2008 PM2.5 Plan

Appendix E 

 

District Additions to the Conceptual Model 
2008 PM2.5 Plan

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Appendix E: District Additions to the Conceptual Model 

2008 PM2.5 Plan

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Appendix E District Additions to the 

Conceptual Model

This Appendix provides supplemental discussion of key meteorological, atmospheric 

and emission issues that affect PM2.5 concentrations. EPA recommends starting the 

PM2.5 SIP analysis by establishing a conceptual description expressing the factors 

involved in creating the PM2.5 concentrations found in the nonattainment area. A full 

conceptual description must include: 

• The natural processes of climate and meteorology that cause PM2.5 to 

accumulate and form additional PM2.5 from precursors 

• Atmospheric factors that cause PM2.5 to form or be removed 

• Emissions patterns and events that require special consideration 

The SJV PM2.5 SIP Modeling Protocol discusses many of the technical issues involved 

in the conceptual model of PM2.5 for the Valley and references a conceptual model 

document that was developed from research conducted for the CRPAQS/IMS 95 study 

to explain the dynamics of both PM10 and PM2.5 events. The discussion in this 

Appendix was developed by the District to further explain key factors associated with 

PM2.5 in the San Joaquin Valley that are not fully addressed by the Protocol or the 

referenced conceptual model document. A variety of special subjects have been 

assessed by the District to provide a comprehensive review of technical issues 

associated with PM2.5 concentrations that occur in the Valley. This discussion 

completes the District conceptual description of PM2.5 as a technical foundation for the 

PM2.5 SIP. 

In addition to use in the development of the SIP, elements of the comprehensive 

conceptual description are in use on a daily basis to support air quality forecasting. 

Understanding conditions associated with increases in PM2.5 concentrations provides 

opportunity to alert the public to conditions that may result in poor air quality and 

supports decision making for smoke management and residential burning curtailments 

to prevent air quality deterioration. 

CONTENTS 

1. Climate and Meteorology 

a. Seasonal Differences 

b. Meteorological Influences on PM2.5 Concentrations 

c. Significant Changes in AQI Due to Meteorological Events 

2. Physical and Chemical Processes 

a. PM2.5 Formation and Removal During Fog Episodes 

b. Dry Deposition 

c. Nitrate Events 

3. Emissions Related Issues 

a. Day of Week and Holiday Effect 

b. Wildfires 

c. Agricultural and Prescribed Burn Management 

d. Residential Wood Burning Program 

e. Recent Research
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1. Climate and Meteorology 

The San Joaquin Valley Air Pollution District (District) analyzed meteorological and air 

quality data in support of the 2008 PM2.5 Plan. PM2.5 data have been collected since 

1999. Currently, there are 11 Federal Reference Method (FRM) PM2.5 air-monitoring 

sites in the San Joaquin Valley, with at least one monitor in each county except Madera. 

The cities of Bakersfield and Fresno/Clovis have three sites each due to federal 

requirements related to their large populations. 

1.a Seasonal Differences 

PM2.5 is a seasonal pollutant that tends to peak during the fall and winter (late 

September into February - when nitrate and seasonal carbon sources dominate) 

compared to the spring and summer when PM2.5 tends to be lower (March into early 

September with mostly geologic soil and road material, elemental carbon and some 

secondary carbon aerosol). The 1999 and 2006 PM2.5 measurements from the Fresno 

1st Street monitoring site are illustrated in Figure 1. Historically, PM2.5 levels were 

higher in 1999 compared to 2006. In 1999, the Fresno-1st site experienced several 

days above the NAAQS, whereas in 2006, only one day was above the 65-microgram 

standard. 

Figure 1 PM2.5 Federal Reference Method (FRM) Measurements for 

Fresno-1st for 1999 and 2006 

The 65 microgram 24-hour average NAAQS is shown on the chart 

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Fresno 1st PM2.5 (μg/m3 )

2006

1999

National Ambient Air Quality Standard of 65 μg / m3 
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1.b Meteorological Influences on PM2.5 Concentrations 

The PM2.5 concentration can be influenced by a variety of meteorological factors. The 

District has identified and evaluated several parameters that merit special attention in 

discussion of the conceptual description of PM2.5 in the Valley: rainfall effects, low wind 

speed stagnation, reduced mixing height inversion layers, 850mb stability (a regional 

temperature profile surrogate) and multi-day episode stability. A final review of the 

850mb stability parameter shows that the PM2.5 level associated with this parameter is 

declining as Valley emissions are reduced. 

1.b.1 Rainfall Effects 

In the warm air mass ahead of an approaching cold front, concentrations of fine 

particulate can significantly increase during periods of light rainfall. Particulate 

can increase or stay at high levels until the new air mass arrives following the surface 

frontal passage. During the Bakersfield precipitation event on February 10 and 11, 

2005, PM2.5 nearly doubled during a nine-hour period of light rainfall, as shown in 

Table 1 and Figure 2. As shown in the weather maps provided in Figures 3 and 4, the 

rain was falling during prefrontal conditions. 

Table 1 PM2.5 Data from Bakersfield - California Monitoring Station and 

Weather Data from Bakersfield - Meadows Airport 

Date Hour PM2.5

(μg/m3)

Weather Temp 

(deg F) 

RH 

(%) 

WD WS 

(mph) 

10 Feb 05 20 38 Haze 53 80 CALM CALM 

10 Feb 05 21 37 Fog 52 86 S 5 

10 Feb 05 22 40 Fog 53 86 ESE 3 

10 Feb 05 23 44 Light Rain/Fog 51 89 NNW 3 

11 Feb 05 0 51 Fog 52 89 NNE 3 

11 Feb 05 1 54 Light Rain 51 92 W 3 

11 Feb 05 2 65 Light Rain 52 86 N 7 

11 Feb 05 3 70 Light Rain 53 83 N 3 

11 Feb 05 4 58 Light Rain 52 89 N 5 

11 Feb 05 5 54 Light Rain 50 94 WNW 7 

11 Feb 05 6 52 Light Rain/Fog 50 94 WNW 6 

11 Feb 05 7 75 Light Rain/Fog 50 94 WNW 5 

11 Feb 05 8 75 Light Rain/Haze 53 83 N 8 

11 Feb 05 9 70 Haze 54 80 N 9 

11 Feb 05 10 68 Haze 58 75 N 12 

11 Feb 05 11 51 Haze 59 67 NNW 9 

Temp = Temperature, RH = Relative Humidity, WD = Wind Direction, WS = Wind Speed 
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Figure 2 PM2.5 During Rain Event in Bakersfield, February 10th and 11th, 2005 

Darker shaded bars indicate precipitation reported during the hour 

PM2.5 DURING RAIN EVENT

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80

20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11

HOUR

PM2.5 (μg/m3)

Figure 3 Surface weather map for 4 PM PST (Hour 16) on February 10, 2005 

Shading indicates light precipitation 
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Figure 4 Surface weather map for 4 AM PST (Hour 4) on February 11, 2005 

 Purple shading indicates light precipitation 

1.b.2 Wind Speed 

Stagnant conditions can lead to a buildup of emissions near the emission source 

area, producing a gradual increase in pollutant concentrations. The longer the 

stagnant period lasts, the higher the particulate concentrations can rise. An 

increase in winds causes an end to the stagnant conditions and can transport the 

polluted air mass downwind, resulting in a lowering of particulate concentrations in the 

emission source area. Figure 5 illustrates that during stagnant (low wind speed 

conditions), PM2.5 levels can be high. During the early morning hours of February 14, 

2006, winds were less than 3 MPH, resulting in poor dispersion conditions and elevated 

PM2.5 readings (generally above 50 μg/m3

). After 10 PST, the winds gradually 

increased and the PM2.5 levels decreased. This plot shows the response of 

particulates to changes in wind speed. 

