Atmospheric Environment 39 (2005) 6765–6776

Local and transported pollution over San Diego, California

M. Luriaa,b, R.L. Tannerc, R.J. Valentec, S.T. Bairaic, D. Koracina,

A.W. Gertlera,

a

Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno NV 89512, USA b

School of Earth Sciences, The Hebrew University, Givat Ram Campus, Jerusalem 91904, Israel c

Environmental Applications Department, Tennessee Valley Authority, Muscle Shoals AL 35662, USA

Received 17 December 2004; received in revised form 15 July 2005; accepted 15 July 2005

Abstract

Ten air-quality samplings flights were performed over the San Diego metropolitan area during July 2003. One of the

objectives of these flights was to assess the contribution of the offshore pollution sources (civilian and military vessels)

to the air quality in the region. The flights, which took place during the mid-day hours in the lower levels of the

boundary layer, originated offshore and proceeded eastwards through a series of concentric arcs covering an area of

approximately 600 km2

. The data from the continuous gas analyzers were translated to image plots that enabled the

determination of the impact of the various sources on air quality. The results revealed that the offshore sources could

often be detected (at least duringfive of the 10 flights) from their SO2 plume. However, the area covered by these plumes

and the concentrations were significantly lower than those measured over other parts of the domain due to sources

south of the region. It was further observed that vehicular traffic contributed in a major source way to the formation of

ozone. During most flights the pollution cloud, originating over the high traffic area in the vicinity of the downtown

area, could be tracked ‘downwind’ until the completion of the photochemical processing. By then, most of the NOX had

been converted to NOZ, and peak O3 levels caused by the local emissions had started to ebb. Also, duringmost flights

boundary layer peak levels of O3 exceeded 100 ppbv, and in one case were above 140 ppbv.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Airborne pollution measurements; Ozone; Sulfur dioxide; Carbon monoxide; Nitrogen oxides; Local sources; Transported

pollution

1. Introduction

The metropolitan area of San Diego is the third

largest in California with a population of more than

2,000,000. Although the climate is quite similar to that

of the Los Angeles metropolitan area, some 150 km

north of San Diego, the air quality in the San Diego area

is significantly better, primarily because of different

topography and emission sources. While numerous

violations from the National Ambient Air Quality

ozone standards were reported for the Los Angeles area

duringthe summer of 2001, none were reported

for the San Diego metropolitan area. Usually the

highest hourly maximum values were observed at

the Alpine inland site some 40 km ENE of downtown.

The highest value observed in 2003 was 125 ppbv, and

was the only 1 h maximum value above the level of the

National Ambient Air Quality standard within San

Diego County (California Air Resources Board, 2004).

During2003 there were 17 1 h events in the entire San

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www.elsevier.com/locate/atmosenv

1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.atmosenv.2005.07.051

Correspondingauthor. Tel.: +1 775 674 7061;

fax: +1 775 674 7060.

E-mail address: alang@dri.edu (A.W. Gertler).
Diego County in which the California standard of

95 ppbv was exceeded.

There are no major industrial or utility sources of

pollutants near the Pacific Coast in San Diego County,

and the main local source of O3 producingprecursors

within the metropolitan area is motorized transportation.

Another source of elevated O3 in the region, which was

already identified in the 1970s, is transport from the

greater Los Angeles area (Kauper and Niemann, 1977).

However, the San Diego harbor is one of the nation’s

largest ports serving both civilian and military vessels.

Whereas some of the military vessels are powered by

nuclear energy with no conventional airborne emissions, a

large number of military and civilian ships burn highsulfur heavy oil at rates equivalent to medium size power

plants (100–200MW). Furthermore, since these sources

employ few, if any, pollution control devices, they have the

potential of beingmajor pollution sources to the region,

especially due to the prevailingonshore westerly wind.

The objectives of this study were to measure the

impact of the offshore sources on regional air quality

and to calibrate a comprehensive chemical dispersion

model. The methodology employed is similar to that

used in power plant plume studies (Luria et al., 2000).

