions continued through December 23, 1999, when two MiniVol 

Samplers, which were part of the California Regional Particulate Air Quality Study 

(CRPAQS) registered the second and third highest concentrations at FresnoDrummond at 168 μg/m3

 and Hanford-South Irwin, at 156 μg/m3. The CRPAQS 

exceedances are not considered for compliance with the standards, but are used in the 

discussion to provide context to the episode. During the period, strong high pressure at 

the surface and aloft resulted in limited afternoon mixing and light offshore wind flow. 

Cool damp mornings and strong stability contributed to the formation of nitrates and 

sulfates during the episode. Chemical composition and meteorological data around 

December 17 and 23, 1999 were evaluated and analyzed to identify the characteristics 

and uniqueness of the exceedances at Corcoran, Fresno-Drummond, and HanfordSouth Irwin. 

Figure 21: Corcoran (PM2.5, PM10 & Coarse)

and Fresno 1st (PM2.5) 

December 17, 1999 Episode

0

20

40

60

80

100

120

140

160

180

12/11/1999

12/12/1999

12/13/1999

12/14/1999

12/15/1999

12/16/1999

12/17/1999

12/18/1999

12/19/1999

12/20/1999

12/21/1999

12/22/1999

12/23/1999

12/24/1999

12/25/1999

12/26/1999

12/27/1999

12/28/1999

12/29/1999

12/30/1999

12/31/1999

1/1/2000

Particulate Matter (micrograms / cubic meter)

Corcoran PM10

Corcoran Coarse

Corcoran PM2.5

FAT-1st PM2.5
36

Figure 22: Fresno Drummond (PM10)

and Fresno 1st (PM2.5) 

December 17, 1999 Episode

0

20

40

60

80

100

120

140

160

180

12/11/1999

12/12/1999

12/13/1999

12/14/1999

12/15/1999

12/16/1999

12/17/1999

12/18/1999

12/19/1999

12/20/1999

12/21/1999

12/22/1999

12/23/1999

12/24/1999

12/25/1999

12/26/1999

12/27/1999

12/28/1999

12/29/1999

12/30/1999

12/31/1999

1/1/2000

Particulate Matter (micrograms/cubic meter)

FAT-DRU PM10

FAT-1st PM2.5

Although PM10 sampling is not conducted daily, information from more frequent PM2.5

sampling combined with an assessment of changes in the meteorological conditions 

suggest that the highest PM10 concentration was not captured on the sample day. Daily 

PM2.5 sampling and meteorological conditions suggested that the highest PM10

concentrations may have occurred on December 18th, when Fresno 1st measured a 

peak PM2.5 concentration of 119 μg/m3. With the lack of chemical composition data for 

the exceedance site, similarities in trends from other sites such as Fresno-Drummond 

were drawn in order to determine the chemical components responsible for the 

December 17th exceedance. The analysis shows that the exceedance was driven by 

high concentrations of ammonium nitrate and sulfate that comprised of 55 to 75 % of 

the PM10 mass. The other major chemical components of the samples were 25 to 30 % 

total carbon and 20 to 25 % geological of the PM10 mass. The size fraction data across 

the Valley indicated that most of the PM10 was in the PM2.5 fraction, with an average 

PM2.5/PM10 mass ratio greater than 0.8. This was further supported by the high PM2.5

concentrations that reached 107 μg/m3 at nearby Fresno-First, (Figure 22). 

The concentration of total carbon (27% of PM10 mass) and coarse material (20% of 

PM10 mass) at Fresno Drummond show the main constituents of the particulate samples 

on December 17, 1999 may have come from local sources (residential wood burning 

and urban/agricultural geologic). Light afternoon mixing and wind flow may have 

allowed some transported contribution from agricultural burning in Merced, Stanislaus, 

and San Joaquin Counties on December 17th. However a no burn day across the rest 

of the Valley, indicates that stagnant conditions favored a larger influence from local 

emissions. Burn variances and noncompliant agricultural burns may have contributed 

to a small portion of the samples. Overnight minimum temperatures in the low 30’s 

suggest that residential wood burning may have been a significant source of PM10. 

Analysis of PM10 and PM2.5 concentrations showed that meteorological conditions led to 

the pollution buildup. Ammonium nitrate and sulfates, carbon, and geological material 

increased prior to the exceedance on December 17, 1999 and then later accumulated 

further leading to two other monitoring sites exceedances on December 23. 
37

Meteorologically, dispersion worsened and particulate formation conditions 

strengthened from December 10th to the 17th. After the passage of a weak cold front on 

December 9th, which brought trace amounts of precipitation to the region, moisture was 

available for atmospheric chemistry reactions. Humidity measurements of 85 – 100 % 

in the morning across the Valley Floor showed a moist atmosphere, with light to dense 

fog and haze being reported. These cool damp mornings and strong stability favored 

the formation of nitrates and sulfates. Synoptically, the eastern Pacific high built over 

the San Joaquin Valley on December 10th and dominated the region’s weather through 

the 31st. The high strengthened and intensified a few days prior to the 17th, further 

tightening the lid and trapping pollutants within the San Joaquin Valley boundary layer. 

With a strong lid in place aloft and maximum high temperatures in the upper 50’s, the 

afternoon hours were marked by limited mixing, resulting in increasing particulate 

potential. Mixing heights at Fresno remained below 500 feet under a strong inversion 

for 18 hours on December 17th, breaking out after 12:00 P.M and reforming shortly 

before 5:00 P.M. During the afternoon hours, there were higher mixing depths 

(maximum mixing depth of 2,000 feet), but the bulk of the day had limited mixing. 

Maximum and minimum temperatures for this episode were slightly above normal, 

further illustrating the intensity of the high pressure system that was controlling the 

region’s weather. Light fog was reported at the Valley surface during the early morning 

hours of December 17th, resulting in slightly lower solar radiation intensities. Along with 

lower solar radiation intensities due to the early morning fog, lowering sun angle, and 

decreasing daylight hours, the atmospheric chemistry reactions may have favored the 

secondary particulate forming regime. 

Chemical composition and meteorological data showed limited mixing depths and light 

and disorganized wind flow resulted in minimal transport and dispersion of pollutants. 

Both primary and secondary pollutants from local emissions around Corcoran 

accumulated resulting in the exceedance of the 24- hour PM10 standard on December 

17, 1999. 

Between December 17th through the 23rd, meteorological conditions remained stable 

leading to elevated particulate conditions continuing across the San Joaquin Valley. 

Without a significant weather disturbance moving through the area to scour out the 

particulates, PM10 concentrations became a region-wide problem and came very close 

to the standard at three monitoring sties (Fresno-1st at 154 μg/m3

, Visalia at 152 μg/m3, 

and Clovis at 151 μg/m3) on December 20, 1999. PM10 concentrations from FresnoDrummond and Hanford climbed between December 17ththrough the 23rd, while PM10 

concentrations at Corcoran lowered during this time frame. Due to the light winds and 

limited mixing, local emission activity around Corcoran may have lowered leading to 

decreasing PM10 levels. Analysis of total carbon and ammonium nitrates and sulfates 

indicates that Fresno-Drummond and Hanford registered higher levels compared to 

Corcoran, indicating potentially that local emissions sources may have impacted these 

sites. A few days later, two MiniVol Samplers, which were collected as part of the 

California Regional Particulate Air Quality Study (CRPAQS), measured the second and 
38

third highest concentrations at Fresno-Drummond at 168 μg/m3 (Figure 22) and 

Hanford-South Irwin, at 156 μg/m3 (Figure 23). The CRPAQS exceedances are not 

considered for compliance with the standards, but are used in the discussion to provide 

context to the episode. 

Figure 23: Hanford (PM10)

and Fresno 1st (PM2.5) 

December 23, 1999 Episode

0

20

40

60

80

100

120

140

160

180

12/11/1999

12/12/1999

12/13/1999

12/14/1999

12/15/1999

12/16/1999

12/17/1999

12/18/1999

12/19/1999

12/20/1999

12/21/1999

12/22/1999

12/23/1999

12/24/1999

12/25/1999

12/26/1999

12/27/1999

12/28/1999

12/29/1999

12/30/1999

12/31/1999

1/1/2000

Particulate Matter (micrograms/cubic meter)

HNF PM10

FAT-1st PM2.5

Although PM10 sampling is not conducted daily, information from more frequent PM2.5

sampling combined with an assessment of changes in the meteorological conditions 

suggest that the highest PM10 concentration was not captured on the sampled day. 

Daily PM2.5 sampling and meteorological conditions suggested that the highest PM10

concentrations may have occurred on December 24th and 25th, when Fresno 1st

measured peak PM2.5 concentrations of 134 and 131 μg/m3

, respectively. The 

exceedances at Fresno-Drummond and Hanford-Irwin were driven by high 

concentrations of ammonia nitrate and sulfate and/or total carbon, which accumulated 

between December 17th and the 23rd. The size fraction data across the Valley 

indicated that most of the PM10 was in the PM2.5 fraction, with an average PM2.5/PM10

mass ratio greater than 0.7. This was further supported by the high PM2.5

concentrations that reached 119 μg/m3

 at nearby Fresno-First, (Figure 22 & 23). 

The concentration at Fresno-Drummond of ammonium nitrate and sulfate (74 μg/m3

), 

total carbon (44 μg/m3), and geological (44 μg/m3), had the highest PM10 mass in the 

Valley. The concentration at Hanford of ammonium nitrate and sulfate (80 μg/m3

), total 

carbon (35 μg/m3), and geological (44 μg/m3), had the second highest PM10 mass in the 

Valley. The main constituents of the particulate samples on December 23, 1999 may 

have come from local sources (residential wood burning and urban/agricultural 

geologic). Agricultural burning may not have contributed as a potential source of PM on 

the exceedance day, because since December 18, 1999 were declared a No Burn 

Day’s District-wide. However, burn variances and noncompliant agricultural burns may 

have contributed to a small portion of the samples. Overnight minimum temperatures in 

the low to mid 30’s suggest that residential wood burning was likely to be a significant 
39

source of PM10. It appears that potentially residential burning may have been a source 

of carbon. PM10 and PM2.5 concentrations showed that meteorological conditions led to 

the pollution buildup. During the buildup time leading to the two separate exceedances 

on December 23, 1999, where ammonium nitrate and sulfates, carbon, and geological 

material increased. 

Meteorologically, dispersion conditions began to slightly improve a few days prior to the 

December 23rd exceedances. The lack of precipitation since December 9th may have 

contributed to the increasing geological component. Humidity measurements of 85 – 

100 % in the morning across the Valley Floor showed a moist atmosphere, with light to 

dense fog and haze being reported. The cool damp mornings and strong stability 

favored the formation of nitrates and sulfates. The eastern Pacific high remained strong 

through the 20th and gradually weakened through the 31st, keeping a tight lid over the 

San Joaquin Valley trapping the pollutants within the Valley boundary layer. 

With a strong lid in place aloft and maximum high temperatures in the low 60’s, the 

afternoon hours were marked by limited mixing, resulting in increasing particulate 

conditions. Mixing heights at Fresno remained below 500 feet under a strong inversion 

for 16 hours on the December 23rd, breaking out shortly after 12:00 P.M and reforming 

before 5:00 P.M. During the afternoon hours, there were higher mixing depths 

(maximum mixing depth of 2,000 feet), but the bulk of the day had limited mixing. 

Maximum temperatures for this episode were above normal, further illustrating the 

intensity of the high pressure system that was controlling the region’s weather. Light 

fog reported at the Valley surface during the early morning hours, resulted in slightly 

lower solar radiation intensities. Along with lower solar radiation intensities due to the 

low sun angle and decreasing daylight hours, the atmospheric chemistry reactions may 

have favored the secondary particulate forming regime. 

In summary, chemical composition and meteorological data showed limited mixing 

depths and light and disorganized wind flow resulted in minimal transport and dispersion 

of pollutants. Both primary and secondary pollutants from local emissions around 

Fresno-Drummond and Hanford-Irwin accumulated resulting in the exceedance of the 

24-hour PM10 standard on December 23, 1999. 
40

10.0 DECEMBER 1999 AND JANUARY 2000 PM2.5 & PM10

METEOROLOGICAL DISCUSSION 

During December 1999 and January 2000 episode, strong high pressure at the surface 

and aloft resulted in limited morning and afternoon mixing and light offshore flow. Cool 

damp mornings and strong stability contributed to the formation of nitrates and sulfates 

during the episode. The December 1999 and January 2000 episode can be divided 

into three distinct events (Figure 24). These events, characterized by limited 

atmospheric dispersion, were separated by trough passages. 

Each event was marked by limited mixing heights with strong inversions of 

approximately 500 feet for 18 or more hours of the day. Maximum mixing depths on the 

highest PM2.5 concentration days ranged between 1,000 to 2,500 feet. Moisture 

availability from recent rainfall and minimal solar insulation, led to prime nitrate and 

sulfate forming conditions. Typically, light fog, with humidity measurements between 85 

– 100% were needed in order to maximize particulate forming potential. The higher the 

humidity, the heavier and wetter the fog, results in the removal of particulates by wet 

deposition. 

Within these three distinct measurement periods, a violation of the Federal 24 hour 

PM2.5 National Ambient Air Quality Standard (NAAQS) of 65 micrograms / meter3

 were 

recorded during the 1st and 3rd events at some locations across the San Joaquin Valley 

(Figure 25). The following analysis will investigate the meteorological and dispersion 

conditions surrounding each event. 

Figure 24: T850 Stability vrs. PM10 and PM2.5 

at Corcoran

 for December 1999 and January 2000

39

81

58 59

49

174

115

135

145 139

113

128

0

20

40

60

80

100

120

140

160

180

200

12/1/199912/2/199912/3/199912/4/199912/5/199912/6/199912/7/199912/8/199912/9/1999

12/10/199912/11/199912/12/199912/13/199912/14/199912/15/199912/16/199912/17/199912/18/199912/19/199912/20/199912/21/199912/22/199912/23/199912/24/199912/25/199912/26/199912/27/199912/28/199912/29/199912/30/199912/31/1999

1/1/20001/2/20001/3/20001/4/20001/5/20001/6/20001/7/20001/8/20001/9/2000

1/10/20001/11/20001/12/2000

PM10 and PM2.5 (micrograms/meter3) 

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

T850 Stability Parameter(Celsius)

FRM PM10

T850 Hanford Stability

FRM 2.5

Event #1 Event #2 Event #3

*No precipitation occurred in Hanford from December 1, 1999 through January 12, 2000.
41

Event #1: December 1 through December 8, 1999 

Atmospheric dispersion worsened and particulate formation conditions became more 

favorable from December 1st to December 6th in the San Joaquin Valley. As is evident 

in Figure 25, event #1 lasted about 9 days and was marked by one main PM2.5 peak, 

which exceeded the NAAQS. After the passage of a cold front on December 1, 1999, 

which brought trace amounts of precipitation across the San Joaquin Valley, moisture 

became available for atmospheric chemistry reactions. Humidity measurements of 85 – 

100 % in the morning across the Valley Floor showed a moist air mass, with light fog 

and haze being reported on the 6th. These cool damp mornings and strong stability 

favored the formation of nitrates and sulfates. Since concentrations rose through the 

episode, it does not appear that the water content of the atmosphere was high enough 

to create deposition of particles that exceeded formation. 

Synoptically, during the first part of event #1, the eastern Pacific ridge built over 

northern California on December 5th then expanded southward on the 6th, bringing 

increasing stability and poor dispersion conditions through the 7th. With a strong lid in 

place aloft and maximum high temperatures in the upper 50’s on the 6th, the afternoon 

hours were marked by limited mixing, resulting in increasing particulate potential. 