1.b.3 Mixing Height and Inversion Layers 

Vertical mixing height is the elevation in the atmosphere into which emissions will freely 

mix and disperse. In Figure 6, the base of the inversion layer is equivalent to the mixing 

height. Vertical mixing is reduced when an inversion layer or temperature inversion is 

present. A temperature inversion is a deviation from the typical temperature profile in a 
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well-mixed atmosphere, where temperature decreases with height. Temperature 

decreases with height in the layer defined by the mixing depth in Figure 6. 

Figure 5 Plot of real-time PM2.5 measurements (μg/m3

) and 

wind speed (mph) at Fresno-1st on February 14, 2006 

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Real-time (BAM) PM2.5 μg / m3

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Real-time (BAM) PM2.5

Wind Speed (MPH)

Figure 6 Temperature sounding plot with an inversion layer present
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Typically, an inversion is a layer of air warmer than the layer below it as shown in Figure 

6. The warmer layer aloft prevents the usual rising airflow that disperses emissions 

from the ground because the cooler air below the inversion layer does not rise through 

the warmer air above. As shown in Figure 6, air contaminants accumulate near the 

ground as emissions continue to be released into the cooler layer near the surface. 

Temperature inversions play a crucial role in trapping PM2.5 near the San Joaquin 

Valley’s surface. An example of a temperature inversion is from an aircraft 

measurement from Fresno on October 24, 2007 in Figure 7. The inversion strength was 

19 degrees Fahrenheit from the surface up to 1,000 feet. Figure 7 is an example of a 

surface based inversion. A surface based inversion is a temperature inversion with the 

base at the earth’s surface. 

Figure 7 Aircraft temperature measurement from Fresno on October 24, 2007 

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50 55 60 65 70 75 80

Temperature (Degrees Fahrenheit)

Feet (Above Ground Level) 

Inversion is from the surface up to 1,000 feet.

Strength is 19 degrees Fahrenheit.

Statistical analysis has shown that the 850 MB Stability Parameter correlates with PM 

levels and is an indicator of how much dispersion is occurring in the atmosphere. The 

District defines the 850 MB Stability Parameter as the inversion located between 

approximately 5,000 feet (850 MB) and the surface. The 850 MB stability parameter is 

calculated by taking the 12 Z (4:00 Pacific Local Time) 850 MB temperature at Oakland 

and subtracting the minimum surface temperature at Fresno (T850 MB (Oakland) – 

TMIN (Fresno)). A positive stability parameter indicates warmer temperatures with 

height. A negative stability parameter shows cooler temperatures with height. Figure 8 

shows the Fresno-1st 850 MB stability parameter and PM2.5 plot for 2006. PM2.5 is 

depicted as the solid line, and the stability parameter is shown as the dots at 

temperature of 75. Figure 8 shows that during periods of strong stability, PM2.5 tends 

to climb or remain high. A stable atmosphere would be characterized as any positive 

temperature difference in degrees Fahrenheit. The graph also shows the seasonality of 

PM2.5 in that during the summer when stability is strong, PM2.5 is low. PM2.5 
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formation in the atmosphere tends to be more pronounced during the fall and winter 

months compared to the summer. 

Figure 8 Graph showing the Fresno-1st stability parameter and 

Federal Reference Method (FRM) PM2.5 measurements for 2006 

PM2.5 tends to follow the atmospheric dispersion conditions of the San Joaquin Valley. 

During multiple days of a stable atmosphere, local emissions accumulate under poor 

dispersion conditions resulting in accumulating PM2.5 concentrations. Preliminary 

analysis of comparable strength meteorology in 1999 / 2000 and 2005 / 2006 shows 

that the stronger stability meteorology in recent years is not resulting in the same high 

levels of PM as in the past. This may be an indicator that District control measures are 

contributing to the lower levels of PM in the present. 

Table 2 illustrates that PM2.5 fluctuates according to local emissions and meteorology. 

Strong high pressure built over the region on February 5, 2006 trapping PM2.5 near the 

surface. PM2.5 levels continued to climb under stagnant dispersion conditions until on 

February 13 when the Fresno-1st monitor recorded 71 μg/m3

. 
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Table 2: Federal Reference Method (FRM) PM2.5 (μg/m3) Measurements 

at Fresno-1st Monitoring Site 

Date PM2.5 Measurement

2/4/2006 22

2/5/2006 26

2/6/2006 30

2/7/2006 26

2/8/2006 38

2/9/2006 49

2/10/2006 56

2/11/2006 51

2/12/2006 56

2/13/2006 71

2/14/2006 34

2/15/2006 8

PM2.5 and the 850 MB stability parameter were analyzed from 1999 to 2006 during the 

peak season (November 10 through January 15), Figure 9. This analysis was 

conducted to determine if meteorology or District control measures played a role in the 

current years’ (2005 / 2006) PM2.5 levels. In Figure 9, stability increases from left to 

right, whereas PM increases from bottom to the top. In 1999 and 2000, high stability 

days tended to result in elevated PM2.5 levels well above the NAAQS. Recently in 

2005 and 2006, high stability days resulted in lower levels of PM2.5, typically near or 

below the NAAQS. 
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Figure 9 PM2.5 measurements at Fresno-1st related to Oakland 850 MB –Fresno 

temperature stability parameter. High stability days (850 MB stability parameter 

is greater than 0) resulted in high PM2.5 measurements from 1999 through 2001. 

Comparable high stability days since 2003 have not caused high PM2.5 

measurements. 

1.c Significant Changes in AQI Due to Meteorological Events 

Very large changes in the Air Quality Index (AQI) and PM2.5 concentration can occur 

over a short time period during the PM2.5 season. Figure 10 presents a plot of the AQI 

in Tulare County (Visalia) during the 2005-2006 PM2.5 season. An AQI improvement of 

134 occurred from the 25th to the 26th of November 2005. Stagnant conditions were in 

place over the San Joaquin Valley Air Basin on the morning of November 25, 2005 as 

shown in the Figure 11 weather maps. A vigorous weather front passed over the region 

during the afternoon and evening of November 25. PM2.5 concentration decreased 

rapidly under the influence of a new clean air mass and moderate NW winds. The 

tightly packed isobars on the weather maps are an indication of a strong pressure 

gradient and the wind flags indicate moderate winds. The Visalia PM2.5 concentration 

decreased rapidly around midnight as shown in Table 3 and Figure 12. Figure 12 

displays how the decrease in the PM2.5 concentration progresses from northwest to 

southeast in the San Joaquin Valley Air Basin, coinciding with the front's progression 

from northwest to southeast across the Valley. The decrease in PM2.5 first occurred in 

Stockton in the north valley. South Valley stations at Corcoran and Visalia recorded the 

decrease in PM2.5 later in the day. 
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Figure 10 AQI in Tulare County (Visalia) 

