AucusT 1995 BIGLER-ENGLER AND BROWN 1863 

Analysis of an Ozone Episode during the San I)iego Air Quality Study: 

'I'he Significance of 'Transport Aloft 

VIRGINIA BIGLER-ENGLER AND I -IAL W. BROWN 

San Diego Air Pollution Control District, San Diego, California 

(Manuscript received 16 March 1994, in final form 8 December 1994) 

ABSTRACT 

San Diego is subject to transport of ozone and precursors from the Los Angeles area, 170 km to the northwest, 

as well as to high ozone concentrations from local emissions. The San Diego Air Quality Study was conducted 

during the summer of 1989. Air quality and meteorological data were obtained at the surface and aloft to 

provide a high quality database for photochemical modeling and control strategy development for both local 

and transport-influenced ozone episodes. Six high-ozone episodes were potentially suitable for photochemical 

modeling, including several from 21 to 29 September 1989; maximum 1-h ozone concentrations exceeded the 

federal ambient standard on six of the nine days and reached 0.17 ppm on two days. From review of preliminary 

surface and routine meteorological data and surface trajectory analysis, 25-26 September 1989 appeared to be 

a local episode with high ozone concentrations downwind from local (i.e., San Diego) emissions sources. This 

paper examines the candidate episode and finds that transport aloft from the Los Angeles area played a 

key role. 

1. Introduction 

The San Diego Air Basin exceeds both California 

and federal ambient ozone standards, of 0.09 and 

0.12 ppm, respectively. In 1993, the most recent year 

of data, the California ambient ozone standard was 

exceeded on 90 days; the federal standard on 14 days, 

with a peak hour of 0.19 ppm. Both California and 

Federal Clean Air Acts require that attainment of 

their respective standards be demonstrated, generally 

by photochemical modeling. San Diego's highest 

concentrations of ozone and precursors result from 

transport from the greater Los Angeles area, with 14 

million residents, 170 km to the northwest, as well 

as from San Diego County emissions. Controls 

placed on San Diego sources may achieve little air 

quality benefit for San Diego under conditions conducive to transport; thus, identifying an episode not 

contaminated by transport is essential to model and 

control strategy development. 

The boundaries of the San Diego Air Basin 

( SDAB ) are the borders of San Diego County, an 

area roughly the size of the state of Connecticut. 

SDAB is bounded on the north by Orange and Riverside Counties and Greater Los Angeles [ South 

Coast Air Basin ( SoCAB )], on the east by Imperial 

County, on the south by Mexico, and on the west by 

the Pacific Ocean (Fig. 1). Mean terrain increases 

Corresponding author address: Virginia Bigler-Engler, San Diego 

Air Pollution Control District, 9150 Chesapeake Drive, San Diego, 

CA 92123-1096. 

from sea level at the coast to more than 1200 m above 

mean sea level (MSL) inland. Isolated peaks rise to 

more than 2100 m. Elevations fall sharply east of the 

mountain crests, to near sea level at the Imperial 

County line, and 60 m below sea level at the desert 

floor 10-20 km farther east. 

Meteorology and air quality in southern California 

and San Diego are dominated by the semipermanent 

Pacific high and the effects of a coastal marine environment. The coastal marine layer is known to intrude more than 160 km over most of California. 

The marine layer extends over San Diego County as 

far inland as the mountain crests. The marine effects 

are modified by intense inland heating that produces 

a thermal low over the southern deserts and by terrain. The result is a mild marine climate at the coast, 

alpine climates 50 km inland, and desert conditions 

east of the mountains. 

A number of studies have documented the structure of the marine layer off the California coast. Mesoscale conditions, with particular attention to description of the inversion wind and temperature 

structure, were described by Bridger et al. ( 1993 ), 

based on the MABLES WC ( Marine Atmospheric 

Boundary Layer Experiments, West Coast) field 

study (1993). CEWCOM-76 (Cooperative Experiment in West Coast Oceanography and Meteorology ) 

(Davidson et al. 1979) and CEWCOM-78 (Fairall et 

al. 1979; Leipper 1979; Niziol et al. 1979; Rosenthal 

et al. 1979 ) provided insight into the structure and 

meteorology of the marine layer over coastal waters 

of southern California, the latter near San Nicolas 

© 1995 American Meteorological Society 
1864 JOURNAL OF APPLIED METEOROLOGY VOLUME 34 

3 

' 2134 

b r 

N ,0 

c 

1829 

o 

o n N 

~ 

D = 0 1524 ~ 

z 

1219 ~ 

0 3 a~i a o fD 0 

D 914 ~ 

3 

Z ~ D ~ 610 

zv X ~ 3 3 

Distance Inland km 

0 10 20 30 40 50 60 70 80 90 100 110 

FiG. 1. (a) Topographical map of Southern California. 

(b) Profile of San Diego County. 

Island. Understanding of transport and diffusion in 

the California shoreline environment has also benefited from research performed in other regions; 

Lyons ( 1975 ) summarized current knowledge and 

the complexity of the shoreline environment in a 

monograph for the American Meteorological Society. 

Lyons discussed plume behavior in the shoreline at 

length, with particular attention to fumigation processes. More recent studies, for example, the Southern California Air Quality Study, the Lake Michigan 

Ozone Study, and the Gulf of Mexico Air Quality 

Study, continue to advance the understanding of 

coastal meteorology, as well as provide data for photochemical modeling of their respective regions, yet 

modeling the meteorology of coastal regions remains 

daunting. 