Mixing heights at Fresno remained below 500 feet under a strong inversion during a 

majority of the day (20 or more hours) on the 6th. During the afternoon hours, there 

were moderate mixing depths of 1,500 feet but the bulk of the day had limited mixing. 

Maximum and minimum temperatures were near to slightly below normal, indicating a 

potential for increasing residential wood burning emission activity, and nitrate forming 

reactions. 

Meteorological data during event #1 showed limited mixing depths and light and 

disorganized wind flow resulted in minimal transport and dispersion of pollutants. Both 

primary and secondary pollutants from local emissions across the San Joaquin Valley 

accumulated resulting in the exceedance of the 24 hour PM2.5 NAAQS at Fresno-1st on 

December 6th. No other sites registered an exceedance on or around December 6th, 

thus indicating that local emissions drove the measurement above the NAAQS. 

A cold front moved into northern California on the 7th and 8th, bringing an end to event 

#1 that lasted from December 1st through the 8th. The disturbance brought moderate 

dispersal conditions across the Valley, allowing pollutants to lower into the 20 to 30 

micrograms per cubic meter range, but was not significant enough to drop PM2.5 levels 

to background. 

Event #2: December 9 through December 31, 1999

The period from December 9, 1999 through December 31st, 1999 was marked by 22 

days of strong stability and poor atmospheric dispersion conditions. Strong high 

pressure over the Intermountain Region dominated the period, leading to light and 

disorganized wind flow and limited dispersion. The period began with the passage of a 
42

weak cold front and trough on the 9th. Between the 10th and the 17th, increasing stability 

and poor dispersion conditions resulted in a PM10 exceedance at Corcoran on the 17th.

Figure 25: PM2.5 at Fresno 1st and Bakersfield California 

for December 1999 and January 2000

0

20

40

60

80

100

120

140

12/1/199912/3/199912/5/199912/7/199912/9/1999

12/11/199912/13/199912/15/199912/17/199912/19/199912/21/199912/23/199912/25/199912/27/199912/29/199912/31/1999

1/2/20001/4/20001/6/20001/8/2000

1/10/20001/12/2000

PM2.5 (micrograms / meter3

)

Fresno-1st Street

Bakersfield-5558 California Avenue

Event #1 Event #2 Event #3

At Corcoran, a 24-hour PM10 (Particulate Matter) concentration of 174 μg/m3

 was 

measured. Table 14 outlines federal reference method (FRM) Daily Average 

Particulate Matter measurements for sites across the San Joaquin Valley (SJV). In 

order to understand the variability of these measurements, an in depth examination of 

the synoptic pattern and surface winds and observations, and aircraft soundings leading 

to the episode were analyzed. 

TABLE 14: Federal Reference Method (FRM) Daily Average Particulate Matter 

measurements for sites across the SJV for December 17, 1999. 

 FRM FRM FRM 

Site Name 24-Avg. Site Name 24-Avg. Site Name 24-Avg. 

10 2.5 10 2.5 10 2.5 

Bakersfield-Gold 92 Modesto 99 93 Corcoran* 174

Bakersfield-CA 111 90 Merced-M St. 75 Stockton** 78

Visalia 114 Clovis 18 Fresno-1st 107

*-Patterson **- Hazleton 

units in μg/m3

 

The meteorological synoptic analysis showed a period of moderately strong 

atmospheric stability from December 10th through the 17th. Ahead of a developing 

trough over the eastern Pacific, a strong mid and upper level ridge developed over the 

region on the 17th. This ridge strengthened the inversion over the San Joaquin Valley 
43

trapping particulates within the Valley boundary layer. The morning surface charts of 

the 17th depicted a strong surface high over the Great Basin and Intermountain Region. 

The 12Z (4 a.m.) surface pressure gradient was –3.9 millibars from San Francisco 

(SFO) to Reno (RNO) (SFO-RNO), with isobars (constant surface pressure) orientated 

west to east. The alignment of the isobars and the –3.9 millibars pressure gradient, 

represented offshore wind flow across the San Joaquin Valley. Many sites across the 

San Joaquin Valley were reporting haze. 

The morning temperature aircraft sounding over Fresno on the 17th showed multiple 

inversions, with a strong inversion (stable layer) of 13 degrees Fahrenheit from the 

surface up to 1,000 feet, with a secondary strong inversion of 11 degrees Fahrenheit 

from 2,500 to 5,000 feet as is evident in Figure 26. The morning temperature sounding 

over Bakersfield also showed multiple inversions, with a strong inversion of 15 degrees 

Fahrenheit from the surface up to 3,000 feet, with a secondary strong inversion of 11 

degrees Fahrenheit from 4,500 feet to 5,000 feet. The temperature sounding on the 

17th, is conducive of elevated PM levels due to low mixing depths and multiple 

inversions, which keep pollutants trapped near the surface. During the early morning 

surface observations across the San Joaquin Valley were cold. The minimum 

temperatures recorded at Fresno and Bakersfield were 33 and 43 degrees Fahrenheit 

respectively. The maximum high temperatures recorded at Fresno and Bakersfield 

were 59 and 57 degrees Fahrenheit respectively. Fresno hourly temperature data 

shows very limited mixing conditions below 500 feet for over 18 hours of the day, 

increasing with minor afternoon heating to a maximum mixing depth of 2,000 feet on the 

17th. 

Figure 26: Atmospheric Temperature Profile at Fresno on December 17, 1999

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

35 40 45 50 55 60 65

Temperature (deg F)

Feet (AGL)

Upper level charts indicated a strong high just west of Santa Barbara, with a ridge 

building northeastward into the Great Basin. A weak trough over the extreme Pacific 

Northwest extended southward to near Eureka. Weak pressure gradients across the 

San Joaquin Valley remained rather flat through the day, leading to light and 

disorganized wind flow across the San Joaquin Valley. 
44

Table 15 shows the 24 hour daily average wind speeds at SJVAPCD monitoring, 

ASOS, and CIMIS sites for December 17, 1999. 

SJVAPCD Monitoring Sites ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

Clovis 2.2Fresno 1.7Shafter/USDA 2.1 Famoso 2.4

Fresno SSP 1.4Bakersfield 3.4Firebaugh/Telles 2.2 Westlands 2.6

Corcoran 2.8Hanford 1.0Stratford 2.4 Panoche 2.6

Edison 3.3 Kettleman 2.6 Arvin-Edison 2.3

Parlier 3.2 Visalia/Americas 2.1 Lindcove 1.8

Arvin 2.2 Parlier 2.2 Kesterson 2.4

Visalia 1.6 Blackwells Corner 2.0 Lodi West 1.8

 Los Banos 2.8 Modesto 2.9

 Manteca 2.4 Fresno State 2.2

Due to the strong stability lasting for over 7 days, PM10 steadily increase region-wide 

until the sampling day on December 17th. With the mid and upper level stability aloft, 

surface based inversion, and light and disorganized wind flow, this weather pattern was 

conducive of an elevated region-wide PM10 measurements. Light south – southeast to 

west – northwest transport wind flow of 4 to 6 miles per hour occurred during a few 

hours on the 17th. At Visalia and Fresno 1st, fine particulates were above 100 μg/m3, 

further suggesting a widespread PM event across the San Joaquin Valley. 

After a weak upper level trough passage on the 18th, strong high pressure rebuilt into 

the region from the eastern Pacific, with increasing stability and poor dispersion 

conditions through the next exceedance date on December 23rd at Fresno-Drummond 

Street and Hanford-Irwin Street. At Fresno-Drummond, a 24-hour PM10 concentration 

of 168 μg/m3 and at Hanford a 24-hour PM10 concentration of 156 μg/m3 was measured. 

 

Table 16 Federal Reference Method (FRM) Daily Average Particulate Matter and Table 

17 California Regional Particulate Air Quality Study (CRPAQS) Measurements shows 

for sites across the San Joaquin Valley (SJV) for December 23. In order to understand 

the variability of these measurements, an in depth examination of the synoptic pattern 

and surface winds and observations, and aircraft soundings leading to the episode was 

done. 
45

TABLE 16: Federal Reference Method (FRM) Daily Average Particulate Matter 

Measurements for sites across the SJV for December 23, 1999. 

 FRM FRM FRM 

Site Name 24-Avg. Site Name 24-Avg. Site Name 24-Avg. 

10 2.5 10 2.5 10 2.5 

Bakersfield-Gold 74 Modesto 119 95 Corcoran* 135 91

Bakersfield-CA 109 72 Merced-M St. 83 Stockton** 79

Visalia 85 Clovis 22 Fresno-1st 119

* -Patterson #-Wagner, **- Hazleton 

TABLE 17: California Regional Particulate Air Quality Study (CRPAQS) Daily Average 

Particulate Matter Measurements for Fresno-Drummond and Hanford-Irwin for 

December 23, 1999. 

 CRPAQS 

Site Name 24-Avg. 

10 

Fresno-Drummond 168 

Hanford-Irwin 156 

units in μg/m3 for Table 16 and 17 

The meteorological synoptic analysis showed after a period of strong atmospheric 

stability from December 18th through the 23rd, increasing stability and poor dispersion 

conditions lead to two exceedances of the PM10 NAAQS measured using CRPAQS 

monitors at Fresno-Drummond and Hanford, Figure 27. The morning surface charts on 

the 23rd depicted a surface ridge extending southwestward across the central San 

Joaquin Valley from strong highs anchored over the Intermountain Region. The 

morning surface pressure gradient was –7.6 millibars from San Francisco to Reno 

(SFO-RNO), with isobars (constant surface pressure) orientated southeast to northwest. 

With the alignment of the isobars and –7.6 millibars pressure gradient, this represented 

light to moderate offshore wind flow across the San Joaquin Valley. Light southeasterly 

transport wind flow of 5 to 8 miles per hour occurred for a few hours of the day. . 

Visibility reports across the San Joaquin Valley indicated hazy conditions throughout the 

day. 
46

Figure 27: T850 Stability and Precipitation vrs. PM10 

at Hanford

December 1999

0

20

40

60

80

100

120

140

160

180

12/1/199912/2/199912/3/199912/4/199912/5/199912/6/199912/7/199912/8/199912/9/199912/10/199912/11/199912/12/199912/13/199912/14/199912/15/199912/16/199912/17/199912/18/199912/19/199912/20/199912/21/199912/22/199912/23/199912/24/199912/25/199912/26/199912/27/199912/28/199912/29/199912/30/199912/31/1999

PM10 (micrograms/meter^3) 

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

T850 Stability Parameter(C)

FRM PM10

CRPAQS PM10

Model PM10

T850 Stability

*No precipitation occurred in Hanford from December 1, 1999 through December 31, 1999. 

The morning temperature aircraft sounding over Fresno depicted a strong inversion 

(stable layer) of 19 degrees Fahrenheit from the surface up to 1,000 feet as is evident in 

Figure 28. Bakersfield showed a strong inversion of 12 degrees Fahrenheit from the 

surface up to 1,000 feet turning isothermal up to 3,000 feet. The temperature 

soundings on the 23rd are indicative of elevated PM levels due to low mixing depths and 

strong inversions, which trap pollutants near the surface. During the early morning 

surface observations across the San Joaquin Valley were cold. The minimum 

temperatures recorded at Fresno and Bakersfield were 34 and 36 degrees respectively. 

The maximum high temperatures recorded at Fresno and Bakersfield were 62 and 64 

degrees Fahrenheit respectively. Fresno hourly temperature data shows very limited 

mixing conditions below 500 feet for over 16 hours of the day, increasing with minor 

afternoon heating to a maximum mixing depth of 2,000 feet on the 23rd

Figure 28: Atmospheric Temperature Profile at Fresno on December 23, 1999

0

1000

2000

3000

4000

5000

35 40 45 50 55 60

Temperature (deg F)

Feet (AGL)
47

Upper level charts indicated a strong high 600 NM west of Santa Barbara, with a ridge 

building northward along the West Coast into the eastern Gulf of Alaska. Pressure 

gradients remained weak throughout the day leading to poor dispersion conditions 

across the San Joaquin Valley. 

Table 18 shows the 24 hour daily average wind speeds at SJVAPCD monitoring, 

ASOS, and CIMIS sites for December 23, 1999. 

SJVAPCD Monitoring Site ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

Clovis 2.5Fresno 1.5Shafter/USDA 2.5 Famoso 2.5

Fresno SSP 2.1Bakersfield 3.1Firebaugh/Telles 2.5 Westlands 3.0

Corcoran 3.3Hanford 1.6Stratford 3.3 Panoche 2.7

Edison 3.7 Kettleman 3.3 Arvin-Edison 2.6

Parlier 3.9 Visalia/Americas 2.5 Lindcove 1.8

Arvin 2.4 Parlier 2.7 Kesterson 2.1

Visalia 1.7 Blackwells Corner 2.1 Lodi West 1.7

 Los Banos 2.8 Modesto 2.4

 Manteca 2.4 Fresno State 2.3

The period of December 9, 1999 through December 31st, 1999, marked one of the 

longest periods of strong stability in recent history. This pattern was similar to the one 

experienced during the California Regional Particulate Air Quality Study. The main 

difference between the two episodes was the CRPAQS event began a week later and 

continued into the early part of January. During the exceedances of 1999, they were 

preceded by an exceptionally dry fall. After a weak frontal passage on the 10th, which 

brought a cold air mass to the Valley, this air remained trapped within the Valley 

boundary layer resulting in strong surface based inversions. Coupled with the synoptic 

and surface pattern, which was indicative of light and disorganized wind flow, PM 

became a region-wide problem until a weak trough scoured out the area on New Year’s 

Eve. As is evident in the PM data, local and regional emissions contributed to the 

exceedance of the NAAQS at Corcoran on the 17th and Fresno-Drummond, and 

Hanford on the 23rd. If monitoring were done on a daily basis, more exceedances would 

have been recorded during this time frame at other San Joaquin Valley locations. 

Event #2 ended when a cold front from the Gulf of Alaska moved through the region 

bringing enhanced dispersion across the San Joaquin Valley, lowering pollutant levels 

to below the PM2.5 NAAQS. 

Event #3 January 1 through January 12, 2000:

Meteorologically, dispersion once again worsened and particulate conditions became 

more favorable from January 1st to January 10th. As is evident in Figure #2, event #3 

lasted about 12 days and was marked by multiple violations of the PM2.5 NAAQS. After 

the passage of a weak cold front on January 1, 2000, which brought trace amounts of 

precipitation to the San Joaquin Valley, moisture became available for atmospheric 
48

chemistry reactions. Humidity measurements of 85 – 100 % in the morning across the 

Valley Floor showed a moist air mass, with light fog and haze being reported. 

Synoptically, during event #3 a weak ridge dominated along the California Coastline 

directing upper level disturbances into the Pacific Northwest. The ridge was strong 

enough to gradually, through a period of a few days, cause a localized PM2.5 problem 

centered at the urban areas, which spread outward toward the rural areas and become 

a regional problem, especially across the central and southern parts of the San Joaquin 

Valley. The fast upper level flow pattern was positioned over the Pacific Northwest 

through event #3, with multiple embedded disturbances within this flow. California 

remained on the southern periphery (stable side) of this flow pattern. With a moderate 

lid in place aloft and maximum temperatures in the mid to upper 50’s, the afternoon 

hours were marked by limited mixing, resulting in increasing particulate potential. 

Mixing heights at Fresno remained below 500 feet under a moderate inversion during 

most of the day (16 hours or more). During the afternoon hours there were moderate 

mixing depths of 1,500 to 2,000 feet but the bulk of the day had limited mixing. 