During the 2005-2006 PM 2.5 Season 

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100

150

200

11/1/200511/8/2005

11/15/200511/22/200511/29/2005

12/6/2005

12/13/200512/20/200512/27/2005

1/3/2006

1/10/20061/17/20061/24/20061/31/2006

2/7/2006

2/14/20062/21/20062/28/2006

DATE

AQI

AQI decrease of 134 due to frontal passage, air mass change & moderate NW winds

 Table 3 PM2.5 concentration in Visalia, November 25th and 26th, 2005 

Date / Hour Visalia PM2.5 

(μg/m3) 

Visalia Municipal Airport 

Wind Direction and Speed (mph) 

25 November 2005 Hour 13 89 ESE 9 

25 November 2005 Hour 14 91 NE 6 

25 November 2005 Hour 15 85 N 5 

25 November 2005 Hour 16 68 N 5 

25 November 2005 Hour 17 72 N 3 

25 November 2005 Hour 18 78 WNW 10 

25 November 2005 Hour 19 88 WNW 12 

25 November 2005 Hour 20 79 NW 10 

25 November 2005 Hour 21 76 NNE 5 

25 November 2005 Hour 22 63 NNW 9 

25 November 2005 Hour 23 39 NNW 10 

26 November 2005 Hour 0 27 NW 10 

26 November 2005 Hour 1 22 NNW 14 

26 November 2005 Hour 2 11 NNW 7 

26 November 2005 Hour 3 6 NW 8 

26 November 2005 Hour 4 2 WNW 5 

26 November 2005 Hour 5 3 NW 7 
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Figure 11 Surface Weather Maps for 4 AM (12Z), November 25th and 26th, 

2005
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Figure 12 PM2.5 Concentration, November 25th and 26th, 2005 

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STOCKTON

BAKERSFIELD

CORCORAN

VISALIA

CLOVIS

FRESNO
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The southern San Joaquin Valley Air Basin experiences significant decreases or 

increases in AQI followed by relatively constant AQI, as may be seen in Kern County 

data presented in Figure 13. The significant decreases can occur in one day. The 

significant increases tend to take several days to build. The northern San Joaquin 

Valley Air Basin experiences more frequent significant increases and decreases in AQI, 

as may be seen in San Joaquin County and Stanislaus County data presented in Figure 

14 and 15. This is likely due to more frequent frontal passages in the northern San 

Joaquin Valley. In the south, a front that may have influenced the north valley may 

dissipate before reaching the southern San Joaquin Valley, resulting more potential for 

longer periods of stagnation than the north Valley. 

Figure 13 AQI in Kern County (Bakersfield) 

During the 2005-2006 PM 2.5 Season 

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11/1/200511/8/2005

11/15/200511/22/200511/29/2005

12/6/2005

12/13/200512/20/200512/27/2005

1/3/2006

1/10/20061/17/20061/24/20061/31/2006

2/7/2006

2/14/20062/21/20062/28/2006

DATE

AQI

AQI decrease of 94 due to pre-frontal conditions & strong SE winds
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Figure 14 AQI in San Joaquin County (Stockton) 

During the 2005-2006 PM 2.5 Season

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200

11/1/200511/8/2005

11/15/200511/22/200511/29/2005

12/6/2005

12/13/200512/20/200512/27/2005

1/3/2006

1/10/20061/17/20061/24/20061/31/2006

2/7/2006

2/14/20062/21/20062/28/2006

DATE

AQI

Figure 15 AQI in Stanislaus County (Modesto) 

During the 2005-2006 PM 2.5 Season

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150

200

11/1/200511/8/2005

11/15/200511/22/200511/29/2005

12/6/2005

12/13/200512/20/200512/27/2005

1/3/2006

1/10/20061/17/20061/24/20061/31/2006

2/7/2006

2/14/20062/21/20062/28/2006

DATE

AQI

AQI decrease of 101 due to pre-frontal conditions & strong SE winds
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The most significant decreases in PM2.5 tend to occur when there is an air mass 

change associated with a vigorous frontal passage, typically in fall and winter. It 

appears that the mechanism that lowers the PM2.5 concentration is an air mass change 

and not necessarily by collision - coalescence or 'washout' by falling precipitation. As 

discussed in section 1.b.1 of this Appendix, precipitation can sometimes be associated 

with a significant increase in PM2.5 in the San Joaquin Valley. 

There is scientific support for why collision - coalescence may not be the primary 

mechanism for PM2.5 removal. Collisions between water drops and particles are likely 

a function of gravitational, electrical and aerodynamic forces. As a water drop falls in a 

cloud, it will collide with a very small amount of the particles in its path because 

aerodynamic forces tend to move the particles to the side, out of the drop path. If a 

collision of a water drop and PM2.5 particle occurs, this collision does not guarantee 

coalescence and removal of PM2.5. Rogers (1976) states that studies of aerodynamic 

forces in cloud drops have indicated that very small drops cannot readily be made to 

collide. A small drop falling though a cloud of smaller droplets will collide with a very 

small percentage of drops in the drop path as long as the drop radius is less than 18μm 

(Rogers, 1976). The drops may bounce away from each other; they may coalesce and 

remain intact; and they may coalesce briefly and separate. The process is a function of 

water drop size, particle size, drop trajectories and external forces such as electrical 

charges. Therefore, some precipitation processes may not be sufficient to cause droplet 

capture of PM2.5. Air mass exchange appears to be a more effective PM2.5 removal 

process in the Valley. 

Very large AQI and PM2.5 concentration changes can also occur over a short time 

period during relatively fair weather conditions. An AQI improvement of 89 occurred 

from the 15th to the 16th of November 2007 in the north San Joaquin Valley (112 AQI to 

23 AQI in Modesto) during a period of benign meteorological conditions. An AQI 

improvement of 67 occurred in the south San Joaquin Valley (in Visalia from the 16th to 

the 17th of November 2007). The improvement in AQI was likely due to a subtle 

improvement in dispersion conditions, as meteorological instrumentation recorded small 

changes in parameters. Water vapor satellite images indicated a very weak frontal 

passage occurred in zonal flow, resulting in a partial air mass change. The weak front 

was not evident in the surface weather maps. Physical evidence of the frontal passage 

was registered by an increase in the mixing height, a slight increase in surface winds 

and some high and middle clouds. 

1.c.1 References for Significant Changes in AQI Due to Meteorological Events 

• Mesowest website, http://www.met.utah.edu/mesowest/, Archived weather data 

• Rogers, R. R. (1976), A short course in cloud physics, Pergamon Press 

• UNISYS website, http://weather.unisys.com/, Archived weather maps 
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2. Physical and Chemical Processes 

A variety of physical and chemical processes occur in the atmosphere causing PM2.5 to 

form or to be removed, including fog processing, dry deposition and nitrate production. 

Due to historical episodes fog relationship with particulate has been a subject for 

extensive research. California and the San Joaquin Valley experience high levels of 

nitrate particulate that is not a common pattern for other states. The District is 

evaluating nitrate patterns and has identified information of potential importance for 

attainment planning. Carbon is another area requiring major continuing evaluation. 

Elemental and organic carbon are directly emitted, transformed by chemical processes 

from gases to particles, and are sampled in error as particles by absorption of gases on 

sampler filters (sampler artifact) making it difficult to assess. 

Some factors have an effect but are considered as either less critical or not well 

quantified for current use in our decision-making processes. Temperature and relative 

humidity affect properties of aerosols but have not been determined to be critical 

factors. Dry deposition of particulates has not been extensively studied. Particle 

deposition and aerosol re-evaporation flux are not well quantified and are not included 

in District forecasting and modeling. 