Airborne samples were taken duringthe middle of the

day in the lower levels of the boundary layer (BL), when

the BL is well mixed. Each flight consisted of several

concentric arcs with increased radius. The dimensions of

the arcs were designed to cover the entire range of the

photochemical evolution from the sources to distances

beyond, where peak O3 formation rates and concentrations were observed. In this paper we present the aircraft

measurement results, and discuss the processes leading

to air quality impacts in the greater San Diego area.

2. Experimental

The air over the city of San Diego and its vicinity was

sampled duringJuly 2003 usingan instrumentation

package operated aboard a twin engine Twin Otter

aircraft. Except for the CO instrument, the gas analyzers

on board were all manufactured by Thermo Environmental Instruments Inc., Franklin, MA (TEII). Instruments included a Model 43S SO2 analyzer and several

other Model 42 and 42S TEII analyzers that were modified

to directly measure NO, NO2, NOy, NOy

* ( 1⁄4 NOy after

samplingthrough a nylon filter, see Tanner et al., 1998)

and O3. By addingor takingthe difference of signals from

these analyzers, the levels of NOx, NOz and nitrate could

be determined. Carbon monoxide was measured with an

Aero-Laser Model AL5002 vacuum UV fluorescence

instrument with 1 s response time. Other continuous

measurements included location, pressure (altitude), temperature, humidity and particle light scattering. The data

from all continuous monitors were recorded at 1 and 5 s

intervals. The 1 s data were used for the calculated

parameters, whereas the data analysis was performed on

the 5 s data set. The response time of the gas analyzers was

less than 5 s for 90% of the signal. The sampling air speed

was 180 km h1

, thus, the special resolution of the data is

approximately 0.25 km. Additional information on the

instruments and their mode of operation is given in

Table 1. Further details regarding the instruments, their

performance, the samplingmanifold, calibration procedures

and the data system are given in previous publications

(Valente et al., 1998; Luria et al., 1999; Luria et al., 2000).

Altogether five preliminary and ten research flights

were performed. The flights took off from Gillespie

airfield near Santee town ship, located some 25 km

northeast of downtown San Diego. The first segment of

the flight consisted of ascent to 6000 ft (1830 m) ASL

and a constant-level flight in the free troposphere (FT)

to a ‘way point’ over the water, just west of the San

Diego harbor. Over this point the aircraft spiraled down

to an elevation of 300 m ASL. The ‘way point’ was the

center of a series of 9–11 concentric arcs taken at a

3–8 km interval, inside the BL at an altitude of

approximately 300 m above the surface. Over the low

land and near the coast, the aircraft maintained constant

elevation. Over the mountainous region frequent

changes in altitude were necessary in order to follow

the terrain features. At the end of the last arc the aircraft

climbed back to 6000 ft ASL for the last 15 min of the

flight and then spiraled down over the airport before

landing. The duration of each flight was approximately

5 h of which 4 h were inside the boundary layer. A

typical flight track (Flight 8) is illustrated in Fig. 1 over a

geographical map of San Diego County.

Table 2 lists the 10 research flights, their duration, and

the extent to which data from the various measurements

are available. For each flight, the data from all

continuous monitors were used to construct image plots

that interpolated the data over the geographical domain.

The interpolation was done by the Kriging method using

commercial software (Surfer by Golden Software,

Golden, CO). Since the prevailingwinds were from the

west and since the aircraft moved eastward at a rate that

was comparable with the mean wind speed

(15–25 km h1

), the image plots (see Fig. 2) approximately represent the chemical composition of an air

mass in an approximately Lagrangian fashion as it

travels inland from the coast.