Maximum and minimum temperatures were near or slightly below normal, indicating a 

potential for higher residential wood burning emission activity and nitrate forming 

reactions. 

Meteorological data during event #3 showed limited mixing depths and light north to 

northwesterly wind flow resulted in minimal dispersion of pollutants. Both primary and 

secondary pollutants from local emissions across the metropolitan areas of the San 

Joaquin Valley accumulated resulting in an exceedance of the 24 hour PM2.5 NAAQS at 

several sites on January 7, 2000. These sites included: Merced, Chowchilla, Fresno1st, Clovis-Villa, Selma, Visalia-Church, Corcoran-Patterson, Kettleman, Pixley Wildlife 

Refuge, Oildale, and Bakersfield Golden and California. Since both rural and urban 

areas registered PM2.5 levels above the NAAQS, these violations continued through the 

10th of January at most locations. A dry cold front moved through the region on January 

12, 2000, providing adequate northwesterly flow, resulting in PM2.5 levels lowering 

below the NAAQS. 
49

Figure 29: Wind Rose at Clovis Air Monitoring Site for 12/17 and 12/23/1999, and 

01/07/2000
50
51

11.0 DECEMBER 2000 AND JANUARY 2001 EPISODE SYNOPTIC 

METEOROLOGICAL DISCUSSION

The CRPAQS fall and winter measurement periods can be divided into seven distinct 

episodes. These episodes, characterized by limited atmospheric dispersion, were 

separated by vigorous trough passages. Analysis of synoptic events shows that once a 

particulate episode begins, a deep trough must traverse the region at both the upper 

levels of the atmosphere and at the surface as a cold frontal boundary in order to 

significantly decrease Particulate Matter (PM) concentrations. These types of troughs 

are normally associated with strong vertical mixing, moderate to high boundary layer 

mixing heights, precipitation, and wind speed and directional sheer within the boundary 

layer. 

The meteorological analysis for CRPAQS utilized synoptic analyses, precipitation 

patterns, and atmospheric stability parameters to determine episode strength and 

periodicity. Analysis of PM2.5/PM10 ratios and nitrate levels reveal that when the 

overnight minimum temperature decreased to below 40 degrees Fahrenheit, coupled 

with significant warming aloft, fine particulate levels climbed. When prolonged periods 

of these conditions occurred, PM2.5 levels climbed above the NAAQS. In even stronger 

more persistent stability regimes, PM10 exceeded the NAAQS. Figure 30 shows 

precipitation, PM2.5, PM10, and stability parameters for the CRPAQS period at the 

Fresno-1st site. General patterns in these parameters where also evident at other sites. 

Precipitation events also influenced PM concentrations during CRPAQS. For the 

CRPAQS period, when a frontal system was strong enough to produce precipitation of 

0.10 inch or more of liquid water, 24 hour averaged PM2.5 concentrations dropped to 

below 15 μg/m3. 
52

Figure 30: Fresno-1st Stability and Pollutant Trends for 

Fall 2000 & Winter 2001 

0

20

40

60

80

100

120

140

160

180

200

10/1/2000

10/4/2000

10/7/2000

10/10/2000

10/13/2000

10/16/2000

10/19/2000

10/22/2000

10/25/2000

10/28/2000

10/31/2000

11/3/2000

11/6/2000

11/9/2000

11/12/2000

11/15/2000

11/18/2000

11/21/2000

11/24/2000

11/27/2000

11/30/2000

12/3/2000

12/6/2000

12/9/2000

12/12/2000

12/15/2000

12/18/2000

12/21/2000

12/24/2000

12/27/2000

12/30/2000

1/2/2001

1/5/2001

1/8/2001

1/11/2001

1/14/2001

1/17/2001

1/20/2001

1/23/2001

1/26/2001

1/29/2001

2/1/2001

-10

-5

0

5

10

15

20

PM10 (micrograms per cubic meter)

Precipitation*100 (inches)

PM 2.5 (micrograms/cubic meter)

T850-T(FAT MIN) (Celsius)

12 3 7 4 6 5

Start of Episode

The synoptic meteorological parameters analyzed to determine the behavior of 

particulate matter were 850 MB temperature stability (T850 Stability 0C), 500 MB height 

(decameters), and precipitation (inches). The 850 MB temperature stability parameter 

was utilize because 850 MB temperature (approximately 5,000 feet) minus the minimum 

surface temperature is a good indicator of the inversion strength, the mixing layer depth, 

and whether air is being warmed from a synoptic subsidence inversion aloft. This 

inversion is commonplace when a ridge builds along the West Coast of California, 

providing general adiabatic warming of subsiding air over the region. In Hackney, et al, 

an analysis was conducted on the relationship of historical 850 MB temperatures and 

PM measurements. Hackney, et al, determined that in the San Joaquin Valley, when 

850 MB temperatures in the fall were above +150

 C and +100C during the winter (late 

November or December to February) an episode was possible. A stronger indication 

for a PM episode to occur is when 850 MB temperatures were above +180

C in the fall 

and above +140

C in the winter. 

500 MB height is also important in that it is directly correlated to the temperature of the 

entire column of air. During periods of high pressure aloft, flows at the surface are 

usually light and accompanied by low mixing depths. The 500 MB height and 

placement of the mean ridge and trough patterns reflect PM amounts and trends. In 

Hackney, et al, it was determined that a height of 5,760 meters in the San Joaquin 

Valley is sufficient to suspect an episode, and that a height of 5,820 meters or more in 

the Valley made an episode occurrence a likelihood. 
53

Precipitation measurements were also analyzed as an indicator of the vigorous trough 

passages, which provided enough vertical mixing to produce rainfall. As mentioned 

before, the strength of the frontal system and trough, was a good indicator of how much 

PM dispersion occurred. 

Table 19 shows for each peak day within the seven episodes, the PM values, 

meteorological parameters, and episode duration. The major episode of CRPAQS, 

when PM10 exceeded the NAAQS, was manifested by cool overnight lows near 32 

degrees Fahrenheit and warm air aloft for an extended period of time. These conditions 

persisted from December 17th through January 7th. During this time, PM steadily 

increased to a peak value at Fresno (PM2.5 = 148 μg/m3 and PM10 = 193 μg/m3 ) and 

Bakersfield-Residential (PM2.5 =133μg/m3

) on the 1st of January. During this period in 

urban areas, initial analysis of speciation data show high carbon and ammonium nitrate 

levels. The combination of an 850 MB temperature ridge aloft along with low minimum 

temperatures resulted in strong inversions trapping pollutants in a shallow mix layer. 

The high carbon levels could have been due to holiday residential wood combustion. In 

addition, under these cold and poor dispersion conditions, nitrate levels measured on 

the real-time instruments were the highest of the study. 

TABLE 19: CRPAQS EPISODE EVENT SUMMARY TABLE AT FRESNO

One basis for understanding the behavior of particulate matter evolution is to analyze 

the synoptic situation that is driving the local effects of dispersion and transport. These 

synoptic descriptions follow. 

Episode 

Number 

Date Peak 

PM10 

(μg / m3) 

Peak 

PM2.5 

(μg / m3) 

T850 

Stability 

(

0

C) 

Minimum 

(

0

F) 

500 MB 

Height 

(dm) 

Episode 

Duration 

(days) 

1 10/1-

10/12/00 

42(6th) 24 (5th & 

6th) 

2.4 63 578 12 

2 10/13-

10/30/00 

67(18th) 47 (18th) 6.8 54 578 18 

3 10/31-

11/15/00 

57 (2nd) 61 (4th) 2.3 47 575 17 

4 11/16-

12/15/00 

114 (20th) 90 (20th) 

89 (28th) 

9.3 

6.2 

35 

46 

576 

572 

28 

5 12/16/00-

01/12/01 

193 (1st) 148 (1st) 12.6 32 574 26 

6 01/13-

01/27/01 

104 (19th) 103(21st) 3.9(19th)

6.8(21st) 

38 (19th) 

33 (21st) 

571(19th) 

569(21st) 

15 

7 01/28-

02/03/01 

47 (31st) 73 (3rd) 3.9(31st) 

6.1(3rd) 

34 (31st) 

39 (3rd) 

568(31st) 

580(3rd) 

7 
54

Episode 1: Sunday, October 1 -Thursday, October 12

From the 1st to the 8th, episode #1 was marked by a mean, broad ridge anchored across 

the eastern Pacific and California, which brought moderate stability and low to moderate 

mixing heights. This ridge was low in amplitude, resulting in moderate subsidence. An 

inverted thermal surface trough developed from Bakersfield curving northwestward to 

Redding. This pressure pattern was representative of calm overnight winds, and 

thermally driven light afternoon northwesterly flow. During this initial episode, PM2.5

values average near 20 μg/m3 and measured PM10 concentration at Fresno was 42 

μg/m3 on the 6th. The 500 MB height of 578 dm on the 6th was conducive of a high PM 

event, whereas, the 850 MB stability parameter of +2.4 Celsius (T850 MB = +200

C) was 

indicative of the likelihood of an event. As is evident in the low T850 MB stability 

parameter, the inversion and resulting stability was not strong enough to trap and 

elevate PM readings, thus resulting in PM levels remaining well below the standard. 

A significant early season trough moved into the region on the 9th, bringing increasing 

instability and boundary layer flow. Within the next couple of days, the 9th to the 12th, a 

series of disturbances embedded within the mean trough traversed central California 

bringing increased wind flow, higher boundary layer mixing heights, and precipitation. 

The most noteworthy precipitation during this period occurred on the 11th, when a 

second short wave embedded within the mean trough across California crossed the 

Valley and produced 0.76 inches of precipitation. 

Episode 2: Friday, October 13 –Monday, October 30

Episode #2 was marked by high pressure rapidly building into the region on the 13th and 

persisting through the 20th. This episode was unique in that the central California was 

under the influence of both the eastern Pacific and Four Corners ridge during the peak 

episode day. This pattern resulted in weak thermally driven flow at the surface and 

general synoptic subsidence aloft. As a result, under light to moderate stability, T850 

stability parameter of +2.8 0

C (T850 MB = 150C), moderately high 500 MB heights (578 

MB) and low mixing depths, PM climbed and reached a maximum concentration in 

Fresno at PM10 – 67 μg/m3 and PM2.5 – 47 μg/m3 and Bakersfield at PM2.5 – 30 μg/m3 on 

the 18th. According to Hackney, et, al, the 850 MB temperature and 500 MB heights 

reached the criteria to suspect an episode. As is evident in the PM measurements the 

state standard was exceeded in Fresno on the 18th. 

The ridge began to breakdown on the 20th as weak disturbances traversed the ridge 

and formed a closed low over the Great Basin on the 24th. Another stronger trough 

moved into the region on the 25th bringing moderate boundary layer flow, vigorous 

vertical mixing, and periods of precipitation through the 30th. With the trough passage 

on the 29th, Fresno recorded the maximum amount of precipitation during the episode of 

0.55 inches. 

Episode 3: Tuesday, October 31 –Wednesday, November 15
55

Episode #3 began on the 31st as a strong ridge developed along the West Coast. The 

ridge axis remained offshore during this episode leading to downstream subsidence and 

increasing stability. During the peak PM event of episode #3, Bakersfield’s PM values 

were lower than Fresno due to a closed low circulation over northern Baja, creating 

some upper level mixing and neutral to light instability over southern California. As a 

result, the peak PM2.5 levels at Bakersfield Residential were 45 μg/m3, with a T850 MB 

stability parameter of +1.4 0C (T850 MB = +9.2 0C) and 500 MB height of 575 dm on the 

3rd and at Fresno First PM10 measurement of 57 μg/m3 on the 2nd and PM2.5

measurement 61 μg/m3

, with a 850 MB stability parameter of +2.3 0C (T850 MB = +10.6 

0

C) and 500 MB height of 575 dm on the 4th. 

In both Fresno and Bakersfield, according to Hackney et, al, the T850 MB and 500 MB 

heights do not suggest an episode under a fall regime; however, the PM10 

measurement in Fresno exceeded the standard on the 2nd. Whereas, comparing 

Hackney et, al, results under a winter regime to the observed T850 MB, their conclusion 

supported a minor PM episode. Due to early season rainfall and a general cooling of 

the lower levels of the atmosphere, the transition from the fall to the winter regime took 

place during this time. 

PM levels steadily lowered as the upper level ridge and surface high pressure gradually 

weakened and moved eastward. An unseasonably cold trough (540 dm at OAK) and a 

vigorous, dry cold front from the Gulf of Alaska moved into the region on the 9th, 

bringing increased boundary layer flow and deep mixing. This system was moisture 

starved and resulted in trace amounts of precipitation across the region. The frontal 

system and accompanying trough were strong enough to bring an end to the 3rd

episode as deep mixing scoured the area. Multiple embedded disturbances within this 

trough traversed the region bringing unstable conditions, good dispersive conditions, 

and moderate boundary layer flow and mixing heights into the 15th. 

Episode 4: Thursday, November 16 –Friday, December 15

Episode #4 began on the 16th and was marked by a moderate ridge building along the 

West Coast bringing increasing stability and offshore flow. During CRPAQS the 4th

episode was the longest period at 28 days. 

Stability and PM values steadily increased to the highest measured values of the 

episode on the 20th and 21st of November, with a secondary peak occurring on the 28th. 

This episode was separated into two peaks due to a trough developing over the Great 

Basin on the 22nd dispersing some of the PM which had built up over the region. Fresno 

and Bakersfield peak PM2.5 measurements were observed on the 20th at 90 μg/m3 and 

on the 21st at 96 μg/m3

, respectively. The highest measured PM10 during this episode 

was at Fresno on the 20th with a value of 114 μg/m3

. Fresno PM2.5 measurement 

reached a secondary peak on the 28th at 89 μg/m3

. This episode was separated into 

two peaks due to a trough developing over the Great Basin on the 22nd dispersing some 

of the PM which had built up over the region. 
56

According to Hackney, et., al, the 850 MB temperature measured +110

C and +140C on 

the 20th and 28th, respectively, were representative of an PM episode occurring, 

whereas, the 500 MB heights of 576 and 572 dm, were indicative of a weak PM 

episode. During this episode, the state standard was exceeded. 

This episode was characterized by a mean mid-tropospheric ridge positioned along the 

West Coast, with brief drop in the PM measurements on the 29th and 30th, when a dry, 

front passed to the north of the area dropping to PM2.5 measurements at Fresno to 35 

μg/m3 on the 30th and Bakersfield to 22 μg/m3 on the 29th. 

The ridge began to breakdown late on the 7th, bringing decreasing stability, better 

dispersion, and lowering PM concentrations. A series of weak disturbances traversed 

the region from the 7th through the 15th, bringing very light precipitation to Fresno (total = 

0.07 inches), increased boundary layer flow and mixing depths, which brought an end to 

the episode #4. 

 

Episode #5: Saturday, December 16 –Friday, January 12

Episode #5 began on the 16th and was marked by the West Coast ridge building into 

California, resulting in lowering boundary layer mixing heights and increasing stability. 

PM region-wide uniformly increased with 500 MB heights and stability, with a minor 

intra-episode peak, with PM10 measurement at Fresno of 110 μg/m3 and PM2.5

measurement at Bakersfield Residential of 58 μg/m3

, occurring on the 20th. A weak 

upper level disturbance briefly broke down the ridge on the 24th resulting in slightly 

lower PM2.5 measurements in Fresno at 49 μg/m3

 and Bakersfield 40 μg/m3. After this 

weak trough passage, the ridge rebuilt in earnest and moderate stability and low mixing 

heights continued through the end of the episode on January 7th. 