Meteorological conditions associated with air pollution episodes over southern California received research attention in the 1950s (Bell 1960; Blake 1960); 

meteorological and climatological analyses implied the 

existence of long-range transport processes soon thereafter ( DeMarrais et al. 1965). Nevertheless, field studies 

to study transport processes and monitor pollutants 

and precursor pollutants did not follow for a decade 

(Schacher et al. 1978; Kauper 1977; Sklarew et al. 

1975). CEWCOM-78 included aerosol measurements, 

as did a 19831idar experiment, the San Diego Oxidant 

Transport Study (McElroy et al. 1983; Smith and 

Lehrman 1984), but gaseous data was liinited or nonexistent. While these studies confirmed ! the transport 

of ozone and precursors from the Los Ahgeles area to 

San Diego County, few specific studies including pollutant measurements over coastal waters have occurred 

due to the extreme expense and logistical problems of 

monitoring a large, uninstrumented, sometimes hostile 

marine region. The result is that data gathering is confined to short temporal and geographical zones, which 

may miss peak episodes and concentrations. 

It is the purpose of this paper to bring these topics 

to knowledge gained from the San Diego Air Quality 

Study. We examine the episode selection portion of 

the study and examine a candidate local episode, using routine regulatory classification and trajectory 

analysis techniques. Finally, we examine intensive 

study data at the surface and aloft to,form the hypothesis that best satisfies the observations. Conclusions are reached regarding the source of contaminants on the 26 September 1989 study day and 

the suitability of the episode for photochemical 

modeling. ; 

I 

2. The San Diego Air Quality Study I 

The San Diego Air Quality Study ( SDAQS ) consists 

of a field study, episode selection, ~ modei; development 

and application, model performance evaluation, and 

future-year control strategy design. The field portion 

of this study was conducted through tlie summer of 

1989, with the objective of acquiring high quality databases for photochemical modeling. Specifically, the 

goal was to obtain extensive mefeorological and air 

quality data for multiple episodes meeting two sets of 

meteorological criteria for the Carbon Bond IV version 

of the Urban Airshed Model (Tesche et al. 1991). 

Three-day intensive data-gathering periods were 

launched on the basis of forecasts by San Diego Air 

Pollution Control District ( SDAPCD ) meteorologists 

from July through October 1989. On iritensive study 

days, two aircraft from North American Weather Consultants (NAWC), a Cessna 340-A and an Aero Commander 680, sampled temperature, ozone, NO x , hydrocarbons, and carbonyl from near-surface elevations 

to roughly 1500 m MSL. These data supplemented a 

28-station aerometric and air quality surface monitoring network, 5 upper-air sites, and a Doppler acoustic 

sounder located midcounty at Kearny! Mesa, 3 km 

south of the National Weather Service (NWS ) rawinsonde site at Miramar Naval Air Station ,( NKX ). Surface sites included NWS airport observations, the 10- 

site SDAPCD moniioring network, and additional 

contract sites operated by Aerovironment. Technical 

& Business Systems, Inc. provided upper-air data via 

balloon soundings four times daily lat five sites 

( SDAPCD 1990a,b ). Two island sites; were instru- 
AuGUST 1995 BIGLER-ENGLER AND BROWN 1865 

TABLE l. SDAQS data collection. 

Abbr. Station Source 

ALP Alpine SDAPCD 

CAT Catalina Island ERCEb 

CVA Chula Vista SDAPCD 

DES Descanso ERCEe 

DMR Del Mar SDAPCD 

DTN Downtown SDAPCD 

DUL Dulzura ERCEb 

ECA El Cajon SDAPCD 

ESC Escondido SDAPCD 

FLN Flinn Springs ERCE° 

JAM Jamacha AERO° 

KMA Kearny Mesa SDAPCD 

LAK Lakeside AERO° 

MRA Mira Mesa AERO° 

MTS Mount Soledad SDAPCD 

MYF Montgomery Field NCDC`•r 

NFG Camp Pendleton NCDC` 

NIM Nimue ERCEb 

NKX Miramar NCDC`,f 

NRS Imperial Beach NCDC` 

NUC San Clemente Island ERCEb 

NZY North Island NCDC` 

OSA Oceanside SDAPCD 

OTM Otay Mesa SDAPCD 

RAM Ramona SDAPCD 

SAN Lindbergh Field NCDC` 

SCR Scripps Pier SDAPCD 

SEE Gillespie Field ERCEb 

SNO San Onofre SDAPCD 

Comments 

03 , NOxa, winds, temperature 

03 , NOxa, winds 

03 , NOxa, ROG`, winds 

03 , winds, temperature 

03 , ROG`, winds 

03 , NOxa, carbonyl, winds 

03 , winds, temperature 

03 , NOxa, carbonyl, winds, temperature 

03 , NOx , ROG`, carbonyl, winds, four soundings daily, temperature 

ROG` carbonyl, winds aloft—four balloons daily 

03 , NOxa, winds, temperature 

03 , NOxa, winds, temperature, winds aloft—Doppler sodar 

03, NOxa, winds, temperature 

03 , NOxe, winds, temperature 

03 , NOxa, winds, temperature 

Airport sfc obs, winds aloft—NWS rawinsonde 

Airport sfc obs 

03 , ROG`, carbonyl, winds aloft—four soundings 

Airport sfc obs, winds aloft—NWS rawinsonde 

Airport sfc obs 

03 , NOxa, ROG`, airport sfc obs 

Airport sfc obs 

03 , NOxe, ROG`, carbonyl, winds, four soundings daily, temperature 

03 , NO a, ROG`, four soundings daily 

03 , ROG`, winds 

Airport sfc obs 

03 

Airport sfc obs; ROG` 

03 , NOxe, winds 

a NOz= NOx , NO, NOZ . 

bData collection under subcontract to project management, ERC Environmental and Energy Services Company. 