3. Results

3.1. Nitric oxide

Nitric oxide (NO) is the best indicator for fresh

emissions from all combustion sources. Because NO

converts rapidly to NO2 (via its titration reaction with

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6766 M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776
O3), its atmospheric lifetime is rather short, usually on

the order of several minutes, dependingon the amount of

O3 available for the titration reaction. Since mobile

sources are the dominant source of NO (and NO2 rapidly

derived there from) emissions, the observed levels can be

attributed to these sources. In all 10 flights the signature

of downtown traffic emissions is evident. No distinct NO

signature from the offshore sources was observed along

the coastline. Peak NO levels measured in the well-mixed

BL were up to 10 ppb, and they decreased rapidly to

below 1 ppb by the time the downtown plume reached

15–20 km from downtown. Fig. 2a shows an image plot

of the NO plume observed on Flight 7.

3.2. Carbon monoxide

Like NO, CO is also an indication of fresh emissions,

almost exclusively from gasoline-powered motor vehicles. Unlike NO, CO has a very longatmospheric

lifetime (on the order of one month). Thus, it is expected

that if both NO and CO have a common source, their

relative plume concentration images will be similar, but

with the CO plume beingbroader. This results from the

fact that its concentrations decrease as the air mass

travels downwind only by dilution, whereas (NO)

decreases also by conversion to other NOY species.

Fig. 2b shows the CO image from Flight 7, which is

consistent with this hypothesis. Duringthe study, CO

levels in the well-mixed BL peaked at 0.5–0.7 ppm and

decreased to below 0.2 ppm at the edges of the study

domain. There were, however, other cases where the

patterns of the images differed significantly, suggesting

additional transport-related complications to be discussed later.

3.3. Total nitrogen oxides (NOY)

While NOY is a surrogate for all combustion sources,

a comparison between the NOY and the CO images for

all 10 flights shows a close similarity. The similarity

between the images is an indication of a common

dominant source (i.e., motor vehicles). The fact that

NOY plume images coincide more closely to the CO

images than do the NO images is expected, since the

atmospheric lifetime of the entire NOY group is

substantially longer than that of NO alone. NOY levels

in the BL maximized at 30 ppb, usually over downtown, and dropped to below 4 ppb at the edges of the

research domain. An illustration of the NOY image from

Flight 7 is given in Fig. 2d.

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Table 1

List of real time measurements performed on board the aircraft

Instrument Maker/model Principal of operation/

sensitivity

Modification/analysis

method

Parameter to be measured

NOx monitor TEI model 42S Chemiluminescence

0.1 ppb

No converter NO

NOx monitor TEI model 42S Chemiluminescence

0.1 ppb

Xe lamp converter

(NOAA/ARL modified)

NO2 (measured as

NO+NO2)

NOx monitor TEI model 42S Chemiluminescence

0.1 ppb

CO on heated gold

catalyst

NOy

NOx monitor TEI model 42S Chemiluminescence

0.1 ppb

CO on heated gold

catalyst plus Nylon filter

Particle NO3

+HNO3

(measured as NOyNOy

*

)

NOx monitor TEI model 42S Chemiluminescence

0.1 ppb

Operated in reversed mode

for fast response

O3

O3 monitor TEI model 49 Light absorption 0.5 ppb Standard configuration O3

a

SO2 monitor TEI model 43S Pulsed fluorescence

0.1 ppb

Standard configuration SO2

CO monitor Aero-Laser GmbH

Model AL 5002

Laser-induced UV

fluorescence 0.2 ppb

Standard configuration CO

3l—Nephelometer TSI Model 3560 Optical scatteringof

particles

Standard configuration Light scattering coefficient

Temperature and

humidity

Custom built 0.1 1C, 1% T and RH

Location GPS 50 m Longitude, latitude

HeadingMagnetic compass 21

Wind speed and

direction

Aventech CAN

Model AIMMS20

GPS and compass 1 m/s,

101

WDD, WDS

Pressure/altitude Custom built 0.2 mb P, Alt

a

For calibrations only, performed at the base of operations.