During this episode the maximum PM measurements of the CRPAQS period were 

observed. The general synoptic pattern during this episode was strong high pressure 

aloft positioned along the West Coast providing a general area of subsidence and stable 

weather over the region, with an accompanying strong surface high located over Idaho. 

With high pressure over the Intermountain Region and lower pressures over the eastern 

Pacific, moderate offshore flow developed. Typically with this type of flow pattern, the 

normal nocturnal inversion at the surface is reinforced with a subsidence inversion aloft, 

created by adiabatic warming off the Sierra’s and general subsidence from the ridge. 

The peak measurements of the fifth episode were recorded on the 1st and 7th with PM10

measurements at Fresno of 193 μg/m3 (1st) and 141μg/m3 (7th). PM2.5 measurements at 

Fresno of 148 μg/m3 (1st) and 101 μg/m3 (7th) and Bakersfield Residential at 133 μg/m3 

(1st) were also recorded. These high PM values corresponded with the highest 850 

MB stability (+13.4 C at Oakland) measured during the CRPAQS period, which occurred 

on the 2nd. The 850 MB temperature was very warm during this episode. At Oakland 

at 12Z, the 850 MB temperature was +13 0

C. This is representative of a strong PM 

episode. The 500 MB height was not indicative of a strong episode. At 574 dm, the 

value was not conducive to a significant PM episode. The combination of weak offshore 
57

flow and mid-tropospheric stability, elevated the PM readings experienced across 

central California. 

A significant trough moved into the region on the 8th, with accompanying rainfall in 

Fresno of 0.31 inches and Bakersfield of 0.29 inches. A series of disturbances within 

the trough traversed the region from the 8th to the 12th adequately dispersing the PM 

that had built up over the region during the episode and lowered PM2.5 measurements 

into the upper teens by the 10th in both Fresno and Bakersfield. 

Episode 6: Saturday, January 13 –Saturday, January 27

High pressure and increasing subsidence once again moved into the region on the 13th

bringing increasing PM levels and leading to the beginning of the episode #6. This 

episode was characterized by ridging through the 17th, resulting in building PM2.5 values 

into the lower 60’s μg/m3

 range. Shortly thereafter, more pronounced ridging and 

decreasing dispersion occurred on the 18th, when 500 MB heights climbed to near 578 

dm with an 850 MB stability indices of +7 °C (850 MB temperature of +70C). The 500 

MB height was conducive of an episode occurring, but the 850 temperature was not 

conducive of an episode. This peak in meteorological parameters corresponded with 

the highest recorded PM10 measurement of the episode at Fresno at 104 μg/m3 on the 

19th. 

PM2.5 peaked at Fresno on the 21st with a measurement of 103 μg/m3 and also in 

Bakersfield Residential on the 22nd at 103 μg/m3. The southward progression of the 

peak PM2.5 values can be accounted for by the pre-frontal / trough stability that 

progressively moved down-valley with the approach of the next system from the eastern 

Pacific on the 23rd. 

A series of disturbances embedded within the trough moved through the region from the 

23rd to the 27th, with adequate boundary layer mixing heights, moderate instability, and 

moderate southeasterly flow, dispersing PM levels into the low teens. From the 23rd to 

the 25th, a series of strong cold frontal systems brought 1.19 inches of rainfall at Fresno, 

bringing an end to the 6th episode. The highest recorded rainfall during the trough 

passage occurred on 0.85 inches on the 24th. 

Episode 7: Sunday, January 28 –Saturday, February 3

The final (7th) episode of CRPAQS began on the 28th, when strong high pressure built 

along the West Coast. Light boundary layer flow and increasing 850 MB stability 

resulted in PM2.5 levels gradually climbing into the early part of February. PM2.5

measurements of 73 μg/m3 in Fresno on the 3rd and 92 μg/m3

 in Bakersfield on the 4th

marked the buildup that occurred during this episode. 
58

12.0 DISPERSION METEOROLOGY FOR THE FALL 2000 AND 

WINTER 2001 CALIFORNIA REGIONAL PM10/PM2.5 AIR QUALITY 

STUDY EPISODE 

During the Fall 2000 and Winter 2001 episode, strong high pressure at the surface and 

aloft resulted in limited morning and afternoon mixing and light offshore wind flow. Cool 

damp mornings and strong stability contributed to the formation of nitrates and sulfates 

during the episode. The California Regional Particulate Air Quality Study (CRPAQS) fall 

and winter measurements can be divided into seven distinct events. These events, 

characterized by limited atmospheric dispersion, were separated by vigorous trough 

passages. Analysis of synoptic patterns shows that once a particulate period begins, a 

deep trough must traverse the region at both the upper levels of the atmosphere and at 

the surface as a cold frontal boundary in order to significantly decrease particulate 

matter (PM) concentrations. These types of troughs are normally associated with 

strong vertical mixing, moderate to high boundary layer mixing heights, precipitation, 

and wind speed and directional sheer within the boundary layer. 

The meteorological analysis for CRPAQS utilized synoptic analyses, precipitation 

patterns, and atmospheric stability parameters to determine episode strength and 

periodicity. Analysis of PM2.5/PM10 ratios and nitrate levels reveal that when the 

overnight minimum temperature decreased to below 40 degrees Fahrenheit, coupled 

with significant warming aloft, fine particulate levels climbed. When prolonged periods 

of these conditions occurred, PM2.5 levels climbed above the National Ambient Air 

Quality Standards (NAAQS). In even stronger more persistent stability regimes, PM10

exceeded the NAAQS. Figure 31 shows precipitation, PM2.5, PM10, and stability 

parameters for the CRPAQS period at the Fresno-1st site. General patterns in these 

parameters were also evident at other sites. Precipitation influenced PM concentrations 

during CRPAQS. For the CRPAQS episode, when a frontal system was strong enough 

to produce precipitation of 0.10 inch or more of liquid water, 24 hour averaged PM2.5

concentrations dropped to below 15 μg/m3. Seven distinct events were identified when 

particulate levels lowered to near background levels. 

Each event was marked by limited mixing heights with strong inversions of 

approximately 500 feet for 18 or more hours of the day. Maximum mixing depths of 

highest PM2.5 concentration days ranged between 1,500 to 2,500 feet. Moisture 

availability from recent rainfall and minimal solar insulation, led to prime nitrate and 

sulfate forming conditions. Typically, light fog, with humidity measurements between 85 

– 100% were needed in order to maximize particulate forming potential. The higher the 

humidity, the heavier and wetter the fog, resulted in the removal of particulates by wet 

deposition. 

Within these seven distinct measurement periods, a violation of the Federal 24 hour 

PM2.5 standard of 65 micrograms/cubic meter were recorded during the 4th, 5th, 6th, and 

7th events at various locations across the San Joaquin Valley. The following analysis 

will investigate the meteorological and dispersion conditions surrounding each event. 
59

 

Figure 31: Fresno-1st Fall 2000 and Winter 2001 Episode 

0

20

40

60

80

100

120

140

160

180

200

10/1/200010/8/200010/15/200010/22/200010/29/200011/5/200011/12/200011/19/200011/26/200012/3/200012/10/200012/17/200012/24/200012/31/20001/7/20011/14/20011/21/20011/28/2001

PM10, PM 2.5 

(Micrograms/cubic^3) and 

precipitation (inches)

-10

-5

0

5

10

15

20

T850 Stability (C)

PM10 (micrograms per cubic meter)

Precipitation*100 (inches)

PM 2.5 (micrograms/cubic meter)

Oakland T850-T(FAT MIN) (Celsius)

12 3 7 4 6 5

Start of Episode

Event #4: November 16 through December 14, 2000

Meteorologically, dispersion worsened and particulate formation conditions became 

more favorable from November 16th to December 10th. As is evident in Figure 31, 

event #4 lasted about 28 days and was marked by two main PM2.5 peaks, which 

exceeded the Federal Standard. After the passage of a weak cold front on November 

14, 2000, which brought a trace to a few hundredths of an inch of precipitation across 

the region, moisture became available for atmospheric chemistry reactions. Humidity 

measurements of 90 – 100 % in the morning across the Valley Floor showed a moist 

atmosphere, with light fog and haze being reported on measured high PM2.5 days. 

These cool damp mornings and strong stability favored the formation of nitrates and 

sulfates. It does not appear that the water content of the atmosphere was high enough 

to create deposition of particles that exceeded formation. 

Synoptically during the first part of event #4, the eastern Pacific high built over the 

Pacific Northwest on November 17th then expanded southward on the 18th, bringing 

increasing stability and poor dispersion conditions through November 30th. From 

November 17th through the 30th, the high strengthened and intensified on November 

19th through the 20th and once again on November 24th through the 28th, further 

tightening the lid and trapping the pollutants within the San Joaquin Valley boundary 

layer. 

With a strong lid in place aloft and maximum high temperatures in the upper 50’s to low 

60’s on the 19th and 20th, and upper 40’s to low 50’s on the 24th through the 27th, the 

afternoon hours were marked by limiting mixing, resulting in increasing particulate 

potential. Mixing heights at Fresno remained below 500 feet under a strong inversion 
60

during a majority of the day (18 or more hours) on days when the fine particulates 

climbed above the standard. During the afternoon hours, there were high mixing depths 

(maximum mixing depth of 1,500 to 2,500 feet), but the bulk of the day had limited 

mixing. Maximum and minimum temperatures from November 19th through the 27th, 

were below normal, indicating a potential for increasing residential wood burning 

emission activity, and nitrate forming reactions. 

Light fog and haze were reported at the Valley surface during the early morning hours of 

November 19th through the 27th, resulting in slightly lower solar radiation intensities. 

Along with lower solar radiation intensities due to the early morning fog and haze, low 

sun angle, and reduced daylight hours, the atmospheric chemistry reactions may have 

favored the secondary particulate forming regime. Light fog would be indicative of 

moderate atmospheric water content that contribute to the formation of ammonium 

nitrate. Heavier fogs result in the removal of particulates by wet deposition. 

Meteorological data during the first part of event #4 showed limited mixing depths and 

light and disorganized wind flow resulted in minimal transport and dispersion of 

pollutants. Both primary and secondary pollutants from local emissions across the San 

Joaquin Valley accumulated resulting in the exceedance of the 24-hour PM2.5 standard 

at Fresno-1st on November 19th, Modesto 14th, Clovis-Villa, Fresno-1st, Visalia-Church, 

Corcoran-Patterson, Oildale, Bakersfield-Golden and California on November 20th, 

Bakersfield-California on November 21st, Fresno-1st on November 24th, BakersfieldCalifornia on November 25th, Bakersfield-Golden and California on November 26th, 

Bakersfield-California on November 27th, and Clovis-Villa, on November 28th. 

A dissipating cold front moved over the San Joaquin Valley on the 29th, bringing an end 

to the first part of event #4 that lasted from November 16th through the 30th. The 

disturbance brought moderate dispersion conditions across the Valley, allowing for 

pollutants to lower into the 25 to 30 micrograms per cubic meter range, but was not 

significant enough to drop PM2.5 levels to background. 

High pressure rebuilt over the region on December 1st, leading to the second part of 

event #4. Meteorologically, dispersion rapidly deteriorated and particulate formation 

conditions became more favorable on December 2nd. Humidity measurements of 90 – 

100 % in the morning across the Valley Floor showed a continued moist atmosphere, 

with light fog and haze being reported on measured high PM2.5 days. These cool damp 

mornings and strong stability favored the formation of nitrates and sulfates. 

Synoptically during the second part of event #4, the eastern Pacific high built along the 

West Coast on the 1st then expanded eastward on the 2nd, bringing increasing stability 

and poor dispersion conditions through the end of event # 4 on December 12. During 

this period, the high strengthened and intensified on December 2nd and once again on 

December 8th through the 10th, further tightening the lid trapping the pollutants within the 

San Joaquin Valley boundary layer. 
61

With a strong lid in place aloft and maximum high temperatures in the upper 40’s to low 

50’s, the afternoon hours of December 2nd through the 8th were marked by limiting 

mixing, resulting in increasing particulate potential. Mixing heights at Fresno remained 

below 500 feet under a strong inversion during a majority of the day (18 or more hours) 

on days when the fine particulates climbed above the standard. During the afternoon 

hours, there were high mixing depths (maximum mixing depth of 1,000 to 2,500 feet), 

but the bulk of the day had limited mixing. 

Maximum and minimum temperatures from December 2nd through the 5th were slightly 

below normal and from December 7th to the 12th were above normal. Light fog and 

haze were reported at the Valley surface during the early morning hours of December 

2nd through the 10th, resulting in slightly lower solar radiation intensities. Along with 

lower solar radiation intensities due to the early morning fog and haze, low sun angle, 

and reduced daylight hours, the atmospheric chemistry reactions may have favored the 

secondary particulate forming regime. Light fog would be indicated of moderate water 

content’s that contribute to the formation of ammonium nitrate. Heavier fogs result in 

the removal of particulates by wet deposition. 

Meteorological data during the second part of event #4 showed limited mixing depths 

and light and disorganized wind flow resulted in minimal transport and dispersion of 

pollutants. Daily filter measurements at Bakersfield-California and Fresno-1st indicated 

that between December 3rd and 4th, PM2.5 levels dropped below the PM2.5 standard in 

response to heavy fog, which lead to the removal of particulates by wet deposition. 

However, on the 7th and 8th the fog lifted to low clouds across the San Joaquin Valley 

causing both primary and secondary pollutants from local emissions across the region 

to accumulate resulting in the exceedance of the 24-hour PM2.5 standard at various 

monitoring sites from December 5th through the 10th. An exceedance of the 24-hour 

PM2.5 standard were measured at Bakersfield California on December 5th through the 

10th, Modesto-14th, Clovis-Villa, Fresno 1st, Visalia, Corcoran, Oildale, BakersfieldGolden, and Edison, on December 8th, and Fresno 1st on December 9th. A moderate 

cold front traversed over the San Joaquin Valley on December 11th bringing upwards to 

0.05 to 0.10 of an inch of precipitation across different parts of the District. This frontal 

passage brought moderate dispersion conditions across the Valley, allowing for 

pollutants to lower to near background levels bringing an end to event #4. 

Event #5: December 15, 2000 through January 11, 2001

Meteorologically, dispersion worsened and particulate formation conditions 

strengthened from December 10th to January 7th. As is shown in Figure 31, event #5 

lasted about 27 days and was marked by a gradual buildup of PM2.5 that lead to multiple 

days the Federal PM2.5 Standard was exceeded. During event #5, strong high pressure 

at the surface and aloft resulted in limited afternoon mixing and light offshore wind flow. 

Cool damp mornings and strong stability contributed to the formation of nitrates and 

sulfates during the episode. 
62

After the passage of a cold front on December 11, 2000, which brought a few 

hundredths of an inch of precipitation to the region, moisture became available for 

atmospheric chemistry reactions. Humidity measurements of 90 – 100 % in the 

morning across the Valley Floor showed a moist atmosphere, with light fog and haze 

being reported on the exceedance days. These cool damp mornings and strong 

stability favored the formation of nitrates and sulfates. 

Synoptically during event #5, the eastern Pacific high built over the San Joaquin Valley 

on December 12, 2000 and dominated the region’s weather through January 7, 2001. 

The event was divided into two periods from December 15 through December 24, 2000 

and December 24, 2000 to January 11, 2001. A trough moved into northern California 

on December 24, allowing for slightly better dispersion conditions and a decrease in fine 

particulate levels across the San Joaquin Valley. 