2.a PM2.5 Formation and Removal During Fog Episodes 

The District has examined fog effects on particulate pollution to understand the role of 

fog on Valley air quality. Scientific investigation of the effect of fog is warranted based 

on historical events that demonstrate a lethal potential for severe fog episodes 

accompanied by PM2.5 high levels of carbon emissions. Probably the most famous 

episode occurred in London, England from 5 to 9 December 1952, resulting in the 

deaths of thousands. In the London episode, a cold fog developed resulting in the 

burning of more coal than usual. The polluted air mass was trapped under a strong 

temperature inversion layer, which acted like a lid on the air mass with coal soot 

accumulating to lethal levels. Research was conducted in the San Joaquin Valley to 

explore the consequences of severe fog events in the Valley and determine the severity 

of PM2.5 during fog episodes. 

While much has been learned about fog interactions with key inorganic aerosol species 

(e.g., ammonium sulfate and nitrate), it is only in recent years that investigators have 

begun examining interactions between fogs and carbonaceous aerosols and volatile 

organic compounds. The District has examined fog effects on particulate pollution 

through integrated projects included in the major field study the California Regional 

Particulate (PM10/PM2.5) Air Quality Study (CRPAQS) and through special technical 

studies and dedicated fog research projects. Special projects related to fog were 

conducted in a preliminary field assessment for CRPAQS known as the 1995 Integrated 

Monitoring Study (IMS95) and in subsequent projects to assess fog and fog chemistry. 

These studies examined PM2.5 formation and removal during fog episodes. Significant 
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CRPAQS findings concerning fog are presented in the following pages and include 

excerpts from a CRPAQS report written by J. Collett, Jr. et al (2005). 

2.a.1 Fog Climatology 

During the winter, persistent high pressure over the Great Basin creates a strong 

subsidence inversion over California’s Central Valley, with a base typically a few 

hundred meters off the Valley floor and below the surrounding mountain ridges (Holets 

et al., 1981). With the help of the mountains, this strong inversion forms a lid over the air 

basin, trapping cool, moist air within the valley. Subsiding air results in clear skies, 

providing excellent conditions for strong radiative cooling at night and, if sufficient 

moisture is present, the formation of dense, widespread radiation fogs. 

2.a.2 Fog Interactions with Gases and Particles, Particle Formation and Removal 

Fog is defined as water droplets suspended near the earth's surface that reduce 

visibility below 1 km (Huschke, 1959). Fog differs from a cloud in that the base of fog is 

at the earth's surface, while clouds are above the surface. Fog water droplets are tiny, 

ranging in size from several micrometers to several tens of micrometers. This 

compares to the diameter of one PM2.5 particle for the smaller dimension, ranging to 

several times the diameter of a PM10 particle for the largest fog particles. These drops 

form by water vapor condensation onto aerosol nuclei known as cloud condensation 

nuclei (CCN). Water-soluble constituents in the CCN determine the initial composition 

of the fog. 

Particles that become fog droplets by absorbing water vapor are subsequently removed 

by gravitational settling. Fog episodes cause a net removal of PM by wet deposition. 

However, fog also provides an aqueous medium for formation of sulfate particulate. 

Fogs have two important, competing effects on aerosol populations: 

• New aerosol mass formation through gas scavenging and chemical reaction in 

the droplets leading to non-volatile species (e.g. conversion of SO2 to sulfate) 

that remain in the particle phase after droplet evaporation 

• Aerosol scavenging followed by deposition through droplet settling and/or 

impaction 

The relative importance of these two processes depends on the environment in which 

the fog forms and can include meteorological conditions, number and composition of 

aerosol particles, and gas phase chemical composition. The net effect of a fog episode 

depends on the pre-fog concentrations of particulate precursors and availability of 

oxidants that are involved in the chemistry of particle formation, and the duration of the 

fog event. The net effect of a fog on atmospheric aerosol concentrations may change 

during a fog event; oxidation could be more important at the beginning of the fog event 
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when reactant concentrations are higher, while deposition rates may increase over time 

with the growth of fog droplets. 

Previous studies of sulfur oxidation in San Joaquin Valley fogs have shown that 

dissolved sulfur dioxide can react rapidly, either being oxidized to sulfate or reacting 

with carbonyl compounds to form hydroxyalkylsulfonic acids. Sulfate is both produced 

and removed by fog. The end result of a fog event may be either and increase or 

decrease in the monitored levels of PM2.5 sulfate. Sulfate particulate is removed by 

wet deposition during fog; however, fog droplets provide the aqueous medium for the 

production of sulfates, and the production of additional sulfate particulate by chemical 

reactions within the fog may over-balance the wet removal and actually increase the 

amount of sulfate PM2.5 in the atmosphere. Whether sulfate formation in fog exceeds 

sulfate removal by fog settling will vary from site to site, depending on the duration of 

event and concentrations of available reactants to form additional sulfate particulate 

matter. Variations in fog drop composition with size are known to influence the rates of 

chemical reactions as well (e.g., Reilly et al., 2001). 

Fog can process particles and gases as shown in Figure 16. Growth of droplets formed 

with aerosol CCN lead to removal of particles and gases from the air mass, which can 

lead to a decrease in PM2.5 concentration. 

Figure 16 Fog Processing of Particles and Gases 

Fog droplet composition is also influenced by dissolution of soluble gases and aqueous 

phase chemical reactions as shown in Figure 17. The competition between removal 

and production processes leads to either an increase or decrease in PM2.5 

concentration. These processes vary with fog characteristics. 
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Figure 17 Fog Droplet Particle and Gas Scavenging Processes 

Deposition due to fog drop sedimentation or impaction has been known to be an 

important removal process for atmospheric pollutants for a long time (Waldman, 1986). 

Some studies have tried to assess the deposition fluxes by fog in the San Joaquin 

Valley by modeling (Lillis et al., 1999) or by measurements (Collett et al., 2001). 

However, there are relatively few measurements for atmospheric removal of fog solutes 

by drop deposition or how drop-size dependent fog composition affects removal rates 

for various chemical species. 

While much is now known about the inorganic composition of San Joaquin Valley fog 

water, little is known about the scavenging and removal of carbonaceous aerosol by 

these fogs. Despite measurement of high total organic carbon (TOC) concentrations 

(e.g., during IMS95), the composition of the organic species making up this TOC is 

largely unknown in the San Joaquin Valley and elsewhere. Although the expected net 

effect of San Joaquin Valley fog episodes is reduced atmospheric loadings of 

carbonaceous aerosol, the magnitude of this removal was unknown prior to CRPAQS. 

Secondary Organic Aerosol (SOA) formation (formation of organic carbon particulate 

from photochemical oxidation reactions of precursor gases) is reduced by decreased 

photochemical activity in circumstances where sunlight is reduced by fog or cloud 

cover. Clouds and fog reduce the photochemical production of secondary organic 

aerosols by a factor of two or three from the maximum clear sky conditions because of 
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reduced availability of photons. The thermodynamic interactions between fog droplets 

and SOA are unknown. 