` Reactive organic gases. 

dAerovironment. 

` Standard surface data; winds, etc. reported to National Climatic Data Center. 

rNational Weather Service upper-air site moved from MYF to NKX 14 September 1989. 

mented, San Clemente Island (120 km west of San 

Diego ) and Santa Catalina Island ( 80 km west of San 

Onofre), as well as the NAWC ketch Nimue, approximately 20 km west of Del Mar ( Table I and Figure 

2). 

3. Episode selection methodology 

a. Synoptic classification 

Regulatory guidance for application of photochemical models recommends modeling episodes for 

all meteorological scenarios that lead to high ozone 

concentrations in an air basin. SDAB ozone episodes 

are associated with two distinctive meteorological 

signatures. The first, a"local episode," is marked by 

a strong marine inversion based around 400-800 m 

above ground level (AGL), moderate onshore pressure 

gradients, and a well-developed sea breeze. Morning 

precursor emissions in the urbanized coastal plain 

chemically react as they are carried inland toward the 

foothills. Trapped by the marine inversion, peak hour 

concentrations exceed the 0.09-ppm state standard and 

may edge above the federal standard at foothill sites, 

typified by Alpine ( ALP ). On the other hand, extreme 

values, above 0.20 ppm, are observed with a"transport 

episode" signature. High pressure builds over the Great 

Basin and southwestern deserts, producing northerly 

mesoscale flow and weak offshore pressure gradients. 

The marine layer may be shallow due to subsidence 

aloft, or the base of the inversion may be depressed to 

the surface along the coast. Ozone and precursors from 

SoCAB are carried offshore, southward over coastal 

waters, and back onshore into San Diego. Any part of 

SDAB may experience high ozone concentrations due 

to transport. ( Bigler-lEngler and Brown 1993). 

b. Source-receptor analysis 

Identification of the synoptic type suggests the 

source of an episode but may not be considered conclusive. Analysis of the flow fields through the duration of an episode and calculation of forward and 

backward air parcel trajectories are commonly used 

means to reveal source-receptor relationships. The 

most common practice is the calculation of surface 
1866 JOURNAL OF APPLIED METEOROLOGY VOLUME 34 

MG. 2. SDAQS photochemical modeling domain 

and aerometric monitoring locations. 

wind trajectories, due to availability of data. The observed surface wind is assumed to represent the 

boundary layer mean wind. Forward trajectories follow a parcel from a potential source region for a 

specified time period. Backward trajectories track a 

parcel to its region of origin. Theoretically, trajectories can be determined accurately if wind fields are 

highly resolved spatially and temporally; .they may 

be developed from observed data or numerical simulation. In practice, they are subject to significant 

error. 

Many studies have explored the uncertainty of 

trajectories, generally in the context of multiday trajectories and long-range transport of pollutants. Differing meteorological regimes have been examined, 

from synoptic-scale cyclones ( Kuo et al. 1985; Doty 

and Perkey 1993 ) to generally persistent flow conditions (Kahl and Samson 1986). Although verification is difficult, several researchers, notably Rolph 

and Draxler (1990 ) and Doty and Perkey (1993), 

have attempted to quantify errors in vertical as well 

as horizontal components. In a paper exploring numerical error, Seibert ( 1993 ) also recommended use 

of an ensemble of trajectories offset 50-100 km to 

reduce error from uncertainty in the wind field. 

These studies agree that the existing NWS rawinsonde network is inadequate for long-range computations and suggest finer temporal and spatial resolution of the meteorological database would improve accuracy. 

c. SDAQS episode selection 

The SDAQS field study yielded six potential modeling episodes with high ozone levels; all were evaluated for photochemical modeling (Tesche et al. 

1991). Each episode was first classified by its synoptic 

signature. Each was subjected to a further series of 

tests, including trajectory and time series analyses, 

to identify a source—receptor relationship for the 

monitored concentrations at each site that exceeded 

the 0.12-ppm federal standard. Hourly analyses of 

surface observations were prepared for episodes of 

particufar interest, as were wind flow',fields, hourly 

at the surface and at 4-h and 300-m ihtervals aloft, 

on intensive days. Trajectories were calculated in 

hourly increments from surface streainlines. Backward trajectories were prepared for all sites exceeding 

the federal ambient ozone standard; forward trajectories were determined for San Diego urban emissions and for SoCAB. Even in smaller-scale analyses, 

the simplifying assumption that the surface wind is 

equivalent to the transport wind can introduce substantial error, so streamlines were reviewed for consistency with winds within the marine layer. The ensemble of source- and receptor-based tr,ajectories and 

use of a streamline field, rather than individual observations, were also designed to reduce error due to 

uncertainty in the wind field, somewhat along the 

lines suggested by Seibert (1993). 