High pressure gradually intensified over the San Joaquin Valley between December 12th

through the 20th, leading to PM2.5 levels exceeding the National Ambient Air Quality 

Standards on December 20th at Fresno-1st. However, by December 22 a weak trough 

moved into northern California providing improving dispersion conditions across the San 

Joaquin Valley and lowering fine particulate measurements to below the National and 

California Ambient Air Quality Standards. 

By December 25th, the combination of increased local emissions due to the Christmas 

and New Year Holiday’s and intensifying stability aloft initiated an eleven day stretch 

were numerous sites across the San Joaquin Valley exceeded the PM2.5 Standard. 

These violations began in the urban areas of Fresno, Modesto, and Bakersfield and 

spread regionally into the rural sections by the end of the episode on January 7. The 

meteorology surrounding this event were classified as an intense high dominating 

central California’s air quality, leading to strong stability and light wind flow. 

With a strong lid in place aloft and maximum high temperatures in the upper 50’s to low 

60’s, the afternoon hours were marked by limited mixing, resulting in increasing 

particulate potential. Mixing heights at Fresno remained below 500 feet under a strong 

inversion for 19 hours on January 1st, breaking out after 4:00 P.M and reforming shortly 

after 7:00 P.M. During the afternoon hours, there were higher mixing depths (maximum 

mixing depth of 1,000 feet), but the bulk of the day had limited mixing. 

Maximum and minimum temperatures for the exceedance day were slightly above 

normal, further illustrating the intensity of the high pressure system that was controlling 

the region’s weather. Light fog was reported at the Valley surface during the early 

morning hours of January 1st, resulting in slightly lower solar radiation intensities. Along 

with lower solar radiation intensities due to the early morning fog, low sun angle, and 

reduced daylight hours, the atmospheric chemistry reactions may have favored the 

secondary particulate forming regime. Light fog would be indicative of moderate water 

content’s that favor the formation of ammonium nitrate. Heavier fogs result in the 

removal or particulates by wet deposition. Chemical composition and meteorological 
63

data showed limited mixing depths and light and disorganized wind flow resulted in 

minimal transport and dispersion of pollutants. 

Meteorological conditions remained extremely stable between January 1st through the 

4th, leading to elevated fine particulate conditions continuing across the San Joaquin 

Valley. Without a significant weather disturbance moving through the area to scour out 

the particulates, PM2.5 concentrations continued to climb and exceeded the NAAQS at 

multiple urban and rural monitoring sites on January 4th. 

With a strong lid in place aloft and maximum high temperatures in the low 60’s, the 

afternoon hours on January 4th, were marked by limited mixing, resulting in increasing 

particulate conditions. Mixing heights at Fresno remained below 500 feet under a 

strong inversion for 20 hours on January 4th. During the afternoon hours, there were 

higher mixing depths (maximum mixing depth of 2,000 feet), but the bulk of the day had 

limited mixing. 

Maximum temperatures for this exceedance day were above normal, further illustrating 

the intensity of the high pressure system that was controlling the region’s weather. 

Light fog reported at the Valley surface during the early morning hours, resulted in 

slightly lower solar radiation intensities. Along with lower solar radiation intensities due 

to the low sun angle and reduced daylight hours, the atmospheric chemistry reactions 

may have favored the secondary particulate forming regime. Meteorological data 

showed limited mixing depths and light and disorganized wind flow resulted in minimal 

transport and dispersion of pollutants. The following day, January 5th, PM2.5

measurements recorded the highest levels during event #5. 

Between January 5th and January 7th, meteorological conditions began to slowly 

improve leading to slightly lower but continued elevated particulate conditions across 

the San Joaquin Valley. Without a significant weather disturbance moving through the 

area to scour out the fine particulates, PM2.5 and PM10 concentrations continued to 

remain high and exceeded the Federal Standard, (Table 20 and Figure 32, 33, 34, and 

35). Humidity measurements of 85 – 100 % on the morning of January 7th, across the 

Valley Floor showed a relatively moist atmosphere, with light fog and haze being 

reported. The cool damp mornings and strong stability favored the formation of nitrates 

and sulfates. Synoptically, the eastern Pacific high began to slowly weaken and move 

into the Intermountain Region on January 7, 2001, slightly weakening the lid over the 

San Joaquin Valley. 

On January 7th a moderately strong lid was in place aloft and maximum high 

temperatures were in the mid 60’s. The afternoon hours were marked by limited mixing, 

resulting in elevated particulate conditions. Mixing heights at Fresno remained below 

500 feet under a strong inversion for 16 hours on January 7th. During the afternoon 

hours, there were higher mixing depths (maximum mixing depth of 2,000 feet), but the 

bulk of the day had limited mixing. Maximum temperatures for this exceedance day 

were above normal, further illustrating the intensity of the high pressure system that was 

controlling the region’s weather. 
64

Light fog reported at the Valley surface during the early morning hours, resulted in 

slightly lower solar radiation intensities. Along with lower solar radiation intensities due 

to the low sun angle and reduced daylight hours, the atmospheric chemistry reactions 

may have favored the secondary particulate forming regime. Chemical composition and 

meteorological data showed limited mixing depths and light and disorganized wind flow 

resulting in minimal transport and dispersion of pollutants. Event # 5 came to an end 

when a significant trough from the eastern Pacific moved through the region on January 

8th, bringing superior dispersion conditions and measurable amounts of precipitation. 

Table 20 PM10 Chemical Composition Data1

 for January 1, 4 and 7, 2001 at selected 

exceedance sites.

Data Collected as part of the Routine and CRPAQS Networks. 

 Concentrations (μg/m3

) Percent of PM10 Mass 

SITE NAME Date PM10 PM2.5 Nitrate/ 

Sulfate 

TC GeologicalPM2.5 Nitrate/ 

Sulfate 

TC Geological

Bakersfield-California 1/1/01 186 133 100 33 71 54 18

Bakersfield-California 1/4/01 190 127 98 30 67 51 16

Bakersfield-California 1/7/01 159 119 81 24 75 51 15

Bakersfield-Golden 1/1/01 205 

Bakersfield-Golden 1/4/01 208 106 38 47 51 18 23

Bakersfield-Golden 1/7/01 174 

Clovis-N. Villa 1/1/01 155 130 77 84 50 

Corcoran-Patterson 1/7/01 165 121 93 73 56 

Fresno-1st Street 1/1/01 193 148 76 51 7 77 40 26 4

Fresno Drummond 1/1/01 186 

Fresno Drummond 1/4/01 159 58 52 29 37 33 18

Hanford-S. Irwin 1/7/01 185 105 57 

Modesto-14th St. 1/7/01 158 136 88 26 86 56 16

Oildale-Manor 1/1/01 158 141 96 89 61 

Oildale-Manor 1/4/01 195 119 28 43 61 14 22

1

 The following fonts were used in the table to distinguish data sources: 

• Regular font for routine data (FRM mass and chemical composition) 

• Underlined font for mass collected using dichotomous sampler 

Italics font for CRPAQS data 
65

Figure 32: Fresno-1st (Coarse) and (PM2.5)

 January 2001 Episode

0

20

40

60

80

100

120

140

160

180

200

12/16/2000

12/18/2000

12/20/2000

12/22/2000

12/24/2000

12/26/2000

12/28/2000

12/30/2000

1/1/2001

1/3/2001

1/5/2001

1/7/2001

1/9/2001

Particulate Matter (micrograms/cubic meter)

FAT-1st Coarse

FAT-1st PM2.5

FAT-1st PM2.5

Figure 33: Bakersfield California (Coarse) and (PM2.5)

 January 2001 Episode

0

20

40

60

80

100

120

140

160

180

200

12/16/2000

12/17/2000

12/18/2000

12/19/2000

12/20/2000

12/21/2000

12/22/2000

12/23/2000

12/24/2000

12/25/2000

12/26/2000

12/27/2000

12/28/2000

12/29/2000

12/30/2000

12/31/2000

1/1/2001

1/2/2001

1/3/2001

1/4/2001

1/5/2001

1/6/2001

1/7/2001

1/8/2001

1/9/2001

Particulate Matter (micrograms/cubic meter)

BFL-CAL Coarse

BFL-CAL PM2.5

BFL-CAL PM2.5

Figure 34: Bakersfield Golden (Coarse), (PM10), and (PM2.5)

and Bakersfield California (PM2.5)

 January 2001 Episode

0

20

40

60

80

100

120

140

160

180

200

220

12/16/2000

12/17/2000

12/18/2000

12/19/2000

12/20/2000

12/21/2000

12/22/2000

12/23/2000

12/24/2000

12/25/2000

12/26/2000

12/27/2000

12/28/2000

12/29/2000

12/30/2000

12/31/2000

1/1/2001

1/2/2001

1/3/2001

1/4/2001

1/5/2001

1/6/2001

1/7/2001

1/8/2001

1/9/2001

Particulate Matter (micrograms/cubic meter)

BFL-Gold PM10

BFL-Gold Coarse

BFL-Gold PM2.5

BFL-CAL PM2.5
66

Figure 35: Hanford, Oildale, and Modesto (PM10)

and Fresno-1st & Bakersfield-California(PM2.5)

January 2001 Episode

0

20

40

60

80

100

120

140

160

180

200

12/16/2000

12/17/2000

12/18/2000

12/19/2000

12/20/2000

12/21/2000

12/22/2000

12/23/2000

12/24/2000

12/25/2000

12/26/2000

12/27/2000

12/28/2000

12/29/2000

12/30/2000

12/31/2000

1/1/2001

1/2/2001

1/3/2001

1/4/2001

1/5/2001

1/6/2001

1/7/2001

1/8/2001

1/9/2001

Particulate Matter (micrograms/cubic meter)

Hnf PM10

Oil PM10

Mod PM10

FAT PM2.5

BFL-CA PM2.5

Event #6: January 12, 2001 through January 24, 2001

Meteorologically, dispersion worsened and particulate formation conditions became 

more favorable from January 14th to January 23rd. As is shown in Figure 31, event #6 

lasted about 12 days and was marked by a gradual buildup of PM2.5 that lead to multiple 

days the Federal PM2.5 Standard was exceeded. After the passage of a trough on 

January 13th, which brought measurable precipitation across the region, moisture 

became available for atmospheric chemistry reactions. Humidity measurements of 90 – 

100 % in the morning across the Valley Floor showed a moist atmosphere, with light fog 

and haze being reported on measured high PM2.5 days. These cool damp mornings 

and strong stability favored the formation of nitrates and sulfates. 

Synoptically during event #6, the eastern Pacific high built over the Pacific Northwest on 

January 15th then expanded southward on the 18th, bringing increasing stability and 

poor dispersion conditions through January 22nd. During this period, the high 

strengthened and intensified on January 18th, further tightening the lid and trapping the 

pollutants within the San Joaquin Valley boundary layer. 

With a strong lid in place aloft and maximum high temperatures in the low 50’s on the 

18th and 19th, and low 60’s on the 20th through the 22nd, the afternoon hours were 

marked by limiting mixing, resulting in increasing particulate potential. Mixing heights at 

Fresno remained below 500 feet under a strong inversion during a majority of the day 

(20 or more hours) on days when the fine particulates climbed above the Standard. 

During the afternoon hours, there were high mixing depths (maximum mixing depth of 

1,500 to 2,500 feet), but the bulk of the day had limited mixing. Maximum and minimum 

temperatures from January 18th through the 21st, were below normal, indicating a 

potential for increase residential wood burning emission activity, and nitrate forming 

reactions. 
67

Light fog and haze were reported at the Valley surface during the early morning hours of 

January 18th through the 22nd, resulting in slightly lower solar radiation intensities. 

Along with lower solar radiation intensities due to the early morning fog and haze, low 

sun angle, and reduced daylight hours, the atmospheric chemistry reactions may have 

favored the secondary particulate forming regime. Light fog would be indicated of 

moderate water content’s that contribute to the formation of ammonium nitrate. 

Meteorological data showed during event #6 limited mixing depths and light and 

disorganized wind flow resulted in minimal transport and dispersion of pollutants. Both 

primary and secondary pollutants from local emissions across the San Joaquin Valley 

accumulated resulting in the exceedance of the 24-hour PM2.5 standard at Fresno-1st on 

January 16th through January 22nd, Visalia-Church, Corcoran-Patterson, Oildale-Manor, 

Bakersfield-Golden, Bakersfield-California, Angiola, and Edison, on January 19th, 

Modesto-14th, Angiola, and Bakersfield-Californian on January 20th, Angiola and 

Bakersfield-California on January 21st, Angiola, Bakersfield-California, CorcoranPatterson, Visalia-Church, and Bakersfield-Golden on January 22nd. A vigorous cold 

front moved over the San Joaquin Valley on January 23rd, bringing and end to event # 6 

that lasted from January 12th through January 24th. This disturbance brought superior 

dispersion conditions across the Valley, allowing pollutants to lower into the 5 to 11 

micrograms per cubic meter range (background levels). 

 

Event #7: January 25th through February 7th, 2003

A series of Pacific storms moved through the region from January 25th through January 

30th, resulting in superior dispersion conditions over the San Joaquin Valley, with PM2.5 

concentrations nearing background levels. As is evident in Figure 31, event #7 lasted 

about 13 days and was marked by a gradual buildup of PM2.5 that lead to multiple days 

the Federal PM2.5 Standard was exceeded. High pressure rebuilt over the region on 

January 31st, leading to building particulate concentrations over the central and 

southern portions of the District. Meteorologically, dispersion rapidly deteriorated and 

particulate formation conditions became more favorable on February 1st. Humidity 

measurements of 90 – 100 % in the morning across the Valley Floor showed a 

continued moist atmosphere, with light fog and haze being reported on measured high 

PM2.5 days. These cool damp mornings and strong stability favored the formation of 

nitrates and sulfates. 

Synoptically during event #7, the eastern Pacific high built along the West Coast on 

February 1st then expanded eastward on the 2nd, bringing strengthening stability and 

poor dispersion conditions through the end of event # 7 on February 6th. During this 

period, the high strengthened and intensified on February 1st, further tightening the lid 

trapping the pollutants within the San Joaquin Valley boundary layer. 

With a strong lid in place aloft and maximum high temperatures in the mid to upper 60’s, 

the afternoon hours were marked by limited mixing, resulting in increasing particulate 

potential. Mixing heights at Fresno remained below 500 feet under a strong inversion 
68

during a majority of the day (18 or more hours) on days when the fine particulates 

climbed above the NAAQS. During the afternoon hours, there were high mixing depths 

(maximum mixing depth of 1,000 to 2,500 feet), but the bulk of the day had limited 

mixing. Maximum and minimum temperatures from February 3rd through the 5th were 

above normal. Light fog and haze were reported at the Valley surface during the early 

morning hours of February 3rd through the 5th, resulting in slightly lower solar radiation 

intensities. 

Along with lower solar radiation intensities due to the early morning fog and haze, the 

atmospheric chemistry reactions may have favored the secondary particulate forming 

regime. Light fog would be indicated of moderate water content’s that contribute to the 

formation of ammonium nitrate. Meteorological data showed limited mixing depths and 

light and disorganized wind flow resulted in minimal transport and dispersion of 

pollutants. An exceedance of the 24-hour PM2.5 standard were measured at Bakersfield 

California on February 3rd through the 5th, Bakersfield Golden on February 3rd and 6th, 

Corcoran on February 3rd, Fresno 1st on February 3rd and 4th, and Clovis on February 

3rd. A cold front traversed over the San Joaquin Valley late in the afternoon on 

February 6th, producing trace amounts of precipitation across different parts of the 

District. This frontal passage brought weak to moderate dispersion conditions across 

the Valley, lowering PM2.5 levels to below the NAAQS. A vigorous upper level system 

moved through the region on the 7th, allowing for pollutants to lower to near background 

levels, bringing an end to event #7. 