2.a.3 Fog Research Results 

In order to improve the understanding of the role fogs play in influencing aerosol 

concentrations in California’s Central Valley, Colorado State University (Collett et al, 

2005) made measurements of the chemical and physical properties of fogs at several 

valley locations during the CRPAQS winter intensive. These observations subsequently 

formed the basis for a data analysis effort aimed at better characterizing fog 

composition and interactions between CRPAQS fogs and pollution levels, especially 

fine particles. The fog measurement campaign and subsequent data analysis phase 

produced a number of significant findings. These include the following major 

observations and conclusions made by J.Collett, Jr. et al (2005): 

• Several fog episodes were characterized during CRPAQS. Most of the fog 

episodes were relatively shallow and featured very large droplets. Rarely did the 

top of the fog layer reach even 100 meters, in contrast to much deeper fogs 

observed during IMS95 and other studies in the San Joaquin Valley. The very 

large drops formed during CRPAQS probably result from strong radiative cooling 

directly from the drops themselves, through the shallow fog layer, driving rapid 

condensational growth. These large drops settle from the fog relatively quickly, 

enhancing the ability of the fogs to rapidly remove scavenged airborne particulate 

matter. 

• The chemical composition of the fogs during CRPAQS was dominated by 

nitrogen species, with important contributions also from organic compounds and 

sulfate. Ammonium and nitrate were the most abundant individual compounds; 

nitrite and sulfate were also found to be present at significant concentrations, as 

were several organic compounds, including formate, acetate, and formaldehyde. 

Abundant gas phase ammonia absorbed by fog drops helps keep the fog pH 

relatively high, with pH values typically well above 6. Material present in fog 

drops is derived from a combination of aerosol particle scavenging, gas 

absorption, and aqueous phase reactions. 

• Comparison of Bakersfield fog composition measurements in CRPAQS and other 

recent San Joaquin Valley fog studies with measurements made in the 1980's 

reveals a statistically significant decrease in fog concentrations of sulfate and an 

increase in fog pH. These changes are consistent with intervening declines in 

SO2 emissions in the southern San Joaquin Valley, which would translate into 

less production of sulfate and greater availability of ammonia to raise fog pH. 

• Comparison of fog composition during CRPAQS at Bakersfield, Angiola, and 

Helm reveals that Angiola and Helm, both rural sites, have generally similar 
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compositions. Urban Bakersfield fog contained greater concentrations of sulfate 

and nitrite.

• The high pH droplets present in CRPAQS fogs make them effective atmospheric 

reactors for dissolved sulfur dioxide. Both oxidation to sulfate and reaction with 

dissolved formaldehyde to produce hydroxymethanesulfonic acid (HMS) are 

important reaction pathways. Numerical simulations using a single drop fog 

chemistry model reveal the importance of considering effects of mass transport 

limitations on aqueous sulfur chemistry in the large fog drops observed in 

CRPAQS fogs. 

• Preferential enrichment of major ion species in small fog drops reduces most 

species’ deposition velocities below the deposition velocity for fog water. Nitrite, 

which was often enriched in large fog drops, exhibited deposition velocities 

higher than fog water. Accounting for drop size-dependent species 

concentrations is essential to accurately modeling rates of pollutant deposition in 

San Joaquin Valley fogs. 

• Previous studies have documented the important role San Joaquin Valley fogs 

play in cleansing the atmosphere via particle scavenging followed by drop 

deposition. The importance of this mechanism was again observed during 

CRPAQS, with fog deposition fluxes capable of reducing boundary layer fine 

particle concentrations of major species (e.g., nitrate and ammonium) at a rate 

on the order of 1 μg/m3

 per hour. The fogs are also effective at scavenging and 

removing sulfate, but this removal is often offset by similar rates of sulfate 

production via aqueous phase oxidation of sulfur dioxide. Assuming a 100 m 

deep fog, Collett et al found that concentrations typically decrease as follows: 

o SULFATE ~ 0.5 μg/m3 per hour 

o NITRATE ~ 1 μg/m3

 per hour 

o AMMONIUM ~ 1 μg/m3 per hour 

o TOTAL ORGANIC CARBON ~ 0.7 μgC/m3

 per hour 

• Results obtained during CRPAQS also indicate the important role that San 

Joaquin Valley fogs play in scavenging and deposition of organic carbon. San 

Joaquin Valley fogs contain a rich mix of organic compounds, with major 

constituents including formaldehyde, formate, and acetate. Many larger organic 

molecules are also observed, including both polar and non-polar compounds. 

Approximately 25% of the fog organic carbon, on average, is present as 

undissolved, suspended material in the droplets. As much as half of the fog 

organic matter may be comprised of high molecular weight compounds, with 

molecular masses exceeding several hundred Daltons. Future studies are 

needed to better characterize this high molecular weight material and determine 

whether it comes mainly from aerosol scavenging or is produced by aqueous 

phase reactions of lower molecular weight precursors.
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• The wide variety of organic compounds observed in the fogs indicates that 

carbonaceous aerosol particles from many source types undergo active fog 

processing. Observations of fog scavenging of organic and elemental carbon 

also indicate the importance of fog processing of carbonaceous aerosol species. 

Organic carbon (OC) was observed to be more actively processed by CRPAQS 

fogs than elemental carbon. Differences were also observed between the 

efficiency of CRPAQS fogs in scavenging different types of fine particle organic 

carbon. Wood smoke particles were scavenged more efficiently and particles 

containing polycyclic aromatic hydrocarbons (PAH) less efficiently than fine 

particle OC as a whole. CRPAQS fogs were also observed to efficiently 

scavenge and remove the pesticide Diazinon. Together, these observations 

suggest that fogs may be more effective cleansing the boundary layer of OC from 

some source types than others, a topic deserving further attention in future 

studies. 

• Deposition fluxes of organic carbon in fog water were significant, yielding 

atmospheric removal rates on the order of 66 μgC/m2

 hr. Because some of the 

fog burden of organic carbon is comprised of soluble, volatile organic compounds 

(VOC), a portion of the deposited carbon may be released back to the 

atmosphere when deposited fog water evaporates following a fog episode. 

• Comparisons of fine particle organic carbon concentrations before and after fog 

episodes also suggests that aqueous phase reactions of dissolved VOC's may 

be important in producing new, secondary organic aerosol matter. Much more 

work is needed to examine this hypothesis in future investigations.

• Measurements of the stability of carbonyl compounds in actual fog water reveal 

the importance of stabilizing these species as soon as possible following sample 

collection. Analysis of field-stabilized samples collected in Fresno fog episodes 

after CRPAQS reveal significant contributions of several carbonyl and dicarbonyl 

compounds to San Joaquin Valley fog organic carbon burdens. Glyoxal and 

methylglyoxal, in particular, were observed at substantial concentrations nearly 

equivalent to formaldehyde. 

• The Carnegie Mellon University fog model, a 14-section size-resolved fog model 

with explicit fog microphysics, was used to simulate a CRPAQS fog episode and 

was able to predict the liquid water evolution, bulk aqueous-phase concentration 

measurements, drop size-resolved trends, and deposition fluxes for a number of 

species in close agreement with observed values. The predicted evolution of the 

distribution of aerosol species like sulfate, nitrate, ammonium, and chloride 

indicated significant differences over the course of fog processing. Different sizedependent processes controlled the size-distribution development at different 

stages of fog processing. Early in processing, the dissolution of soluble gases 

and rapid aqueous phase reactions control the development of the size 

distribution. At later times, the effects of deposition begin to dominate, and push 
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the peak of the distribution towards smaller diameters. Model predictions of the 

evolution of most key species approached bulk aqueous-phase concentration 

measurements. During the early stages of the fog, the behavior of species 

originating partially in the gas phase (NO3

-

, SO4

2-) was heavily influenced by 

dissolution into the droplets and rapid aqueous phase reactions. Following these 

initial peaks in aqueous phase concentrations, deposition began to dominate, 

and the species were gradually depleted from the fog layer. 