Each candidate modeling episode was required to 

fulfill additional criteria. A high-ozorie event, with 

extreme ozone concentrations with respect to the 

federal ambient standard, would be pieferred to an 

episode marginally exceeding the fedeial or California standards, as does the typical local SDAB local 

episode. For each 2-day ( minimum) inodeling episode, a robust database with at least one day of intensive data was a prerequisite, preferably on the 

peak day. i 

4. September 1989 episode selection and analysis 

Federal and state ambient ozone standards 

were violated on nine consecutive days in September 1989 (Table 2). Highest concentrations 

were monitored on the first six days I of this siege, 

with a maximum of 0.17 ppm. Concentrations on 

the final three days, 27-29 September, exceeded only 

the more stringent California standard and at fewer 

sites. ~ 

a. Synoptic summary and classificationl 

The period 21-24 September was maiked by strong 

ridging over the western United States, subsidence, and 

northerly flow. Triggered by SDAPCD's forecast of potential transport conditions, 3-day intensive data sampling began; significant subsidence was evident in the 

first soundings early on the first intensive day, 21 September. Figure 3 displays the characteristic features of 

the transport meteorological signature at the surface 

and 500 mb on 22 September 1989, when the pattern 

was well established. As the broad ridg'e moved into 

the western states, surface pressure gradients became 
AuGusT 1995 BIGLER-ENGLER AND BROWN 1867 

TABLE 2. Daily maximum 1-h average ozone concentration in parts per million for 21-29 September 1989. 

Bold values are basin maximum. 

Site 

Day ALP CVA DMR DTN ECA ESC KMA MTS OSA RAM 

21 0.11 0.12 0.13 0.14 0.11 0.16 0.14 0.13 0.09 0.12 

22 0.15 0.07 0.07 0.07 0.12 0.10 0.09 0.07 0.06 0.11 

23 0.14 0.08 0.10 0.09 0.11 0.09 0.08 0.10 0.08 0.10 

24 0.16 0.13 0.14 0.14 0.13 0.17 0.16 0.16 0.12 0.12 

25 0.15 0.07 0.07 0.08 0.10 0.10 0.10 0.08 0.06 0.12 

26 0.17 0.05 0.04 0.06 0.10 0.12 0.08 0.09 0.05 0.12 

27 0.12 0.06 0.07 0.07 0.08 0.08 0.07 0.06 0.06 0.09 

28 0.10 0.09 0.07 0.09 0.10 0.08 0.09 0.08 0.06 0.09 

29 0.10 0.07 0.07 0.07 0.07 0.08 0.12 0.06 0.06 0.09 

offshore. The thermal trough was pushed off the California coast; its southern section, into northern Mexico. Maximum ozone concentrations were associated 

with this phase. 

A transition to a local episodic signature began 25 

September. By 26 September (Figs. 3c,d), a long-wave 

trough approached the West Coast, and the upper ridge 

moved on to the Rocky Mountains. At the surface, the 

thermal low became established once again over the 

southern California desert. The meteorology of 26 

September shows a return to the more typical mesoscale 

conditions described by Bridger et al. (1993). This 

pattern resembles their 9 August 1978 example day, 

with a strong marine inversion and an inland thermal 

trough over most of California. 

All three high-ozone events, 21, 24, and 26 September, were evaluated for photochemical modeling. The 

2-day period 20-21 September was developed as a 

transport episode; 23-24 September was rejected for 

lack of intensive data and its weekend occurrence ( for 

which there is no emissions inventory, the catalog of 

hourly emissions basinwide). Although 26 September 

1989 was the sixth of the nine days of ozone violations, 

it was one of the two dates on which the extreme 0.17 

ppm was monitored, at Alpine, and the first day apparently with a local synoptic signature. Because of 

this, 25-26 September received further attention as a 

candidate for photochemical modeling, possibly as a 

local episode. 

b. Source-receptor analysis 

1 ) SURFACE TRAJECTORY ANALYSIS 

A surface wind trajectory was used to trace the plume 

back to a precursor source region at sunrise. One-hour 

increments were calculated from flow fields generated 

by means of surface streamlines. Figure 4 exhibits the 

backward surface trajectory for ALP. The urban emissions of the populous coastal plain are represented by 

a density plot of emissions of oxides of nitrogen; two 

freeway corridors from the less-populated North 

County can be discerned, as can the large urbanized 

core region. A forward trajectory was also calculated 

for morning emissions downtown. This trajectory was 

consistent, moving morning emissions toward the 

foothills on the sea breeze. Use of the 1-h average winds 

of the SDAQS database, as well as the relatively high 

resolution of the data in the coastal plain, and the short 

transit period would, presumably, minimize uncertainty in the surface trajectory; thus, there is a strong 

suggestion that the surface trajectory originates in the 

San Diego Air Basin in the urbanized coastal region. 

There is, however, significant uncertainty attached to 

the assumption that the surface wind and, accordingly, 

the surface trajectory are representative of the mean 

boundary layer wind, in this case, the marine layer 

overlying the basin throughout the episode. The assumption must be valid from early morning when 

conditions may be stable through midday when they 

may be unstable inland, at least. 

NWS rawinsonde data were inconclusive. Shown in 

Fig. 5, the 1200 UTC (0400 PST) NKX sounding of 

26 September showed southerly winds above 800 mb 

under the influence of the approaching trough; nevertheless, the marine inversion was still surface based, 

winds at and below 850 mb remained northerly, and 

low-level winds were missing. By 0000 UTC, 4 h after 

the ozone peak at Alpine, winds had backed to the 

southwest at all levels, and the base of the marine inversion had risen to 500 m. 