 

 

 

 
69

13.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE JANUARY 2001 EPISODE 

The January 2001 particulate episode was characterized by a prolonged period (three 

and a half weeks) of strong stability and light wind flow, which resulted in poor 

atmospheric dispersion conditions across the San Joaquin Valley. Coarse and fine 

particulates accumulated during the period leading to three separate days exceeding 

the National Ambient Air Quality Standards (NAAQS). These exceedances occurred on 

January 1, 4, and 7, 2001 and were measured by Federal Reference Monitoring (FRM) 

sites and MiniVol Samplers, which were part of the California Regional Particulate Air 

Quality Study (CRPAQS). The CRPAQS exceedances are not considered for 

compliance with the Federal Standards, but are used in the discussion to provide 

context to the episode. PM10 and PM2.5 concentrations, chemical composition, and 

meteorological data around January 1, 4, and 7, 2001 were evaluated and analyzed to 

identify the characteristics and uniqueness of the exceedances at the FRM and 

CRPAQS PM10 monitoring sites. 

PM10 and PM2.5 Mass Concentrations

Daily PM2.5 sampling and meteorological conditions suggest that PM10 exceedances 

began on December 31, 2000 and continued through January 7, 2001 at various 

monitoring sites across the San Joaquin Valley. PM2.5 exceedances began a few days 

earlier on December 25 and 26, 2000, when real-time monitors captured readings 

above the Federal Standard of 65 μg/m3. Although PM10 sampling is not conducted 

daily, information from more frequent PM2.5 sampling combined with an assessment of 

changes in the meteorological conditions suggest that the highest PM10 concentrations 

were not captured on the sampled days. The highest PM10 concentrations may have 

occurred on January 1st and January 6th, when Fresno 1st measured a peak PM2.5

concentration of 148 and 129 μg/m3, respectively. PM10 samples were available on 

January 1st. However, no PM10 data was available on January 6th. Particulate material 

concentrations were dominated by fine particulates (PM2.5) of ammonia nitrate and 

sulfate and were most prevalent across the entire San Joaquin Valley. 

Although dispersion was poor, given the length of the episode and the large 

contributions from secondary components, there was an underlying regional component 

to this episode as it progressed through January 7th. Local carbon and geologic 

contributions added to this regional component and influenced site to site concentration 

variations. As the episode continued, PM and precursors became more homogeneous 

across the region. This resulted in PM2.5 concentrations at rural sites lagging those of 

urban sites, and rural concentrations continued to build throughout the episode. 

The FRM sites which exceeded the Federal PM10 standard on January 1, 2001, were 

Clovis-Villa 155 μg/m3, Fresno-1st 193 μg/m3, Fresno-Drummond 186 μg/m3, 

Bakersfield-California 186 μg/m3

, Bakersfield-Golden 205 μg/m3, and Oildale-Manor 158 

μg/m3

 (Figure 36, 37, 38, and 39 and Table 21). PM10 at other Valley locations were 

elevated but not over the Standard. For example, Stockton-Wagner and Visalia-Church 
70

recorded 119 μg/m3 and 143 μg/m3, respectively. The size fraction data on January 1st

across the Valley indicated that most of the PM10 mass was in the PM2.5 fraction, with 

an average PM2.5/PM10 mass ratio greater than 0.7. This was further supported by the 

high PM2.5 concentrations that reached 148 μg/m3 at Fresno-First and 133 μg/m3 at 

Bakersfield-Golden (Figure 36 and 38). 

Stagnation conditions continued through January 4, 2001, when three MiniVol 

Samplers, which were part of the California Regional Particulate Air Quality Study 

(CRPAQS) and one federal reference monitor (FRM) registered exceedances of the 

Federal Standard. The CRPAQS monitors measured at Fresno-Drummond, 159 μg/m3, 

at Bakersfield-Golden, 208 μg/m3, and at Oildale-Manor, 195 μg/m3 (Figure 38 and 39

and Table 21). The FRM PM10 concentration at Bakersfield-California was 190 μg/m3

(Figure 37 and Table 21). The size fraction data across the Valley on January 4th

indicated that most of the PM10 was in the PM2.5 fraction, with an average PM2.5/PM10

mass ratio greater than 0.8. This was further supported by the high PM2.5

concentrations that reached 106 μg/m3

 at Fresno-First and 127 μg/m3 at BakersfieldCalifornia (Figure 36, 38, & 39). 

Poor dispersion conditions continued through January 7, 2001, when five FRM monitors 

registered exceedances of the Federal Standard. The FRM sites, which exceeded the 

Federal PM10 Standard on January 7th, were Bakersfield-California, 159 μg/m3, 

Bakersfield-Golden, 174 μg/m3, Corcoran-Patterson, 165 μg/m3, Hanford-Irwin, 185 

μg/m3

, and Modesto-14th, 158 μg/m3 (Figure 37, 38, and 39 and Table 21). The size 

fraction data across the Valley on January 7, 2001 indicated that most of the PM10 was 

in the PM2.5 fraction, with an average PM2.5/PM10 mass ratio greater than 0.75. This 

was further supported by the high PM2.5 concentrations that reached 101 μg/m3 at 

Fresno-First and 119 μg/m3

 at Bakersfield-California (Figure 36 & 37). Chemical 

composition data was than analyzed to identify the characteristics and uniqueness of 

the exceedances at the FRM and CRPAQS PM10 monitoring sites. 

Figure 36: Fresno-1st (Coarse) and (PM2.5)

 January 2001 Episode

0

20

40

60

80

100

120

140

160

180

200

12/16/2000

12/18/2000

12/20/2000

12/22/2000

12/24/2000

12/26/2000

12/28/2000

12/30/2000

1/1/2001

1/3/2001

1/5/2001

1/7/2001

1/9/2001

Particulate Matter (micrograms/cubic meter)

FAT-1st Coarse

FAT-1st PM2.5

FAT-1st PM2.5
71

Table 21 PM10 Chemical Composition Data1

 for January 1, 4 and 7, 2001 

at selected exceedance sites. 

Data collected as part of the Routine and CRPAQS Networks. 

 Concentrations (μg/m3

) Percent of PM10 Mass 

SITE NAME Date PM10 PM2.5 Nitrate/ 

Sulfate 

TC GeologicalPM2.5 Nitrate/ 

Sulfate 

TC Geological

Bakersfield-California 1/1/01 186 133 100 33 71 54 18

Bakersfield-California 1/4/01 190 127 98 30 67 51 16

Bakersfield-California 1/7/01 159 119 81 24 75 51 15

Bakersfield-Golden 1/1/01 205 

Bakersfield-Golden 1/4/01 208 106 38 47 51 18 23

Bakersfield-Golden 1/7/01 174 

Clovis-N Villa 1/1/01 155 130 77 84 50 

Corcoran-Patterson 1/7/01 165 121 93 73 56 

Fresno-1st Street 1/1/01 193 148 76 51 7 77 40 26 4

Fresno Drummond 1/1/01 186 

Fresno Drummond 1/4/01 159 58 52 29 37 33 18

Hanford-S Irwin 1/7/01 185 105 57 

Modesto-14th St. 1/7/01 158 136 88 26 86 56 16

Oildale-Manor 1/1/01 158 141 96 89 61 

Oildale-Manor 1/4/01 195 119 28 43 61 14 22

 

1

 The following fonts were used in the table to distinguish data sources: 

• Regular font for routine data (FRM mass and chemical composition) 

• Underlined font for mass collected using dichotomous sampler 

• Italics font for CRPAQS data 
72

Figure 37: Bakersfield California (Coarse) and (PM2.5)

 January 2001 Episode

0

20

40

60

80

100

120

140

160

180

200

12/16/2000

12/18/2000

12/20/2000

12/22/2000

12/24/2000

12/26/2000

12/28/2000

12/30/2000

1/1/2001

1/3/2001

1/5/2001

1/7/2001

1/9/2001

Particulate Matter (micrograms/cubic meter)

BFL-CAL Coarse

BFL-CAL PM2.5

BFL-CAL PM2.5

Figure 38: Bakersfield Golden (Coarse), (PM10), and (PM2.5)

and Bakersfield California (PM2.5)

 January 2001 Episode

0

20

40

60

80

100

120

140

160

180

200

220

12/16/2000

12/18/2000

12/20/2000

12/22/2000

12/24/2000

12/26/2000

12/28/2000

12/30/2000

1/1/2001

1/3/2001

1/5/2001

1/7/2001

1/9/2001

Particulate Matter (micrograms/cubic meter)

BFL-Gold PM10

BFL-Gold Coarse

BFL-Gold PM2.5

BFL-CAL PM2.5

Figure 39: Hanford, Oildale, and Modesto (PM10)

and Fresno-1st & Bakersfield-California(PM2.5)

January 2001 Episode

0

20

40

60

80

100

120

140

160

180

200

12/16/2000

12/17/2000

12/18/2000

12/19/2000

12/20/2000

12/21/2000

12/22/2000

12/23/2000

12/24/2000

12/25/2000

12/26/2000

12/27/2000

12/28/2000

12/29/2000

12/30/2000

12/31/2000

1/1/2001

1/2/2001

1/3/2001

1/4/2001

1/5/2001

1/6/2001

1/7/2001

1/8/2001

1/9/2001

Particulate Matter (micrograms/cubic meter)

Hnf PM10

Oil PM10

Mod PM10

FAT PM2.5

BFL-CA PM2.5

Chemical Composition Data

Ammonium nitrate and sulfates increased prior to the exceedance day on January 1, 

2001, whereas total carbon and geological remained stable. Chemical composition 
73

data on January 1, 2001 showed the exceedances were driven by high concentrations 

of ammonium nitrate and sulfate that comprised of 40 to 60 % of the PM10 mass. The 

other major chemical components of the samples were 18 to 26 % total carbon and 4 to 

20 % geological of the PM10 mass (Table 21). At Fresno-First the concentration of 

total carbon (26 % of PM10 mass) and coarse material (4 % of PM10 mass) indicated the 

constituents of the particulate samples on January 1st may predominantly come from 

local sources, such as residential wood burning (Table 21). Since December 2000 No 

Burn Day’s were declared District-wide; consequently, agricultural burning was probably 

not a contributing source of PM on the exceedance days in January. However, burn 

variances and noncompliant agricultural burns may have contributed to a small portion 

of the samples. Overnight minimum temperatures in the low 30’s on January 1, 4 and 

7, 2001 suggested that residential wood burning may have been a significant source of 

PM10. The potential of increased residential wood burning on New Year’s Day, may 

have led to elevated concentrations of total carbon in the samples. 

Local emission source activity and meteorological conditions led to PM10 concentrations 

at Bakersfield-Golden, Oildale-Manor, and Bakersfield-California climbing between 

January 1st and the 4th. Fresno-Drummond PM10 concentrations lowered during this 

same period. Because limited mixing and light winds remained over the area, this 

decrease in PM10 at the site may have been due to lower emission activity there. 

Analysis of ammonium nitrates and sulfates indicates that Oildale-Manor and 

Bakersfield-California registered higher levels compared to non-exceedance locations, 

indicating potentially that local emissions sources may have impacted these sites. The 

exceedances at Fresno-Drummond, Oildale-Manor, and Bakersfield-California and 

Golden on January 4th, were driven by high concentrations of ammonia nitrate and 

sulfate and/or total carbon. 

The concentration at Bakersfield-Golden of ammonium nitrate and sulfate (106 μg/m3

), 

total carbon (18 μg/m3

), and geological (23 μg/m3), was the highest PM10 mass in the 

Valley on January 4th (Table 21). The concentration at Oildale-Manor, BakersfieldCalifornia, Fresno-Drummond, were the second, third and fourth highest PM10 mass in 

the Valley (Table 21). The constituents of the particulate samples on January 4, 2001 

may predominantly come from local sources (residential wood burning and urban / 

agricultural geologic). The lack of precipitation since December 9th may have also 

contributed to the increasing geological component. PM10 and PM2.5 concentrations 

showed that meteorological conditions led to the pollution buildup. Ammonium nitrate 

and sulfates, and geological increased prior to the exceedance on January 4, 2001, 

whereas total carbon remained stable. 

Local emission source activity and meteorology led to PM10 concentrations at HanfordIrwin, Corcoran-Patterson, and Modesto-14th climbing between January 4th and the 7th, 

while PM10 concentrations at Bakersfield-Golden and California lowered. Because 

dispersion conditions remained poor, local emission activity around Bakersfield may 

have decreased prior to January 7th leading to lowering PM10 levels. Both Bakersfield 

California and Golden measured lower ammonium nitrate and sulfate levels compared 

to the 4th, further indicating that potentially local emission source activity may have 
74

decreased on the 7th at these sites. Analysis of ammonium nitrates and sulfates 

indicates that Modesto-14th, Hanford-Irwin, Corcoran-Patterson, registered higher levels 

compared to non-exceedance locations, indicating potentially that local emissions 

sources may have impacted these sites. 

The exceedances at Bakersfield-California, Bakersfield-Golden, Corcoran-Patterson, 

Hanford-Irwin, and Modesto-14th were driven by high concentrations of ammonia nitrate 

and sulfate and/or total carbon, which remained high and elevated between January 4th

and January 7th. Ammonium nitrate and sulfates, and total carbon increased prior to 

the exceedance on January 7, 2001 at Modesto-14th, Corcoran-Patterson, and HanfordIrwin whereas at Bakersfield-Golden and California ammonium nitrate and sulfates, and 

total carbon slightly decreased. The concentration at Hanford-Irwin of ammonium 

nitrate and sulfate (105 μg/m3) and no total carbon and geological measurements, had 

the highest PM10 mass in the Valley on January 7th (Table 21). The concentration at 

Bakersfield-Golden, Corcoran-Patterson, Bakersfield-California, Modesto-14th, had the 

second, third, fourth, and fifth highest PM10 mass in the Valley (Table 21). The 

constituents of the particulate samples on January 7, 2001 may predominantly come 

from local emission sources (residential wood burning and urban / agricultural geologic). 

Dispersion Meteorology

During the January 2001 episode, strong high pressure at the surface and aloft resulted 

in limited afternoon mixing and light offshore wind flow. Cool damp mornings and 

strong stability contributed to the formation of nitrates and sulfates during the episode. 

Meteorologically, dispersion worsened and particulate formation conditions 

strengthened from December 10th to January 7th. After the passage of a weak cold front 

on December 9, 2000, which brought a few hundredths of an inch of precipitation to the 

region, moisture became available for atmospheric chemistry reactions. Humidity 

measurements of 90 – 100 % in the morning across the Valley Floor showed a moist 

atmosphere, with light fog and haze being reported on the exceedance days. These 

cool damp mornings and strong stability favored the formation of nitrates and sulfates. 

Synoptically, the eastern Pacific high built over the San Joaquin Valley on December 

10, 2000 and dominated the region’s weather through January 7, 2001. The high 

strengthened and intensified a few days prior to January 1st, further tightening the lid 

and trapping pollutants within the San Joaquin Valley boundary layer. 