2.a.4 Fog References 

• Collett, J.L.J., Herckes P., Chang H., and Lee T., Pandis, S.N., and Fahey K. (2005) 

Examination of the Influence of California Regional Particulate Air Quality Study Fog 

Episodes on Aerosol Formation and Removal, Submitted to: San Joaquin Valleywide 

Air Pollution Study Agency, January 13, 2005

• Collett J. L., Sherman D. E., Moore K. F., Hannigan M. P., and Lee T. (2001) 

Aerosol particle processing and removal by fogs: observations in chemically 

heterogeneous central California radiation fogs. Water, Air, Soil Poll: Focus. 1, 303-

312. 

• Holets, S. and Swanson, R.N., 1981. High inversion fog episodes in central 

California. Journal of Applied Meteorology, 20: 890 - 899. 

• Huschke, R. E. (1959), Glossary of Meteorology, American Meteorological Society

• Lillis, D., Cruz, C.N., Collett, J., Richards, L.W. and Pandis, S.N., 1999. Production 

and removal of aerosol in a polluted fog layer: model evaluation and fog effect on 

PM. Atmospheric Environment, 33(29): 4797-4816. 

• Pandis S. N., Cruz C. (1997) Technical Support Study 11. Investigating the 

dynamics and chemistry of fog formation and dissipation. Prepared for the California 

Air Resources Board 

• Reilly, J. E., Rattigan, O. V., Moore, K. F., Judd, C., Sherman, D. E., Dutkiewicz, V. 

A., Kreidenweis, S. M., Husain, L. and Collett, J. L., Jr., 2001. Drop size-dependent 

S(IV) oxidation in chemically heterogeneous radiation fogs. Atmospheric 

Environment, 35(33): 5717-5728. 

• Waldman J. M. (1986) Depositional aspects of pollutant behavior in a fog. Ph.D. 

Thesis, Caltech. Pasadena, California 

2.b Dry Deposition 

Fine particles are small enough that the random forces from collisions with gas 

molecules largely overcome gravitational forces. Fine particles tend to follow air 

streams and are typically not removed by impaction. As a result, they are not easily 

traced back to their individual sources. 

Fine particles formed from accumulation processes are significantly larger than gas 

molecules and their diffusion velocity is low. These particles have very long half-lives in 
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the atmosphere, travel long distances, and tend to be more uniformly distributed over 

large geographic areas than coarse-mode particles. Removal by dry deposition is 

inefficient since they do not readily diffuse through the boundary layer of still air next to 

surfaces. The atmospheric half-life of fine accumulation particles with respect to dry 

deposition is on the order of weeks; however, removal occurs when the particles absorb 

water, grow into cloud droplets, grow further to raindrops, and fall out as rain. This 

process reduces the typical atmospheric half-life to a few days. 

Ultrafine or nuclei-mode particles, formed by nucleation of low saturation vapor pressure 

substances, tend to exist as disaggregated individual particles for very short periods of 

time (less than minutes) in the ambient atmosphere and tend to age rapidly into larger 

accumulation particles that may be dispersed more widely over long distances. 

Secondary fine particles are formed by atmospheric transformation of gases to particles. 

Atmospheric transformation can take place locally during stagnations or during transport 

over long distances. Fine particles have very low dry deposition velocities, which 

contribute to their uniformity throughout the air mass. Aerosol effects on visibility and 

climate, through light scattering and changes in cloud microphysics, primarily arise from 

fine particles. 

Nitrates and carbon are the two largest components of PM2.5 and their atmospheric 

behavior is entirely different. 

“High concentrations of PM organic carbon (OC) were spatially limited to 

core urban sites while high concentrations of PM ammonium nitrate were 

regionally distributed throughout the SJV. Concentrations of PM and its 

precursors were typically lower at the elevated sites surrounding the SJV 

than at monitoring sites located on the SJV floor....At distances more than 

50 km from the urban areas, OM concentrations typically declined by a 

factor of three or more. Emissions of OM at the urban core are either not 

rapidly transported to the rural sites or are diluted too much to 

substantially impact rural sites. Concentrations of OM at elevated sites 

were comparable to concentrations at rural sites on the Valley 

floor....Overall, these spatial patterns of OM suggest that the impact of 

emissions was largely confined to the local area and OM concentrations 

were unevenly distributed over the duration of the episode. 

The contrast in spatial variability between the ammonium nitrate and OM 

components of PM2.5 in the SJV winter episodes provides information on 

the spatial extent of the production of ammonium nitrate. PM2.5 OM and 

ammonium nitrate are both subject to the same meteorological transport 

conditions, yet ammonium nitrate concentrations are relatively 

homogeneous and OM concentrations are much higher in the urban 

source areas. In addition, OM and ammonium nitrate components are 

expected to have the majority of their mass in a similar size fraction 

(PM0.1 to PM1) (Lighty et al., 2000; Hughes et al., 1999; Bench et al., 
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2002) and, therefore, the rates of removal should be approximately the 

same. In summary, the likely explanation for the difference in spatial 

variability is the spatial distribution of the emissions or precursors. Primary 

OM emissions occur predominantly from mobile sources and wood smoke 

located in urban areas. The formation of ammonium nitrate from NOx 

precursors (Lurmann et al., 2004) must occur throughout the SJV to 

account for its spatial homogeneity. 

High concentrations of PM organic carbon were spatially limited to core 

urban sites while high concentrations of PM ammonium nitrate were 

regionally distributed throughout the SJV. The regional homogeneity of 

ammonium nitrate concentrations coupled with the stagnant wind 

conditions provides evidence that production of ammonium nitrate occurs 

at similar rates throughout the valley. In contrast, the OC component of 

PM indicates that production rates were much higher in the urban areas 

than at rural sites.” (reference citation follows in 2.b.1) 

2.b.1 Dry Deposition Reference 

• Background and Boundary Conditions for Particulate Matter and Precursors in the 

San Joaquin Valley in Winter, Technical Memorandum STI-902325-2779-TM, 

Michael C. McCarthy, Hilary R. Hafner, Steven G. Brown, Fredrick W. Lurmann, 

Paul T. Roberts, Sonoma Technology, Inc., 1360 Redwood Way, Suite C, 

Petaluma, CA 94954-1169, July 29, 2005 

2.c Nitrate Events 

Nitrate particulates are generally low in summer and fall, partially due to breakdown of 

the particles back into gaseous precursors. Photochemical reactions reduce nitrates; 

therefore, overcast conditions support increase in nitrate production. Valley summers 

have infrequent cloud cover. Smoke from fires whether aloft or at the surface can 

reduce photochemical energy penetration to the surface allowing more particle 

formation. As with all other particulates, accumulation to high concentrations is also 

dependent on stagnation events to allow the material to accumulate. 