2) DIURNAL PATTERNS OF OZONE GENERATION 

Emissions moving downwind from a local source 

region after sunrise should display an expected pattern 

of ozone generation that reflects the flow field and rate 

of chemical reaction. Figure 6 presents Alpine and 

Oceanside ( OSA ) hourly ozone, monitored against the 

monthly mean hour. Upwind of San Diego's core 

emissions area and 30 km southwest of the Orange 

County line, OSA is a receptor for transport from 

SoCAB, while ALP lies downwind of San Diego urban 

emissions. Mean September concentrations for both 

sites rise after sunrise to an afternoon peak, then decline 

during the night to a background level. On 26 Septem- 
lti 

H 

FiG. 3. Synoptic maps for (a) 500 mb 0400 PST 22 Septe 

(c) 500 mb 0400 PST 26 September 1989, 

i 

1868 JOURNAL OF APPLIED METEOROLOGY VOLUME 34 

ber, OSA ozone remained low, generally following the 

expected profile. In contrast, ozone at ALP rose gradually after sunrise but nearly doubled in I h before 

reaching its 0.17-ppm maximum in the next. Rather 

than evidence of a local episode, this is an anomaly 

that hints of a sudden event: the arrival of some ozone 

"front" or fumigation of pollutants aloft. 

Alpine ozone reached 0.17 ppm at 1200 PST 

(1-h average, 1200-1300 PST ). This site is 42 

km east of the downtown San Diego urban corridor, at 604 m MSL. Hourly ozone concentrations 

at the 10 SDAB network sites are presented in Table 

3. No other site exceeded the federal 'standard, although Escondido (ESC) (0.12 ppm), El Cajon 

( ECA )( 0.10 ppm), and Ramona ( RAM )( 0.12 ppm) 

exceeded the state ozone standard. These are all in 

inland areas known for high afternoon temperatures. Like Alpine, Ramona is in the foothills at 

424 m MSL. j 

Station data were reviewed with regard to site location, hourly temperature, and behavior of the sea 

breeze. Alpine ozone nearly doubled iri the hour in 

which the maximum temperature w'as observed, 

33.3°C, while the mean stood at 32°C for 2 h. Maxi- 
AUGUST 1995 BIGLER-ENGLER AND BROWN 1869 

FiG. 4. Surface trajectory for a parcel of air that moves from urban 

emissions sources to Alpine on 26 September 1989. The surface trajectory assumes observed surface winds are representative of the mean 

boundary layer wind. 

mum temperatures of 32°-35°C were observed in the 

same hour at ESC, ECA, and RAM. Unfortunately, 

ozone data are missing for El Cajon for this hour. At 

Alpine, concentrations exceeded the state standard for 

an extraordinary 7 h, four of them 0.14 or more. At 

Escondido, ozone fell in the 1100-1200 PST hour as 

the sea breeze arrived and the temperature dropped 

more than I°C. Ambient temperature began to recover 

in the next hour, and ozone nearly doubled to 0.11 

ppm. At ESC and RAM, the episode lasted 5 h; concentrations finally declined to moderate levels in the 

evening, as temperatures dropped below 27° or 28°C. 

In contrast, ozone remained low at coastal sites near 

sea level [ OSA, Del Mar ( DMR ), Downtown ( DTN ), 

Chula Vista (CVA)], where the maximum was near 

24°C. At 219 m MSL on the windward side of the 

mountain, the 0.09 ppm recorded at Mount Soledad 

was twice the peak hour monitored at the other coastal 

sites, 200 m below. In the coastal plain with moderate 

temperatures less than 30°C, the ICMA ( 13 5 m MSL) 

peak was 0.08 ppm. 

Ozone transported from SoCAB at low levels will 

cause high concentrations at one or more SDAB 

coastal sites, most frequently OSA and DMR, impacting additional sites successively as the "cloud" 

or "front" advances southward and inland. Concentrations on 24 September 1989 ( Table 2) exhibit this 

pattern. The dramatic increase in ozone concentrations and duration at inland sites during intense surface heating, the low concentrations near sea level 

at coastal sites, and an increase in concentration at 

higher elevations on 26 September, however, suggest 

an episode in which vertical processes and pollutants 

aloft are significant. 

c. SDAQS data analysis 

The second 3-day intensive sampling program of the 

extended episode began Monday, 25 September. Early 

morning aircraft sampling began shortly after 0500, 

ending about 0830 PST. Afternoon takeoffwas at 1400 

and landing at about 1700 PST. Data are available for 

both aircraft early on 26 September but only for the 

Aero Commander on the evenings of 25 and 26 September. Sampling included ambient temperature as 

well as continuous ozone and reactive organic gases 

( ROG ) to permit additional ozone sampling and ROG 

grab samples, for example, near the base and within 

the marine inversion. Logistical and technical problems 

(e.g., limited number and location of samples, lack of 

reproducibility in laboratory analysis, etc.) precluded 

use of most of the ROG data, and they will not be 

discussed further. Soundings were taken at approximately 0500, 1000, 1400, and 2100 PST at Nimue, 

( NIM )( offshore ), Oceanside, Otay Mesa ( OTM ) 

(known also as Brown Field), Escondido, and Flinn 

Springs ( FLN ) . 