M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6767
3.4. Ozone

Peak O3 levels observed duringthis study were in fact

higher than those observed at any of the ground

monitoringstations. This is expected since O3 is

removed by dry deposition near the surface (Van Valin

et al., 1994). In most cases the highest O3 levels were

detected within the domain, with lower levels at the

edges. This observation is consistent with processes

dominated by local sources. However, in a few cases

there were indications that the elevated O3 levels

resulted from transport of sources north of the region

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Table 2

Summary of San Diego Flights—July 2003

Flight (Ft) number Date Flight start–end (Dec. hours) Low level start–end (Dec. hours) Data available

Ft 6 7/7/2003 11.42–16.58 11.78–15.87 All, some CO data missing

Ft 7 7/9/2003 12.28–17.71 12.81–17.00 All

Ft 8 7/11/2003 11.51–17.06 11.99–1620 All

Ft 9 7/13/2003 11.43–16.86 11.83–16.30 All

Ft 10 7/17/2003 11.58–16.93 12.16–16.50 All, limited wind data

Ft 11 7/19/2003 11.50–16.64 12.17–16.12 All

Ft 12 7/21/2003 11.30–16.53 11.84–16.00 All

Ft 13 7/22/2003 11.39–15.64 12.00–15.14 All

Ft 14 7/24/2003 11.42–16.88 11.93–16.34 All

Ft 15 7/25/2003 11.33–16.79 11.88–16.22 All

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Flight 8, San Diego, 11 July 2003

0 20 40 km

SD Harbor

Downtown SD

Fig. 1. Tracks of Flight 8 (11 July) presented over a map of San Diego and vicinity.

6768 M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776
(see Table 3). The differentiation between local and

transported sources is based on the wind direction, the

location of the O3 cloud plume, and the relationship

between the NOY and O3 plumes. Fig. 2f shows a typical

case (Flight 7) where the entire area of excess O3 is

within the domain of the study. Table 3 summarizes the

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2

4

6

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NO CO

NO2 NOY

SO2 O3

Nitrate Visibility (m-1)

Fig. 2. Image plots of NO, CO, NOY, Ozone, SO2 and visibility. All plots are taken from Flight 7 (9 July).

M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6769
O3 information from all 10 flights. The highest values

observed at the air monitoringsite on the ground were at

the Alpine site on July 5 and 8, with a 1 h max value on

both days of 108 ppb (California Air Resources Board,

2004).

3.5. Sulfur dioxide

Because of strict emission controls, SO2 emissions in

California are very low. It was assumed that the SO2

traces from offshore sources could be used to identify

them. Indeed, duringseveral flights SO2 emissions,

which originated in the vicinity of the San Diego

Harbor, were detected as the air mass drifted eastward.

A typical SO2 image is shown in Fig. 2e for Flight 7.

Similar SO2 plumes were detected duringatleast half of

the research flights with peak values of up to 5 ppb (see

Fig. 4 below). Background levels were significantly low,

usually below 0.4 ppb. Unlike other spatially correlated

pollution images discussed earlier, the SO2 images were

quite different. During half of the flights, high levels of

SO2 were measured over a wide area at the eastern edge

of the domain as shown in Fig. 2e. These, relatively high

levels of SO2 were not accompanied by elevated levels of

any other pollutant.

3.6. Visibility

The only means available for estimatingparticle levels

in this study was a TSI Model 3560 3-l light scattering

instrument. The image plots of the light scattered in the

green wavelengths region showed that visibility over

most of the region was good (light scattering

o1  105 m1

, i.e., o10Mm1). The light scattering

image (Fig. 2h) was similar in many ways to the nitrate

image (Fig. 2g), suggesting that particulate nitrate is a

main component of the particulate load. Furthermore,

the lack of any correlation between light scattering and

SO2 at the eastern edge of the domain suggests that the

SO2 cloud plume observed is rather fresh and that

significant conversion to particulate sulfate has not yet

occurred.