2.c.1 Relationship Between Carbon and Hydroxyl Radicals and Nitrate PM2.5 

Nitrate particulate levels in the San Joaquin Valley declined sharply after the 2000-2001 

winter. It is difficult to isolate meteorological variations from other factors to determine a 

strong cause and effect relationship. However, the most pronounced change (during 

the time of year that the nitrate levels increase) was the introduction of different smoke 

management practices including the voluntary curtailment (commencing in 2001) and 

mandatory curtailment (commencing in 2003) of residential wood combustion when air 

quality conditions were forecast to be unfavorable. 
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Since the primary emissions from residential wood combustion are carbon and VOC, 

the link between reduced burning and reduced nitrate particulate levels is not 

automatically evident. EPA has established that hydroxyl radicals, released by 

chemical reaction from organic VOC, are the primary mechanism for much of the 

atmospheric chemistry for both ozone and particulates. The role of hydroxyl radicals in 

ozone and particle formation is a key concept in EPA's "One Atmosphere" technical 

evaluation. Nitrate particulates require hydroxyl radicals to form particulate. This 

establishes the possible effect of receiving multiple benefits from reducing carbon and 

organic carbon emissions, in that not only will carbon be reduced but also secondary 

particulate formation including nitrates and secondary organic aerosol (SOA). 

Additional evidence supports the relationship between carbon and nitrate particulate 

formation. A sharp increase in nitrate appears to occur during periods when the air 

shed is being significantly impacted by wildland fire smoke. During several wildfires that 

occurred during the summer and fall, when nitrate is usually at very low concentrations, 

elevated levels of nitrates occurred on a daily basis. This conveys that either the fires 

produce much more nitrate than expected and transport it to distant areas or that the 

carbon and hydroxyl radicals promote nitrate formation in the local area. Either 

circumstance connects fires to nitrates. The carbon in the smoke plume provides 

carbon nuclei and/or organic carbon radicals that can initiate nitrate particle formation. 

The District has conducted preliminary evaluation of two case studies that provide 

supporting information. Further evaluation of these events and other similar events may 

be warranted to establish a confirmed finding. 

Figure 18 Daily average nitrate and maximum hourly nitrate by day at Fresno - 

First Street monitoring station affected by Day Fire 

Nitrate Concentrations - Fresno - First Street

0

3

6

9

12

15

18

21

9/27/20069/28/20069/29/20069/30/2006

10/1/200610/2/2006

10/3/200610/4/2006

10/5/200610/6/200610/7/2006

10/8/2006

10/9/2006

10/10/2006

10/11/2006

10/12/200610/13/2006

10/14/2006

10/15/2006

Nitrate (ug/m3) 

Average Maximum Hour
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Figure 18 shows high nitrate concentrations during a period of smoke production from 

large wildland fires. Typical nitrate levels at this time of year (September and early 

October) are less than ten micrograms. 

Figure 19 is a smoke plume simulation using the CANSAC MM5 meteorological model. 

At the time of this analysis, 10:00 PST on October 10, 2006, the smoke plume was 

projected to be over the Fresno area. The CANSAC simulation has not been confirmed 

for accurate fire inputs for this simulation. The value of assessing this type of event is 

that the path and origin of emissions provide a more direct linkage to a fire event than 

when the emissions path crosses through large cities. Events of this type do not occur 

frequently and events that contain emissions mixed from many source areas are more 

difficult to resolve into cause and effect relationships. 

Figure 19 Wildfire smoke simulation for October 10, 2006, at 10:00 PST, 

displaying peak particulate concentration over the Fresno area 
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In Figure 20, the observed increase in nitrates occurred in the hours immediately after 

the projected arrival of the smoke plume in Fresno. This indicates that the smoke 

plume itself was not high in formed nitrates, but provided carbon nuclei and/or organic 

carbon radicals that initiated nitrate particle formation. After departure of the smoke 

plume, nitrate levels rapidly returned to ambient levels that were measured before the 

arrival of the smoke plume. 

Figure 20 Hourly nitrate (ug/m3) at Fresno - First Street monitoring station, 

displaying increase in nitrate on October 10, 2006, 

in the hours following arrival of the smoke plume 

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

10/10/2006

The case studies, and unexpected decline in nitrates after establishing stronger burning 

restrictions, establish a strong likelihood that burning controls provide more than a 

carbon benefit for the District. Understanding and confirming this relationship through 

future research and technical evaluation is valuable for effective PM2.5 planning. 

3. Emissions Related Issues 

High levels of PM2.5 may occur anytime throughout the year if influenced by unusual 

local or regional emission activity. Wildland fires and 4th of July firework activity have 

been noted as causing PM2.5 levels to be higher than what is typically experienced 

during the summer and fall. Day of the week activity variations and holidays during the 

fall and winter have the potential to cause higher PM2.5 levels due to residential wood 

burning. 
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3.a Day of week and Holiday Effect 

• Day Of The Week Use of fireplaces tends to increase on cold winter weekend 

nights. PM2.5 can increase on Friday, Saturday and Sunday when residential wood 

combustion may be higher than during the workweek. 

• Holiday On winter holidays, residential wood combustion from fireplaces can 

substantially increase, resulting in a significant increase in the particulate matter 

concentrations. Days around Thanksgiving, Christmas and New Year’s Day tend to 

have higher PM2.5 concentrations. 

• 4th of July PM 2.5 concentrations increase significantly during the evening of July 4 

and continue into the early morning hours of July 5 due to fireworks emissions. This 

effect is especially pronounced during periods of stagnant weather conditions. On 

July 4, 2007, PM2.5 increased to very high concentrations during the time of day 

when fireworks were used, as shown in Figure 21. This coincided with a period of 

stagnant conditions, which resulted in PM2.5 concentrations remaining high into the 

early morning hours of July 5. Fireworks emissions are categorized as an 

Exceptional Event by EPA policy, but the District is proposing a feasibility study to 

investigate the possibility for controls on fireworks (see Chapter 6). 

Figure 21 Fireworks Emissions Effect on PM2.5, July 4th and 5th, 2007 

Data in this plot is preliminary and subject to revision 

0

200

400

600

800

1000

1200

13:0014:0015:0016:0017:0018:0019:0020:0021:0022:0023:00

0:001:002:003:004:005:006:007:008:009:00

10:0011:0012:00

4-

Jul07

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Jul07

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Jul07

4-

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4-

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Date and Time

PM2.5 (ug/m3)

Turlock

Modesto

Fresno

Clovis

Corcoran

Visalia

Bakersfield California Street

Bakersfield Golden State Highway
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3.b Wildland Fires 

Wildland fires can have a significant effect on particulate air pollution. Wildland fires 

can be ignited by human activity and by lightning. These fires are either suppressed 

(put out) or managed (allowed to burn) as a controlled burn. Particulate from wildland 

fires can increase surface PM 2.5 concentrations. Figure 22 shows smoke from the 

Day Fire being transported to the San Joaquin Valley. It should be noted that large 

wildfires may release particulates in amounts that dwarf the District daily emission 

inventory, causing impacts that cannot be fully remediated by District action to curtail 

local emissions. The impact of a wildfire will depend on whether the smoke passes aloft 

over the District or mixes into the surface air. Wildfires release carbon, NOx and VOC's. 

Some of the VOC's that are emitted by wildfires are classified as air toxics. 