1) SDAQS UPPER-AIR DATA, 25-26 SEPTEMBER 

1989 

(i) Evening, 25 September 1989 

The soundings taken 25 September show a strong 

marine inversion and northerly flow. Each of the five 

700 

1 ~ 

750 2500f ~ 

Jj ~ 

800 2000 ,r ~ 

850 1500 ~ 

900 1000M 

~ 

950 ~ 

500 -~ 

MB m, m 

MSL 1 ~ 

20- 30 am Wind pm 

04PST - - - 16PST_ Temp,°C 

FIG. 5. Miramar (NKX) sounding 26 September 1989. 
1870 JOURNAL OF APPLIED METEOROLOGY , VoLUME34 

a ❑ ALP'observed ALP mo. mean 

0.18 

0.16-n 

0.14 

0.12 

ppm 0.1 

0.08 

0.06 

0.04 

0.02 

0 

FDJR 00 Ot 02 03 04 05 06 07 08 09 10 1112 13 14 15 16 17 18 19 20 2122 23 

0 

ppm 

~ 

FIG. 6. Comparison of hourly observed ozone versus monthly mean hour ozone for 26 September 1989 for (a) Alpine and (b) Oceanside 

evening soundings taken between 2030 ( ESC) and 2127 

PST ( NIM ) shows a shallow surface layer 1-200 m 

deep. The surface temperature range from 20 °(OSA) 

to 24 °C (FLN) reflects surface influences ranging from 

cool sea surface temperatures to radiative cooling inland; otherwise, the Nimue sounding is representative 

of all five, showing a consistent structure for the marine 

layer overlying the basin from coastal waters to mountain crests 90 km east of the coastline. Figure 7 presents 

two soundings at Nimue, 20 km west of Del Mar. The 

Nimue sounding at 2127 PST 25 September shows 

continued northerly flow from the surface to more than 

1500 m aloft and moderate winds of 5-7 m s -' within 

the inversion. Winds above the stable layer drop off to 

1-2 m s-'; they increase and become southeasterly 

above 2300 m AGL. 

(ii) Early morning, 26 September 1989 

Near dawn at 0502 PST, the inversion is surface 

based at all sites, the surface temperature near 18 °C. 

At Nimue, the top of the strong inversion is found at 

about 500 m; it slopes gradually upward to near 700 m 

AGL inland. Whether at Nimue at sea level or 30 km 

inland at Flinn Springs at an elevation of 202 m MSL, 

the profile is remarkably similar, with a stable layer 

that continues to around 1200-1300 m AGL. Surface 

winds at several inland sites show light (1 m s -' ) easterly or southeasterly components due to nighttime 

drainage, but northerly winds continue within the inversion to 4.0 m s-' . Winds peak at 4.6 m s -' near 

the top of the stable layer, which is nearly isothermal 

at Nimue, a suggestion of the California coastal jet described by Bridger et al. (1993). 

( iii ) Midmorning, 26 September 1989 

The marine layer dominates the San Diego Air Basin. The soundings taken about 1000 PST and 1400 

PST are presented as cross sections of the basin. This 

means of presentation discloses the essential structure 

of the marine layer, which maintains its integrity despite terrain effects and intense heating, even 50 km 

east of the ocean. Figure 8 demonstrates the plane of 

projection, roughly perpendicular to the;coastline. Interestingly enough, this plane also approximates a cross 

section through the northerly nighttime flow field previously discussed. Figure 9 presents soundings taken 

between 0930 and 1004 PST for Nimue, Oceanside 

(37 m MSL), Escondido (197 m), and Flinn Springs 

(202 m MSL) from the surface to 1200 'm MSL. The 

surface at Oceanside has been slow 6 warm, even 

compared to the boat; the inversion rexnains surface 

based. A shallow sea breeze can be seen and there is a 

northerly component within the inversion and deep 

stable layer to at least 900 m MSL. The top of the 

inversion, at 500-600 m MSL for both the offshore 

NIM and coastal OSA, is near 28 °C. It retains its character as a deep, nearly isothermal layei of 28°C to 

nearly 1300 m MSL, despite the higher mixing heights 

due to surface temperatures of 32 ° -35' °C (versus a 

temperature of 18°C at OSA). The inversion slopes 

upward gradually with increasing terrairi until the intense heating of the more arid inland aieas produces 

a dramatic gradient in mixing heights pa'st Escondido. 

It is in or near this hour that maximum !temperatures 

were observed through most of the suiface network 

and, soon after, that ozone rose dramatically. 

( iv ) Afternoon, 26 September 1989 ; 

i 

Figure 10 displays the afternoon profiles along with 

ozone measured by aircraft. By 1400 PST, the marine 

inversion has lifted and weakened in response to the 

approaching trough; winds have backed to the west 

within the inversion, cutting off any poteritial for ozone 

and precursor transport. The peak heating and temperatures of the day are past. Cooling of about 2°C 

can be discerned in the inversion andl above in all 

soundings. Over the ocean and along the coast, the 

inversion is elevated and a little weaker in response to 

the cooling of the air mass. Inland, surface; temperatures 

are still very warm, and the structure and slope of the 

inversion are still indicative of the intense heating that 

has occurred; nevertheless, the strength of the marine 

layer is such that the inversion is already becoming 
ro 

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reestablished. Winds have become westerly aloft. ( In 

both 1000 and 1400 PST soundings, the temperature 

lapse becomes nearly adiabatic, and winds above 1200- 

1300 m are southerly to the end of the release, about 

3000 m MSL.) 

2) SDAQS AIRCRAFI' DATA, 25-26 SEPTEMBER 

1989 

The Aero Commander 680 aircraft collected ozone 

data by means of a Dasibi 1003-AAS sampler. This 

aircraft flew spirals to 1500 m MSL over each of the 

SDAQS sites (in order: CVA, OTM, ECA, ALP, RAM, 

ESC, OSA ) to locate the inversion and gather temperature, precursor, and ozone data over the basin. 

Equipped with a Scintrex LOZ-3 ozone sampler, the 

Cessna 340-A established boundary values of ozone 

and precursors for the boundaries of the modeling domain: offshore (west), SoCAB (northwest), ridgeline 

( east ), and Mexican border ( south ). Flying in a terminal control area prevented sampling much below 

1400 m over land. Descent to low levels could only be 

accomplished where the aircraft could make an approach to an airport. Offshore, continuous ozone samples were taken to 1-200 m MSL. 