4. Discussion

4.1. Evolution of photochemical smog

The evolution of photochemical smogin the San

Diego downtown plume is demonstrated in Fig. 3 for

Flight 11. As shown, there is one distinct source of NO

over the downtown area (Fig. 3a). Similar to the

measurements from Flight 7 (Fig. 2), the NO plume is

very short lived and rapidly converts to more extensive

NO2 and NOY plumes (Fig. 3b and c). The CO trace

(not shown) is again very similar to that of the NOY.

The conversion of reactive NOY species (NOX 1⁄4

NO+NO2) to less reactive species (nitrate, nitric acid

and PANs) occurs quite rapidly as can be seen from the

nitrate image on Fig. 3d when compared with that of the

entire ‘odd nitrogen’ group (NOY, Fig. 3c).

The conversion of NOX to NOZ (NOZ 1⁄4 NOY–NOX)

is illustrated in Fig. 3e, which shows the so-called

‘chemical age’ (NOZ/NOY) of the nitrogen plume

(Trainer et al., 1993). Low chemical ages (o0.4) are

indicative of fresh emissions, while high values (40.7)

indicate that the photochemical processes producing

NOz are nearly complete, and little or no new additional

O3 can be produced (Olszyna et al., 1994). Fig. 3e shows

that the reactive zone is confined to near the downtown

area and it becomes less reactive as the plume travels to

the northeast. O3 is formed as the air mass ages and

reaches its peak level where the chemical age approaches

0.8 (Fig. 3f).

4.2. Transport from the south

The phenomenon of an isolated SO2-rich region,

observed in at least five of the 10 flights, was most

evident on Flight 9. Over this area (an isolated ‘island’,

inland east of 116.5 1C W longitude) SO2 levels exceed

4 ppbv, significantly higher than those observed over the

rest of the domain. The latter were typically below

0.5 ppb, except for a small area covered in plumes from

the coastal sources. The area over which elevated SO2

levels were observed is defined by very sharp borders, as

determined not only from the SO2 image but also by

three other independent measurements—wind direction,

O3 and CO. Fig. 4 shows the images of these four

parameters.

The wind direction in the ‘island’ area is from the

southeast, in contrast to the rest of the domain in which

the airflow is northwesterly (except for the coastal area

where the flow is from the west-southwest sector).

Moreover, at least two other independent measurements

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Table 3

Summary of peak O3 values observed duringthe flights

Flight Peak O3,

ppb

Probable source

Ft 6 120 Local

Ft 7 135 Local

Ft 8 141 Local and transport of O3 from the north

Ft 9 135 Local

Ft 10 133 Local and transport of O3 from the north

Ft 11 88 Local

Ft 12 105 Local

Ft 13 112 Local

Ft 14 104 Local and transport of O3 from the north

Ft 15 104 Local

6770 M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776
(CO and O3) showed similar behavior, with almost

identical geographical coverage. Within the ‘island’, O3

concentrations dropped to below 60 ppb and CO levels

were almost as low as remote troposphere background

levels of approximately 100 ppb.

The ratio SO2/NOY over the ‘island’ is greater than

4.0—which is very unusual for major sources within the

USA. Even in the plume of a power plant that uses highsulfur coal, such as the TVA Cumberland Plant in

Tennessee, a typical ratio of SO2 to NOY is less than 0.1.

This ratio was approximately 2 before the installation of

scrubbers that removed 95% of the SO2 in the mid1990’s (Luria et al., 2003). Because of the location, the

wind direction and the high ratio, it is highly unlikely

that the source for this polluted air is from within the

USA. This source likely originates from a metal refining

operation or similar smelter-type source with limited or

no pollution controls located across the border in

Mexico. The chemical age (NOz/NOy ratio) is relatively

low (lower than observed inside the San Diego urban

plume) suggesting that the source is relatively nearby.