With a strong lid in place aloft and maximum high temperatures in the upper 50’s to low 

60’s, the afternoon hours were marked by limited mixing, resulting in increasing 

particulate potential. Mixing heights at Fresno remained below 500 feet under a strong 

inversion for 19 hours on January 1st, breaking out after 4:00 P.M and reforming shortly 

after 7:00 P.M. During the afternoon hours, there were higher mixing depths (maximum 

mixing depth of 1,000 feet), but the bulk of the day had limited mixing. Maximum and 

minimum temperatures for the exceedance day were slightly above normal, further 

illustrating the intensity of the high pressure system that was controlling the region’s 

weather. Light fog was reported at the Valley surface during the early morning hours of 

January 1st, resulting in slightly lower solar radiation intensities. Along with lower solar 
75

radiation intensities due to the early morning fog, low sun angle, and reduced daylight 

hours, the atmospheric chemistry reactions may have favored the secondary particulate 

forming regime. Light fog would be indicative of moderate water content’s that favor the 

formation of ammonium nitrate. Heavier fogs result in the removal or particulates by 

wet deposition. Chemical composition and meteorological data showed limited mixing 

depths and light and disorganized wind flow resulted in minimal transport and dispersion 

of pollutants. Both primary and secondary pollutants from local emissions around 

Clovis-Villa, Fresno-1st, Fresno-Drummond, Bakersfield-California, Bakersfield-Golden, 

and Oildale-Manor accumulated resulting in the exceedance of the 24-hour PM10

standard on January 1, 2001. 

Meteorological conditions remained extremely stable between January 1st through the 

4th, leading to elevated particulate conditions continuing across the San Joaquin Valley. 

Without a significant weather disturbance moving through the area to scour out the 

particulates, PM10 concentrations continued to climb and exceeded the NAAQS at both 

the FRM and CRPAQS PM10 monitoring sites on the second measured exceedance day 

on January 4th. With a strong lid in place aloft and maximum high temperatures in the 

low 60’s, the afternoon hours on January 4th, were marked by limited mixing, resulting in 

increasing particulate conditions. Mixing heights at Fresno remained below 500 feet 

under a strong inversion for 20 hours on January 4th. During the afternoon hours, there 

were higher mixing depths (maximum mixing depth of 2,000 feet), but the bulk of the 

day had limited mixing. Maximum temperatures for this exceedance day were above 

normal, further illustrating the intensity of the high pressure system that was controlling 

the region’s weather. Light fog reported at the Valley surface during the early morning 

hours, resulted in slightly lower solar radiation intensities. Along with lower solar 

radiation intensities due to the low sun angle and reduced daylight hours, the 

atmospheric chemistry reactions may have favored the secondary particulate forming 

regime. Chemical composition and meteorological data showed limited mixing depths 

and light and disorganized wind flow resulted in minimal transport and dispersion of 

pollutants. Both primary and secondary pollutants from local emissions around FresnoDrummond, Bakersfield-Golden, Oildale-Manor, and Bakersfield-California accumulated 

resulting in the exceedance of the 24-hour PM10 standard on January 4, 2001. The 

January 4th exceedances recorded the highest PM10 levels during the January 2001 

episode. 

Between January 4th and January 7th, meteorological conditions began to slowly 

improve leading to slightly lower but continued elevated particulate conditions across 

the San Joaquin Valley. Without a significant weather disturbance moving through the 

area to scour out the particulates, PM10 concentrations continued to remain high and 

exceeded the Federal Standard at five FRM PM10 monitoring sites on the third 

measured exceedance day on January 7th. The FRM PM10 concentrations at 

Bakersfield-California (159 μg/m3

), Bakersfield-Golden (174 μg/m3), Corcoran-Patterson 

(165 μg/m3

), Hanford-Irwin (185 μg/m3), and Modesto-14th (158 μg/m3) were captured 

on January 7th (Figure 37, 38, and 39). Humidity measurements of 85 – 100 % on the 

morning of January 7th, across the Valley Floor showed a relatively moist atmosphere, 

with light fog and haze being reported. The cool damp mornings and strong stability 
76

favored the formation of nitrates and sulfates. Synoptically, the eastern Pacific high 

began to slowly weaken and move into the Intermountain Region on January 7, 2001, 

slightly weakening the lid over the San Joaquin Valley. 

On January 7th a moderately strong lid was in place aloft and maximum high 

temperatures were in the mid 60’s. The afternoon hours were marked by limited mixing, 

resulting in elevated particulate conditions. Mixing heights at Fresno remained below 

500 feet under a strong inversion for 16 hours on January 7th. During the afternoon 

hours, there were higher mixing depths (maximum mixing depth of 2,000 feet), but the 

bulk of the day had limited mixing. Maximum temperatures for this exceedance day 

were above normal, further illustrating the intensity of the high pressure system that was 

controlling the region’s weather. Light fog reported at the Valley surface during the 

early morning hours, resulted in slightly lower solar radiation intensities. Along with 

lower solar radiation intensities due to the low sun angle and reduced daylight hours, 

the atmospheric chemistry reactions may have favored the secondary particulate 

forming regime. Chemical composition and meteorological data showed limited mixing 

depths and light and disorganized wind flow resulting in minimal transport and 

dispersion of pollutants. Both primary and secondary pollutants from local emissions 

around Bakersfield-California and Golden, Corcoran-Patterson, Hanford-Irwin, and 

Modesto-14th, accumulated resulting in the exceedance of the 24-hour PM10 standard 

on January 7, 2001. 

Conclusion

The 2001 episode was marked by a prolonged period of very strong stability and limited 

mixing, resulting in widespread exceedances of the Federal PM10 Standard across the 

San Joaquin Valley. Although dispersion was poor, given the length of the episode and 

the large contributions from secondary components, there was an underlying regional 

component to this episode as it progressed through January 7th. Local carbon and 

geologic contributions added to this regional component and influenced site to site 

concentration variations. As the episode continued, PM and precursors became more 

homogeneous across the region. This resulted in PM2.5 concentrations at rural sites 

lagging those of urban sites, and rural concentrations continued to increase throughout 

the episode. Historically, the strength of the stability and length of the episode has not 

occurred often. The January 2001 exceedances were unique in that the meteorology 

was so strong and lasted for so long, that PM remained trapped and accumulated in the 

San Joaquin Valley, leading to widespread exceedances of the Federal PM10 Standard 

by the end of the episode. 

 
77

14.0 NOVEMBER 9, 2001 EPISODE SYNOPTIC METEOROLOGICAL 

DISCUSSION 

October 31, 2001- November 10, 2001

The period from October 31 through November 10, 2001 was marked by 11 days of 

strong stability and poor atmospheric dispersion conditions. Strong high pressure both 

at the surface and aloft over the Intermountain Region dominated the period, leading to 

light and disorganized wind flow and limited dispersion. The period began with the 

passage of a vigorous cold front and upper level trough on the 31st, Figure 40. 

Between the 31st and the 10th, strong stability, light and disorganized wind flow, and 

poor dispersion conditions resulted in a PM10 exceedance at Hanford on November 9th. 

Figure 40: T850 Stability and Precipitation vrs. PM10 

at Hanford

Fall and Winter 2001

0

20

40

60

80

100

120

140

160

10/1/200110/8/2001

10/15/200110/22/200110/29/2001

11/5/2001

11/12/200111/19/200111/26/2001

12/3/2001

12/10/200112/17/200112/24/200112/31/2001

PM10 

(micrograms/meter^3) and 

Precipitation (inches)/100

-12

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

14

T850 Stability Parameter 

(C)

PPT

 PM10

T850 Stability

At Hanford, a 24-hour PM10 (Particulate Matter) concentration of 155 μg/m3

 was 

measured. Table 22 outlines federal reference method (FRM) Daily Average 

Particulate Matter measurements for sites across the San Joaquin Valley (SJV). In 

order to understand the variability of these measurements, an in depth examination of 

the synoptic pattern and surface winds and observations, and aircraft soundings leading 

to the episode were analyzed. 
78

TABLE 22: Federal Reference Method (FRM) Daily Average Particulate Matter 

measurements for sites across the SJV for November 9, 2001. 

 FRM FRM FRM 

Site Name 24-Avg. Site Name 24-Avg. Site Name 24-Avg. 

10 2.5 10 2.5 10 2.5 

Bakersfield-Gold 102 Modesto 115 69 Corcoran* 117 46

Bakersfield-CA 38 Merced-M St. 69 34 Stockton** 97 55

Visalia 74 38 Clovis 68 32 Fresno-1st 70 46

Hanford 155 Fresno-Drum 97 Turlock 124

*-Patterson **- Hazleton 

units in μg/m3

 

The meteorological synoptic analysis showed a period of moderately strong 

atmospheric stability from October 31st through the November 10th. Ahead of a 

developing trough over the eastern Pacific, a strong mid and upper level ridge 

developed over the region on the 9th. This ridge strengthened the inversion over the 

San Joaquin Valley trapping particulates within the Valley boundary layer. The morning 

surface charts of the 9th depicted a strong surface high over the Intermountain Region, 

with a surface ridge extending southwestward across central California. The 12Z (4 

a.m.) surface pressure gradient was –4.1 millibars from San Francisco (SFO) to Las 

Vegas (LAS), with isobars (constant surface pressure) orientated southeast to 

northwest. The alignment of the isobars and the –4.1 (SFO-LAS) millibars pressure 

gradient, represents light southeasterly wind flow across the San Joaquin Valley. 

Visibilities throughout the day across the San Joaquin Valley were reporting haze. 

The morning temperature aircraft sounding over Fresno on the 9th showed a strong 

inversion (stable layer) of 16 degrees Fahrenheit from the surface up to 1,000 feet, 

turning isothermal (constant temperature) up through 2,500 feet as is evident in Figure 

41. The morning temperature sounding over Bakersfield showed a very strong 

inversion of 22 degrees Fahrenheit from the surface up to 1,500 feet. The temperature 

sounding on the 9th, is conducive of elevated PM levels due to low mixing depths and 

very strong surface based inversions, which keep pollutants trapped near the surface. 

During the early morning surface observations across the San Joaquin Valley were 

cold. The minimum temperature recorded at Fresno was 49 degrees Fahrenheit. The 

minimum temperatures recorded at Bakersfield and Hanford were 45 degrees 

Fahrenheit. The maximum high temperatures recorded at Fresno and Hanford were 80 

degrees Fahrenheit. The maximum high temperature recorded in Bakersfield was 83 

degrees Fahrenheit. Fresno hourly temperature data shows very limited mixing 

conditions below 1,000 feet for over 18 hours of the day, increasing with afternoon 
79

heating to a maximum mixing depth of 2,000 feet on the 9th. 

Figure 41: Atmospheric Temperature Profile at Fresno on November 9, 2001

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

50 55 60 65 70 75

Temperature (deg F)

Feet (AGL)

Upper level charts indicated a strong high over the Intermountain Region, with a ridge 

extending westward across central California. An upper level trough 750 NM west of 

San Francisco developed and moved closer to the Californian coastline on the 9th. 

Weak pressure gradients over the region remained rather flat through the day, leading 

to light southeasterly wind flow across the San Joaquin Valley. 

Table 23 shows the 24 hour daily average wind speeds at SJVAPCD monitoring, 

ASOS, and CIMIS sites for November 09, 2001. 

SJVAPCD Monitoring Sites ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

Clovis 2.4Fresno 1.5Shafter/USDA 3.0 Famoso 2.3

Fresno SSP 2.2Bakersfield 5.3Firebaugh/Telles 2.3 Westlands 4.1

Corcoran 2.3Hanford 1.2Stratford 1.8 Panoche 2.4

Edison 3.6 Madera 3.9Kettleman 2.8 Arvin-Edison 3.0

Parlier 2.9 Merced 3.6Visalia/Americas 2.1 Lindcove 2.1

Arvin 2.5 Parlier 1.2 Kesterson 2.4

Visalia 1.4 Blackwells Corner 3.5 Lodi West 1.3

 Los Banos 2.5 Modesto 3.4

 Manteca 2.8 Fresno State 2.3

Due to the strong stability lasting for over 10 days, PM10 steadily increase region-wide 

until the sampling day on November 9th. With the mid and upper level stability aloft, 

surface based inversion, and light southeasterly wind flow, this weather pattern was 

conducive of elevated PM10 measurements. As is evident in the PM data, local 

emissions contributed to the exceedance of the National Ambient Air Quality Standards 

(NAAQS) at Hanford on the 9th. The preceding day (the 8th) had higher atmospheric 

stability compared to the 9th, thus PM measurements would have been higher Valley 
80

wide. Due to decreasing stability and increasing dispersion conditions into the 10th, PM 

gradually lowered with the approach of the trough from the eastern Pacific. Widespread 

showers developed across the region on the 10th, bringing an end to the November 9th

PM event. 
81

15.0 INTEGRATED ANALYSIS OF ATMOSPHERIC CHEMISTRY AND 

METEOROLOGY DURING THE NOVEMBER 9, 2001 EPISODE 

The November 9, 2001 particulate episode was characterized by a medium length 

period (a week and a half) of strong stability and light wind flow, which resulted in poor 

atmospheric dispersion conditions across the San Joaquin Valley. Coarse and fine 

particulates accumulated during the period, leading to an exceedance of the 24-hour 

standard at one air monitoring site on November 9, 2001. Concentrations were 

dominated by the coarse particulates (PM10) and were most prevalent across Kern 

County early in the episode then progressively spread northward to Stanislaus and San 

Joaquin Counties toward the end of the episode. The highest PM10 measured was 155 

μg/m3

 (Figure 42) at Hanford-S Irwin. PM10 at other Valley locations were lower 

between 115 to 125 μg/m3

 (Figure 43). During the period, strong high pressure at the 

surface and aloft resulted in limited afternoon mixing and light offshore wind flow. Cool 

damp mornings and strong stability contributed to the formation of nitrates and sulfates 

during the episode. These meteorological conditions led to elevated particulate 

concentrations. Chemical composition and meteorological data around November 9, 

2001, was evaluated and analyzed to identify the characteristics and uniqueness of the 

exceedance at Hanford-S Irwin. 

Figure 42: Hanford (PM10)

and Fresno 1st (PM2.5) 

November 9, 2001 Episode

0

20

40

60

80

100

120

140

160

10/28/2001

10/29/2001

10/30/2001

10/31/2001

11/1/2001

11/2/2001

11/3/2001

11/4/2001

11/5/2001

11/6/2001

11/7/2001

11/8/2001

11/9/2001

11/10/2001

11/11/2001

Particulate Matter (micrograms/cubic meter)

HNF PM10

FAT-1st PM2.5

Although PM10 sampling is not conducted daily, information from more frequent PM2.5

sampling combined with an assessment of changes in the meteorological conditions 

suggest that the highest PM10 concentration was not captured on the sampled day. 

Daily PM2.5 sampling and meteorological conditions suggested that the highest PM10

concentrations may have occurred on November 8th, when Fresno 1st measured a peak 

PM2.5 concentration of 67 μg/m3

. No chemical composition data was available for the 

exceedance day. However, the size fraction data across the Valley indicated that most 

of the PM10 was in the coarse fraction, with an average PM2.5/PM10 ratio of slightly 

greater than 0.40 at nearby Corcoran (Figure 43). 
82

Figure 43: Corcoran (PM2.5 & Coarse)

and Fresno 1st (PM2.5) 

November 09, 2001 Episode

0

20

40

60

80

100

120

140

160

10/28/2001

10/29/2001

10/30/2001

10/31/2001

11/1/2001

11/2/2001

11/3/2001

11/4/2001

11/5/2001

11/6/2001

11/7/2001

11/8/2001

11/9/2001

11/10/2001

11/11/2001

Particulate Matter (micrograms / cubic meter)

COR Coarse

COR PM2.5

FAT-1st PM2.5

The concentration of coarse material may have been highest at the exceedance site as 

indicated by the higher coarse fraction at nearby Corcoran compared to other sites in 

the Valley. In November, many agricultural land preparing and harvesting activities 

were occurring, potentially contributing to the elevated concentrations of geological 

material found in the sample. Agricultural burning probably did not contribute to PM on 

the exceedance day, because November 8 and 9, 2001 were declared No Burn Day’s 

District-wide. However, burn variances and noncompliant agricultural burns may have 

contributed to a small portion of the samples. Overnight minimum temperatures in the 

mid to upper 40’s suggest that residential wood burning may have been a source of 

PM10. The PM2.5 temporal patterns across the Valley showed highest concentrations in 

Bakersfield on November 3 and 4 followed by peak concentrations on November 5 at 

Fresno, then the following day on November 6 at Corcoran. A few days later on 

November 9, the exceedance day at Hanford, peak PM2.5 concentrations were recorded 

in the northern portions of the Valley (Modesto and Stockton). As discussed below, 

both PM10 and PM2.5 concentrations showed that meteorological conditions led to the 

temporal variability and pollution buildup. 