( i) Afternoon, 25 September 1989 

The Aero Commander made the only flight. Between 

takeoff at 1403 and landing at 1648 PST, the aircraft 

reported relatively uniform concentrations of 0.05-0.08 

ppm, except a layer of 0.10 ppm (1615 PST) found 

over Escondido at 788 m MSL, the lowest altitude allowed before the aircraft was required to climb out. 

(ii) Morning, 26 September 1989 

From takeoff at Montgomery Field (MYF) at 0507 

PST, the Cessna climbed to the southwest boundary 

of the domain. A spiral from 1300 m down to 229 m 

MSL found ozone concentrations of 0.06-0.07 ppm. 

At 0600, the plane turned eastward, climbed from 250 

m MSL over the ocean to about 1400 m MSL along 

the Mexican border, then turned northward along the 

mountain ridgeline. At 0630, at 1484 m, 0.09-ppm 

ozone was monitored north of Alpine. The plane 

turned westward, to Nimue, and descended over the 

water to 90 m(0.05 ppm) and the western boundary. 

The next leg climbed northeastward back to Oceanside. 

In level flight over Oceanside at 0734 PST, a band of 

ozone 5-10 km wide was found (0.10 ppm, 1294 m 

MSL); concentrations around 0.08 ppm continued to 

the Riverside County line. High concentrations (0.11 

ppm) were again encountered on the descent back to 

OSA and for 15 km over nearby waters at 506 m(0748 

PST). Finally, the aircraft retraced its path, first to the 

western boundary then back to the coast, this time at 

an altitude of about 200 m MSL. Ozone on the order 

of 0.06 ppm was seen enroute over the water until 

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° 1872 JOURNAL OF APPLIED METEOROLOGY VOLUME34 

F7G. 7. Ntmue sounding for (a) 2127 PST 25 September and (b) 0502 PST 26 September 1989. 

landing at MYF at 0831 PST. Surface stations monitored low ozone concentrations of 0.02-0.03 ppm at 

this time. 

Expected "clean" background ozone is 0.04-0.05 

ppm; ozone aloft of 0.10-0.11 ppm is a significant 

finding given that, just 2 h after sunrise, photochemical 

reactions have just begun and will continue for a number of hours. 

(iii) Afternoon, 26 September 1989 

The Aero Commander took off from Montgomery 

Field at 1412 PST. Spirals were flown over SDAQS 

sites beginning at Chula Vista for periods of 2( ESC ) 

to 12 (CVA, ECA) min. Longer, more detailed spirals 

were possible near airport sites (OSA, MYF, OTM, 

ECA, RAM ) than over Escondido, where the plane 

was required to remain above 800 m MSL. Where possible, additional ozone data were collected at the base 

and within the inversion as identified by airborne temperature observations. Spirals were flown over SDAQS 

sites (start time, PST:altitude, m MSL) as follows: CVA 

(1439:1461-529 m); OTM (1455:165-922 m); ECA 

(1519:222-941 m); ALP (1536:1488-769 m); RAM 

(1548:368-1056 m); ESC (1608:1269-870 m); OSA 

(1618:1289-37 m). At 1633 PST, the plane climbed 

to 750 m to head back to MYF, encountering the most 

concentrated ozone a few minutes later (0.22 ppm at 

450 m MSL) just north of the airport. 

In Fig. 10, the afternoon spiral data are presented 

with the sounding data for 1400 PST. Mean terrain 

increases from left to right from Nimue at sea level, 

eastward, and inland; elevation increases more rapidly 

past Alpine, which at 604 m is above the level of the 

mean terrain. Ozone is plotted at the station over which 

I 

the spiral was flown. The aircraft sampling time is 1 

( OTM ) to 3 h( OSA ) after the 1400 sounding. While 

little ozone is seen in the south ( 0.04-0.08 ppm above 

CVA and OTM), high concentrations are found near 

the base and witfiin the inversion over all of the remaining sites from mid- to north county' OSA, MYF, 

ECA, and ALP. Southward from Oceanside (0.11-0.15 

ppm, 300-400 m MSL), (0.19-0.22 pprri, 350-500 m 

MSL ) to MYF, and eastward to ECA ( 0.1' 1-0.14 ppm, 

400-800 m MSL), there is evidence of a large pool of 

ozone over the basin. The layer of significant ozone 

clearly slopes upward with terrain, following the upward trend of the inversion base showri in the four 

afternoon temperature soundings. The he

I

ight at which 

i 

FiG. 8. Projected cross section for SDAQS stations. 
AuGUST 1995 BIGLER-ENGLER AND BROWN 1873 

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FIG. 9. SDAQS sounding data 26 September 1989 for 

(a) vertical temperature profiles and (b) winds aloft. 

the high concentrations reside is directly related to the 

location of the inversion. 

Extreme concentrations remain over the coastal zone 

from north to midcounty (0.15 ppm at OSA to 0.22 

ppm near MYF) an hour before sunset. Mixing heights 

were, at most, a few hundred meters in this region. 

Due to further cooling during the evening and the already strong marine inversion, no mechanism exists 

to bring these pollutants to the surface. As shown in 

Table 3, surface ozone maxima were 0.06 ppm or less 

at all coastal sites: DTN, OSA, DMR, and CVA. It is 

important to note that coastal mixing heights remained 

low throughout the day; thus, flow within the extremely 

stable marine layer aloft (500 m MSL) remained decoupled from processes near the surface. 