Also, the air samples were taken only at the edge of the

‘island’ leavingthe possibility that higher SO2 levels

prevail further east. In addition, as stated earlier, the low

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NO NO2

NOY Nitrate

NOZ O3 /NOY

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0

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4

6

8

10

12

14

Fig. 3. Image plots of NO, NO2, NOY, nitrate NOZ/NOY and O3 calculated from the measurements performed on 19 July (Flight 11).

M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6771
light scattering in the ‘island’ provides another indication that the SO2 emissions are fresh, and that only

limited conversion to particulate sulfate could have

occurred.

The hypothesis of transport of pollutants from the

south is supported by atmospheric mesoscale simulations for 11 July (Kahyaog ̆lu-Koracˇin, 2004) using

version 3.6.2 of non-hydrostatic Mesoscale Model 5

(MM5) (Grell et al., 1994). Detailed evaluation of MM5

in simulatingcoastal marine atmospheric boundary

layer flows is shown by Koracˇin and Dorman (2001)

and Koracˇin et al. (2004). The model grid consisted of a

coarse grid (centered at 33.41N, 118.41W) with

75  75  35 grid points with a horizontal resolution of

15 km and a nested grid with 61  55  35 grid points

with a horizontal resolution of 5 km. Selected models

physics options include the Grell cumulus parameterization (Grell et al., 1994), mixed-phase microphysics,

parameterization of long- and short-wave radiation

fluxes includingcloud–radiation effects, and the Eta

planetary boundary layer scheme includingthe prognostic equation for the turbulence kinetic energy.

To make sure that the meteorology was well

presented, we evaluated the results of the MM5 model

usingdata from the meteorological stations at San

Diego (32.73N, 117.14W) and Glendale (32.4N,

118.23W), which represents the Los Angeles area. Fig.

5 shows the time series comparison of wind speed and

direction between the observations at these sites and the

model results for 10 July 2003. It is evident from this

figure that, the model predicts the observed temporal

variations of the wind speed and the wind direction at

both locations, its magnitude and most importantly the

shift in wind direction which occurred at 1800 UTC

(1100 local time).

The model results, presented in Fig. 6, show the

estimated wind field pattern for this day at 1700 (UTC

0000 on 12 July) at height of 0.3 km. This figure shows

that at that time the western part of the domain was

under the influence of the westerly sea breeze, which had

a southern component over the city and was purely

westerly north of the city. The sea breeze was blocked at

approximately 40–50 km from the coast by a southeasterly airflow. The latter was shifted southwesterly at

the northeastern corner of the domain. This simulation

is consistent with the observation of ‘SO2 island’ that

penetrated deeper into the middle of the domain as

shown in Fig. 4.

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0

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1

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Fig. 4. Image plots of SO2, CO, O3 and wind direction calculated from the measurements performed on 11 July (Flight 9).

6772 M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776
The phenomenon of air mass transport from the south

is most likely the result of the local topography. The

Laguna mountain chain that is located some 50 km east

of San Diego has peak elevations of nearly 2 km, which

blocks the predominantly westerly sea breeze. The valley

on the other side of the mountain chain Imperial Valley,

is affected by the sea breeze which originates over the

coast of Baja California and has a strongsoutherly

component. Thus, the western parts of the domain are

impacted by the San Diego coastal pollution sources

while the eastern areas are impacted by sources from the

south (i.e., Mexico).

4.3. Transport from the north

The data from Flight 9 seem to show that the region

can be affected not only by transport from the south but

also by sources north of the domain (i.e., Los Angeles).

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Fig. 5. A comparison of wind speed and direction between MM5 and observations on 10 July 2003 at San Diego (upper panel) and

Glendale (lower panel).