Meteorologically, dispersion worsened and particulate formation conditions 

strengthened from November 2nd to the 7th. However, on November 8th the day before 

the exceedance day at Hanford, dispersion conditions began to slightly improve under 

weaker stability. After the passage of a cold front on October 30th, which brought 0.16 

inches of rainfall to Hanford, moisture was available for atmospheric chemistry 

reactions. Humidity measurements of 80 – 95% in the morning across the Valley Floor 

showed a moist atmosphere, with light fog and haze being reported. The cool damp 

mornings and strong stability favored some formation of nitrate and sulfate particulates. 

The eastern Pacific high built over the San Joaquin Valley on November 2nd, and 

dominated the region’s weather through the 10th. The high began to slowly break down 

on the 8th, but remained strong through the exceedance day (November 9th), trapping 

pollutants within the San Joaquin Valley boundary layer. 
83

With a strong lid in place and maximum high temperatures in the upper 70’s to low 80’s 

on November 9, the afternoon hours were marked by limited mixing, resulting in 

elevated particulate conditions. Mixing heights at Fresno remained below 500 feet 

under a strong inversion for 16 hours on November 9, 2001 breaking out by 10:00 A.M. 

and reforming by 5:00 P.M. During the afternoon hours, there were higher mixing 

depths (maximum mixing depth of 2,000 feet), but the bulk of the day had limited 

mixing. Maximum temperatures were above normal for the November 2001 episode, 

further illustrating the intensity of the high pressure system that was controlling the 

region’s weather. Light fog being reported at the Valley surface on November 9th, 

resulted in slightly lower levels of solar radiation intensity. Along with lower solar 

radiation intensities due to the low sun angle and decreasing daylight hours, 

atmospheric chemistry reactions may have favored the secondary particulate forming 

regime. 

Chemical composition and meteorological data showed that limited mixing depths and 

light and disorganized wind flow resulted in minimal transport and dispersion of 

pollutants. Due to the seasonality of the November 9th exceedance, the PM10

concentration at Hanford had a dominant coarse fraction compared to exceedances, 

which occur later in the winter. The duration, moisture availability, light winds, and 

maximum and minimum temperatures influenced the atmospheric chemistry reactions, 

during the episode. This led to higher coarse fractions and geological components 

compared to episodes later in the year. With stagnant weather conditions, local 

geological sources around Hanford led to the exceedance. Both primary and secondary 

pollutants from local emissions around Hanford-S Irwin accumulated, resulting in the 

exceedance of the 24-hour PM10 standard on November 9, 2001. 
84

16.0 MAY 19 AND 20, 2002 EPISODE SYNOPTIC METEOROLOGICAL 

DISCUSSION 

May 19, 2002

A wind blown dust event occurred across parts of the central and southern San Joaquin 

Valley on May 19, 2002. At McFarland, a 24-hour PM10 (Particulate Matter) 

concentration of 407 μg/m3

 was measured. The McFarland measurement was taken 

8AM to 8AM daily, whereas all other sites are measured midnight to midnight daily. 

Table 24 outlines the Peak and Daily Average Particulate Matter Measurements at 

McFarland, Corcoran, Clovis, and Fresno-1st. In order to understand the variability of 

these measurements, an in depth examination of the synoptic pattern and surface winds 

and observations, aircraft soundings, pibal (balloon) wind measurement, and lower air 

profiler measurements leading to the episode were analyzed. 

TABLE 24: Peak and Daily Average Particulate Matter Measurements for sites across 

the SJV for May 19, 2002

 PM10-Teom PM10-BAM PM2.5-BAM 

Site Name Peak 24-Avg. Peak 24-Avg. Peak 24-Avg. 

McFarland * 407# * * * * 

Corcoran 235 39 674 84 27 13 

Clovis 106 29 * * * * 

Fresno-1st * * * * 8 4 

*-N/A 

units in μg/m3

 

The meteorological synoptic analysis showed that an unseasonably, deep trough 

developed across the eastern Pacific, the morning of the 19th. The trough slowly 

pushed southeastward across California during the afternoon hours. The morning 

surface charts depicted a surface high pressure ridge draped across central California 

to Reno, with a thermal low near Las Vegas. The 12Z surface pressure gradient was 

+9.4 millibars from San Francisco (SFO) to Las Vegas (LAS). A +9.4 millibars pressure 

gradient means onshore or down-valley flow, which results in strong northwesterly 

winds across the San Joaquin Valley. A developing cold front curved southward into 

the eastern Pacific from a moderate low 600 NM west of Eureka. 

The morning temperature soundings over Fresno showed an isothermal (slightly stable) 

layer up to 4,000 feet. At Bakersfield the temperature sounding showed a similar 

isothermal layer up to 3,000 feet. The morning pibal wind measurement from Fresno, 

depicted strong northwesterly flow decoupled from the surface at 37 knots at 1,000 feet 

continuing northwesterly up to 4,000 feet, then shifting westerly up to 5,000 feet, and 

then backing southerly above 5,000 feet. 
85

Upper level charts indicated a strong low 750 NM west of Eureka, with a trough digging 

southward across the eastern Pacific. A strong temperature gradient (packing) at 850 

MB and an intense upper level (300 MB) jet were evident across northwestern California 

on the 12Z analysis maps. The strong temperature gradient aloft, along with an intense 

upper level jet, manifested the surface pressure gradient, resulting in strong to gusty 

winds measured at several hourly-surface meteorological monitoring sites and aloft at 

the Visalia and Lost Hills lower air profiler sites. Table 25 shows the 24 hour daily 

average wind speeds at SJVAPCD air monitoring, ASOS, and CIMIS sites for May 19, 

2002. 

Table 25: 24 hour average wind speeds at SJVAPCD air monitoring, ASOS, and CIMIS 

sites for May 19, 2002. 

SJVAPCD Air Monitor ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

BFLD-Gold 6.8 BFLD Meadows 11.6 FivePoints 11.2 Famoso 5.2

Clovis 7.5 Fresno YI 9.4 Shafter/USDA 6.3 Orange Cove 5.5

Fresno SSP 4.4 Hanford 9 Firebaugh/Telles 5.9 Madera 7.2

Madera Pump 7.5 Madera-AP 10.2 Stratford 7.5 Belridge 6.8

Maricopa 9.2 Merced-AP 8.7 Kettleman 5.2 Merced 4.4

Merced-Coffee 6.2 Modesto-AP 6.7 Visalia/Americas 5.1 Patterson 6.8

Parlier 8.4 Stockton-AP 6.6 Parlier 6.6 Lodi West 2.2

Tracy 9.0 Blackwells Corner 14 Tracy 7.3

Turlock 4.8 Los Banos 8.6 Porterville 4.9

Visalia LAP 6.8 Manteca 4.6 Lost Hills 9.1

 Modesto 6.4 Delano 6.1

 Fresno State 6.7 Westlands 8.1

 Lindcove 3.9 Panoche 6.8

 Kesterson 6.1 Arvin-Edison 6.1

Lower air profiler data from Visalia and Lost Hills showed that with solar insulation 

(heating) the stable isothermal layer present in the morning began to mix out by 11-12 

PDT (18Z-19Z), resulting in strong winds aloft mixing to the surface (Figure 44 and 45). 

The ASOS hourly observations showed northwesterly flow across the San Joaquin 

Valley (down-valley). At Bakersfield, Hanford, and Lemoore, winds were light and 

variable in the morning. As the strong temperature gradient aloft and upper level jet 

began to nose into the region, combined with the strong surface gradient and afternoon 

heating, gusty northwesterly winds developed across the southern San Joaquin Valley 

during the afternoon; with Hanford ASOS reporting winds at 10.4 mph gusting to 20 

mph and Lemoore with winds at 12.7 mph gusting to 23 mph at the 13:55 PDT 

observation. Reduced visibilities of 5 miles were later noted at Hanford around 16:53 

PDT when winds were at 17.3 mph gusting to 26.5 mph. Gusty winds continued into 

the early evening hours and began diminishing after 21:00 PDT. TEOM and BAM 

hourly measurements, at Corcoran immediately showed the diminishing wind speeds, 

by measurements lowering to 20 μg/m3

. 
86

Figure 44: Visalia Lower Air Profiler image for May 19-May 20th, 2002. 

Figure 45: Lost Hill Lower Air Profiler Image for May 19-May 20th, 2002. 
87

The wind event on May 19th, 2002 lasted six to eight hours and resulted in one real time 

monitoring site at McFarland (407 μg/m3) exceeding the Federal 24-hour PM10 standard. 

Since other real time particulate monitoring sites did not register above the Federal 24-

hour PM10 standard, the McFarland measurement signifies local wind and emission 

characteristics. The meteorology did suggest that winds were strong enough to reduce 

visibilities and impact locally other southern San Joaquin Valley communities during the 

peak of the dust event. 

May 20, 2002

Isolated cases of wind blown dust continued across parts of the central and southern 

San Joaquin Valley on the 20th. At Bakersfield Golden, a 24-hour PM10 concentration of 

189 μg/m3

 was measured, whereas at Bakersfield California, the PM10 concentration 

was 65 μg/m3

. Table 26 outlines the Peak and Daily Average Particulate Matter 

Measurements at McFarland, Corcoran, Clovis, Fresno-1st, Bakersfield Golden and 

California. In order to understand the variability of these measurements, an in depth 

examination of the synoptic pattern and surface winds and observations, aircraft 

soundings, pibal balloon measurement, and lower air profiler measurements leading to 

the episode were analyzed. 

TABLE 26: Peak and Daily Average Particulate Matter Measurements for sites across 

the SJV for May 20, 2002. 

 10-Teom 10-BAM 2.5-BAM FRM-10 

Site Name Peak 24-

Avg. 

Peak 24-

Avg.

Peak 24-

Avg. 

24- 

Avg. 

McFarland * 45 * * * * * 

Corcoran 45 12 45 20 22 11 15 

Clovis 21 6 * * * * 8 

Fresno-1st * * * * 10 1 7 

Bak-Gold * * * * * * 189 

Bak-Cal * * * * * * 65 

*-N/A 

units in μg/m3

 

The meteorological synoptic analysis showed the jet stream carving a deep trough 

across central California on the morning of 20th. The morning surface charts depicted a 

strong surface cold front draped southwestward from Tonopah to Big Sur. 

The 12Z surface pressure gradient was +4 millibars from San Francisco (SFO) to Las 

Vegas (LAS). A +4.0 millibars pressure gradient means onshore or down-valley flow. 

The morning temperature sounding over Fresno showed a weak inversion of 2 degrees 

Fahrenheit from the surface up to 500 feet turning unstable through 5,000 feet. At 

Bakersfield the temperature sounding showed an unstable atmosphere. The morning 

pibal measurement from Fresno depicted moderate to strong south-southwesterly flow 

from the surface up to 7,000 feet. 
88

Upper level charts indicated a strong low 250 NM west of Eureka, with a broad trough 

covering the eastern Pacific and California. A strong temperature gradient at 850 MB 

and an intense upper level (300 MB) jet were evident across southern California on the 

12Z analysis maps. Along with daytime heating, scattered thunderstorms developed 

across the southern San Joaquin Valley on the 20th. These thunderstorms coupled with 

an intense upper level jet, brought gusty winds across isolated parts of the Valley. In 

Madera, a small tornado touched down. Table 27 shows the 24 hour daily average 

wind speeds at SJVAPCD air monitoring, ASOS, and CIMIS sites for May 20, 2002. 

Table 27: 24 hour average wind speeds at SJVAPCD air monitoring, ASOS, and CIMIS 

sites for May 20, 2002. 

SJVAPCD Air Monitor ASOS CIMIS 

 WS WS WS WS 

 MPH MPH MPH MPH

BFLD-Gold 7.3 BFLD Meadows 11.2 FivePoints 10.4 Famoso 6.6

Clovis 5.9 Fresno YI 9.3 Shafter/USDA 8.1 Orange Cove 5.2

Fresno SSP 5.5 Hanford 8.9 Firebaugh/Telles 5.7 Madera 6.0

Madera Pump 6.3 Madera-AP 10.2 Stratford 7.9 Belridge 7.7

Maricopa N/A Merced-AP 7.2 Kettleman 4.5 Merced 3.3

Merced-Coffee N/A Modesto-AP 7.2 Visalia/Americas 4.2 Patterson 8.3

Parlier 8.2 Stockton-AP 6.7 Parlier 6.3 Lodi West 2.9

Tracy 7.9 Blackwells Corner 14.2 Tracy 5.8

Turlock 6.5 Los Banos 5.8 Porterville 4.6

Visalia LAP 7.3 Manteca 4.6 Lost Hills 11.2

 Modesto 7.2 Delano N/A

 Fresno State 5.9 Westlands 6.1

 Lindcove 4.6 Panoche 5.8

 Kesterson 7.5 Arvin-Edison 5.3

Bakersfield Golden had higher wind speeds on the 20th compared to the 19th. Figure 

46 shows on the 19th wind characteristics at Bakersfield Golden were marked by a short 

duration event of elevated winds. On the other hand, the 20th was marked by gusty, 

strong winds occurring for most of the day. Peak one hour average wind speed 

measurements at Bakersfield California measured 13.8 mph at 18-19 PDT, with a 11.8 

mph wind speed measured at 17-18 PDT and 19-20 PDT. At the Bakersfield ASOS a 

peak one hour wind speed measurement was 26 mph on the 20th. The wind event on 

the 20th at Bakersfield Golden resulted in a PM10 measurement of 189 μg/m3

; exceeding 

the Federal 24-hour PM10 standard. Since Bakersfield California and other real time 

and Federal Reference Method (FRM) PM monitoring sites did not measure PM above 

the Federal 24-hour PM10 standard, the Bakersfield Golden measurement signifies local 

wind and emission characteristics. 
89

Figure 46: Hourly Average Wind Speeds at Bakersfield Golden 

on May 19 and 20, 2002

0

1

2

3

4

5

6

7

8

0123456789101112131415161718192021222301234567891011121314151617181920212223

May 19 | May 20 

Wind Speed (meters per second)

Wind Speeds at Bakersfield Golden(mps)
90

17.0 REFERENCES 

Hackney, Richard, Larimer Xo, and Ranzieri, Andrew. 1991. Analysis of Upper Air 

Synoptic Parameters Relating to PM10 Episodes in the San Joaquin Valley. California 

Air Resources Board, Sacramento, California. 

http://www.arb.ca.gov/airways/CRPAQS/DA/PMEpisodes.htm. June 24, 2004. PM 

Episodes: Central California Air Quality Studies – CCOS & CRPAQS. California Air 

Resources Board. Sacramento, California.