Hot inland temperatures led to higher mixing heights 

and erosion of the base of the inversion. Concentrations 

of ozone aloft at.Ramona and Alpine continue above 

ambient standards but are less than the extremes sampled aloft near the coast within the next hour. Interestingly, they are comparable to the concurrent surface 

observations. At RAM and ESC, surface ozone has declined little from the day's maximum at this hour, and 

surface temperature has fallen only 1°C. The Alpine 

spiral, flown 3 h after the 0.17-ppm surface peak, still 

shows 0.13 ppm aloft. At the same time, 1500 PST, 

surface temperature has fallen 3°C from the day's high, 

and ALP surface ozone at 0.13 ppm is in the fifth hour 

of a 7-h episode. Concentrations fall from 0.11 to 0.06 

ppm in the 1800-1900 PST hour as the surface temperature finally cools to 25°C (approximately, the 

diurnal maximum in the coastal area). 

S. Conclusions 

The uniformity of observed concentrations between 

the surface and reestablished inversion at Alpine and 

Ramona, and ECA ( aloft ) and ESC ( surface ) observations, is suggestive of mixing of elevated pollutants 

into an expanding surface boundary layer. This mixing 

continues so long as surface sensible heat is sufficient. 

The near-doubling of surface ozone in 1 h at Alpine 

and at Escondido, are anomalous, as is the 5-7-h duration of high concentrations. This leads to the conclusion that fumigation of pollutants from aloft begins 

when the threshold temperature of the polluted stable 

layer is reached. High concentrations are brought to 

the surface at inland sites with high temperatures and 

mixing heights; dilution of concentrated ozone aloft 

occurs in the process. Low surface concentrations in 

the cool coastal zone with extreme coastal ozone levels 

aloft are consistent with a hypothesis of fumigation 

into the surface layer of pollutants transported from 

SoCAB in the stable layer aloft. Lyons' (1975 ) obser- 
1874 JOURNAL OF APPLIED METEOROLOGY VOLUME 34 

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FiG. 10. SDAQS afternoon aircraft and sounding data for (a) vertical temperature profiles, 

(b) winds aloft, and (c) ozone. 

vations of shoreline fumigation in southern Wisconsin, 

of individual power plant emissions (p. 156) as well 

as photochemical pollutants transported aloft from 

Chicago, are pertinent. Although the depth, strength, 

and geographical scale of the marine layer cause fumigation much farther inland than Lyons observed (1- 

5 km), processes in the shallow surface layer and aloft 

are decoupled. Photochemical smog fumigation was 

found at similar levels above the shallow lake breeze, 

and high concentrations from continuous fumigation 

can continue for hours. 

Intensive data have shown that northerly flow 

continued through the night and into the morning 

of 26 September. The northerly wind flow field, persistent until midmorning on 26 September precludes 

any possibility that San Diego was the 'source of elevated ozone and precursors. The northerly flow and 

observations of concentrated ozone within the inversion, seen as high concentrations from 0.13 to 

0.22 ppm over the air basin, give evidence of a broad 

pool of ozone transported southward from SoCAB 

in the stable layer aloft into San Diego County. The 

25-26 September 1989 episode is actually a complex 

transport episode unsuitable for photochemical 

modeling. 

Trajectory analysis is routinely usedl to establish 

accountability for source emissions in regulatory 

programs. Use of finely resolved data of observations 

in a single diurnal cycle of transport should minimize 

uncertainty but may give false confidence in re- 
AUGUST 1995 BIGLER-ENGLER AND BROWN 1875 

FIG. 11. Trajectory analyses for air parcels at the surface and aloft. 

The surface trajectory is as in Fig. 4. The trajectory aloft is based on 

SDAQS intensive winds aloft. It represents the path of a parcel trapped 

within the base of the marine inversion. 

sults. Reliance on the surface trajectory in Fig. 4 

would improperly attribute the 25-26 September 

episode solely to San Diego emissions sources. Figure 

11 presents a new trajectory along with the original. 

Prepared using the SDAQS soundings, it represents 

flow within the base of the inversion on 26 September 

1989. This trajectory, for a parcel reaching Alpine 

at the maximum hour, originates off the southern 

coast of Orange County ( part of SoCAB, a source 

region of 6000 km-2early on 26 September). Since 

the Los Angeles source region is several hundred kilometers long, there are many possible regional-scale 

trajectories, but no two-dimensional trajectory can 

adequately describe the episode. The example trajectories serve to demonstrate that flow within the 

marine layer aloft may be independent and very 

different than that determined by surface analysis. 

Routine wind and air quality network data sampling 

inadequate to satisfy needs for source attribution. Even 

aircraft data provides limited temporal and spacial resolution data, at best, and is prohibitively expensive. 

Inferences can be made about the presence of transported species, but neither extent nor magnitude can 

be quantified. Acquisition of both continuous wind and 

air quality information aloft by remote sensing techniques is needed. 

Inaccurate attribution resulting from oversimplification and erroneous assumptions can have serious consequences on air quality programs and communities in downwind air basins. Control programs 

can force expensive regulatory burdens on local 

communities without accompanying air quality 

benefit. 

Acknowledgments. SDAQS was supported by a grant 

from the California Secretary of Environmental Affairs, 

as part of the Coastal County Offshore Energy Assistance Program. Field study project management was 

provided by ERC Environmental and Energy Services 

Company. Project manager for SDAPCD was Mahmood Hossain. 

Many thanks are due Barry Speer for his many hours 

of assistance. The authors also wish to express their 

appreciation to the reviewers whose comments greatly 

improved this paper. 

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