Figure 22 Smoke from the Day Fire Transported to the San Joaquin Valley 
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3.c Agricultural and Prescribed Burn Management 

The District's agricultural, hazard reduction, and prescribed burn programs permit, 

regulate, and coordinate the use of burning in the San Joaquin Valley air basin to 

minimize smoke impacts on the public. The goal of these smoke management 

programs is to minimize air quality impacts by allowing agriculture, hazard reduction, 

and prescribe burning to occur on days when dispersion is adequate enough to carry 

emissions away from sensitive receptors. The District has developed a state of the art 

Smoke Management System (SMS) that allows the District to permit agricultural burning 

in some areas of the District and not allow agricultural burning in others, based on 

dispersion conditions and air quality. The SMS reduces smoke impacts by preventing 

burning from occurring in a concentrated area or in locations that are forecast to have 

poor air quality, while allowing agricultural burning in areas that have adequate 

dispersion and good air quality. The District manages the burn programs daily to 

determine if the potential emissions will impact the air quality in the air basin. On days 

when the air quality is predicted to be marginal or poor, the District severely restricts, or 

does not allow, agricultural, hazard reduction, and prescribed burning to occur. When 

air quality conditions improve, the District allows open burning as dispersion conditions 

and air quality dictate. 

Applicable District rules regulating burning activity:

Rule 4103 Open Burning - Agriculture 

Rule 4106 Prescribed Burning and Hazard Reduction Burning 

3.d Residential Wood Burning Program 

The purpose of the District's residential wood burning program is to protect the public by 

limiting the emission of particulate matter from wood burning fireplaces and wood 

burning heaters when air quality conditions are unfavorable for dispersion of the 

emissions. The District discourages the public from burning when the Air Quality Index 

(AQI) is greater than 100. An episodic curtailment for a geographic region is declared 

whenever an Air Quality Index value of 150 or greater is predicted for the geographic 

region (county). An episodic wood burning curtailment may be issued during the 

months of November through February. Operation of wood burning fireplaces and 

wood burning heaters is prohibited in the specified region when the District declares a 

curtailment, except for certain exemptions (e. g. device is sole source of home heat). 

Applicable District rule regulating burning activity:

Rule 4901 Wood Burning Fireplaces and Wood Burning Heaters 
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3.e Recent Particulate Research Studies 

Evaluation of Rule 4901 This study (Lighthall, 2007) is designed to provide the District 

with a comprehensive evaluation of Rule 4901, which regulates residential wood 

combustion. Rule 4901 set in place an AQI-triggered ban on wood burning in areas of 

the San Joaquin Valley that have natural gas service and are below 3,000 ft elevation. 

In particular, the study will assess the contribution of the Rule to reduced PM2.5 

emissions in urban areas of the region and to corresponding reductions in health 

impacts (mortality and morbidity). In addition, to contributing to the larger body of 

knowledge about the health effects of PM2.5 exposure, the study will provide the District 

staff and Board with a solid scientific basis for determining whether additional 

restrictions in wood burning will be justified. 

PM Emission Factors and Inventories from Leaf Blowers The District funded the 

University of California at Riverside - College of Engineering Center for Environmental 

Technology (CE-CERT) to design a study and perform measurements and data 

analysis to determine particulate matter emissions from leaf blowers and to obtain a PM 

emission inventory from their operation in the District. The following is an excerpt from 

the executive summary of the final report (Fitz, 2006). 

The approach used to measure emissions from leaf blowers and alternative devices 

(vacuums, rakes, and brooms) was to operate the devices over a measured area in a 

tent-like enclosure. In this enclosure, the leaf blower (or other device) could be used in a 

normal manner while allowing the PM emissions to be confined for quantification. PM 

concentrations were measured with real time sensors. Measurements were made for 

total suspended particulate matter (TSP), particulate matter (PM) with an aerodynamic 

diameter less than ten microns (PM10) and particulate matter with an aerodynamic 

diameter less than 2.5 microns (PM2.5). The amount of PM produced per unit area was 

then calculated by multiplying the concentration once it stabilized (when it became 

uniformly mixed) by the volume of the enclosure and dividing by the area treated. To 

directly compare the PM emission characteristics of blowing, vacuuming, raking, and 

sweeping, the surface to be treated in the enclosure was loaded with surrogate debris. 

To develop the composition of this surrogate material, bulk samples were collected from 

areas on the University of California, Riverside (UCR) campus where leaf blowing was 

about to be conducted to determine the mass of soil and vegetative matter present 

where these cleaning activities are conducted. The test system was then used to 

measure emissions from leaf blowing over surfaces where leaf blowing is typically 

conducted and over surfaces where a surrogate mixture of soil (obtained from the San 

Joaquin Valley) and vegetative matter was deposited by our staff. A number of emission 

tests were performed using the natural/indigenous material at the CE-CERT facility in 

Riverside and at the UC Kearney Agricultural Center in Parlier, CA. 

Health Effects of Airborne Fine Particles (PM2.5) and Ozone The goal of this study 

(Tyner, 2007) is to determine whether exposures to airborne fine particles (PM2.5) and 

ozone (O3) induce lung inflammation that is related to exacerbations and severity of 
San Joaquin Valley Unified Air Pollution Control District April 30, 2008

Appendix E: District Additions to the Conceptual Model 

2008 PM2.5 Plan

E-34 

disease in nonatopic asthmatics and, reflecting the multi-factorial web of causation of 

asthma, how this relation is modified by viral infection and genetic predisposition. The 

development of urinary PAH-quinone biomarkers to monitor individual exposure to 

PM2.5 provides an enhanced method for examining associations between exposures 

and health outcomes. In addition, measurement of organic species (2-3 ring PAH and 

oxygen derivatives) and wood-smoke tracers (levoglucosan) in ambient PM2.5 samples 

and subject urine samples provides information on the impacts of PM composition and 

reactivity on respiratory health and potential associations with exacerbation of asthma. 

Ambient 24-hr PM2.5 filter samples will be obtained from the Clovis and Winery 

monitoring sites throughout the study period and analyzed by project investigators 

(Hasson Lab). Filter samples will be collected 1-in-3 days from October 2007 – March 

2008 and 1-in-6 days from April 2008– September 2008 per US EPA PM2.5 monitoring 

schedule. Following gravimetric analysis by the Ventura County Air Pollution Control 

District Lab, filters will be archived for 30 days (stored under controlled 

temperature/humidity) after which filters will be returned to the District. Filters will then 

be retrieved from the District by project investigators and filters will be extracted in 

organic solvent and solute composition (PAH, PAH-quinone, levoglucosan) and 

reactivity (ROS generation) will be determined. 

3.e.1 Recent Particulate Research Study References 

• Fitz, D. R., Pankratz D., Chitjian, M., Bristow. J., Peterson S. (2006) Particulate 

Matter Emissions Factors and Emissions Inventory from Leaf Blowers in use in the 

San Joaquin Valley, CE-CERT, UC Riverside

• Lighthall, D (2007) Email communication 

• Tyner, T. (2007) Email communication 

Additional links to peer-reviewed publications that complement the staff reports and 

memos cited in this Appendix: 

Basic Characterization: 

http://www.arb.ca.gov/airways/CRPAQS/PM_DA/final/Chowetal2006JGRChemC

ompSpatioVar.pdf

Fog: 

http://www.arb.ca.gov/airways/CRPAQS/PM_DA/final/Herckesetal2005JAWMAF

ogOverview.pdf

Secondary Aerosols: 

http://www.arb.ca.gov/airways/CRPAQS/PM_DA/final/Lurmannetal2006JAWMAS

JVFresnoSecAerosol.pdf

Transport: 

http://www.arb.ca.gov/airways/CRPAQS/PM_DA/final/MacDonaldetal2006JAWM

ASJVPMTransport.pdf