M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6773
In all other flights the highest CO levels were observed in

the vicinity of the fresh NO emissions. On Flight 9 the

highest CO concentrations were not associated with

fresh emissions but were co-located with an O3 peak

with high chemical age (NOZ/NOY40.8). In this specific

case, it is unlikely that the source of the emissions was

downtown San Diego, and that even after travel times of

several hours the CO levels were still higher than those

observed over the traffic sources near downtown. Fig. 7

shows that the CO image (Fig. 6a) overlaps with the

NOY image (Fig. 6b) and that the ratio (NOZ/NOY,

Fig. 6c) is rather high (40.8) indicatinga photochemically aged air mass. The only feasible explanation is that

the air in this region has mixed with another air mass

originating north of the domain. Indeed, the onboard in

situ wind measurement (Fig. 3d) indicates that the

airflow at this location is from the north-northwest

direction, which may suggest transport from the Los

Angeles metropolitan area.

The wind fields duringthe flights were also more

closely examined using a Lagrangian random

particle dispersion model (Koracˇin et al., 1998, 1999,

2000). The dispersion model used the wind data

obtained from the MM5 results described earlier

and continuously released particles at a Los Angeles

area source beginning at UTC 1200 (0500 local

time) on 10 July 2003 (about 24 h before the sampling).

The area is bordered in latitude by 33.51N and 34.21N

and in longitude by 117.01W and 118.51W. The

particles were released approximately at the middle of

the atmospheric boundary layer (500 m) covering

various effective plume heights and mixing at the

Los Angeles’ emission sources. This height was selected

to represent the main characteristics of regional scale

transport. Sixty particles were released at each time step

of 60 s from each of the sources and continuously traced

in time and space for the duration of the simulations. As

shown in Fig. 7d, a significant fraction of the emissions,

which originated over the greater Los Angeles basin,

drifted to the south toward San Diego, either along the

coast or inland through the Santa Terresa Valley. This

air mass transport is most likely responsible for the

highest O3 levels observed in this study duringthis

flight, and still retained the signature of high CO

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Fig. 6. Wind field data derived from MM5 model simulations for the San Diego area on 11 July 2003. The arrows represent wind

direction and the length of the arrows—wind speed.

6774 M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776
concentrations, which resulted from transport of vehicular emissions from the LA area.

5. Conclusions

 Offshore sources (commercial and military vessels)

can be detected through their SO2 plume which is

very easily identified due to the low-SO2 regional

background

 The majority of SO2 in the region is transported from

sources south-southeast of San Diego, most likely

from Mexico. The ratio of SO2/NOY suggests

emissions are from sources with limited or no

controls.

 Vehicular transportation is the main source of

precursors leadingto the formation of photochemical

smog in the region. During most flights it was

observed that the entire process of daytime conversion of the fresh NO emissions leadingto the

formation of ozone and NOz (HNO3, nitrate and

PANs) occurred over the metropolitan area within

the domain of this study.

 In several cases, there is evidence of transport of O3

precursors and its precursors O3 from sources in the

Los Angeles area.

Acknowledgment

The authors wish to acknowledge Vince Van Pelt and

David Branscomb for their assistance with the measurements, Dr. John Lewis for providingthe meteorological

forecast before the flights, Dr. Julide Kahyaog ̆luKoracˇin, Travis McCord, and Ramesh Vellore for

performingthe MM5 runs, and Dr. William Stockwell

for valuable discussions. This study was sponsored by

the Strategic Environmental Research and Development

Program (SERDP), under contract CP-1253, Dr. Robert

Holst program manager.

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NO CO Y

0

5

10

15

NOZ/NOY

0.05

0.25

0.45

0.65

0.85

60

110

160

210

260

310

360

32.6

32.8

33

33.2

33.4

-117.8 -117.6 -117.4 -117.2 -117 -116.8 -116.6 -116.4 -116.2

32.6

32.8

33

33.2

33.4

-117.8 -117.6 -117.4 -117.2 -117 -116.8 -116.6 -116.4 -116.2

32.6

32.8

33

33.2

33.4

-117.8 -117.6 -117.4 -117.2 -117 -116.8 -116.6 -116.4 -116.2

Fig. 7. (a–c) Image plots CO, NOY and NOZ/NOY calculated from the measurements performed on 11 July (Flight 8) and (d) the

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