Atmospheric Environment 39 (2005) 6765–6776



# Local and transported pollution over San Diego, California

M. Luria a,b 

## , R.L. Tanner c 

## , R.J. Valente c 

## , S.T. Bairai c 

## , D. Koracin a



## , A.W. Gertler a, 

## 

aDivision 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

cEnvironmental 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 km 2 . 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 SO 2 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 NO X had been converted to NO Z, and peak O 3 levels caused by the local emissions had started to ebb. Also, duringmost flights boundary layer peak levels of O 3 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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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 O 3 producingprecursors within the metropolitan area is motorized transportation. Another source of elevated O 3 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, alarge 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 O 3 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 analyzerson board were all manufactured by Thermo Environmental Instruments Inc., Franklin, MA (TEII). Instruments included a Model 43S SO 2 analyzer and several other Model 42 and 42S TEII analyzers that were modified to directly measure NO, NO 2 , NO y , NO y * ( 1⁄4 NO y after samplingthrough a nylon filter, see Tanner et al., 1998 ) and O 3 . By addingor takingthe difference of signals from these analyzers, the levels of NO x, 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 continuousmeasurements 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 h

1 , 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 h

1 ), 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 NO 2 (via its titration reaction with 

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M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6766O 3 ), its atmospheric lifetime is rather short, usually on the order of several minutes, dependingon the amount of O 3 available for the titration reaction. Since mobile sources are the dominant source of NO (and NO 2 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 NO Y 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, suggestingadditional transport-related complications to be discussed later.

3.3. Total nitrogen oxides (NO Y )

While NO Y is a surrogate for all combustion sources, a comparison between the NO Y 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 NO Y plume images coincide more closely to the CO images than do the NO images is expected, since the atmospheric lifetime of the entire NO Y group is substantially longer than that of NO alone. NO Y 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 NO Y 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

NO x monitor TEI model 42S Chemiluminescence 0.1 ppb No converter NO

NO x monitor TEI model 42S Chemiluminescence 0.1 ppb Xe lamp converter (NOAA/ARL modified) NO 2 (measured as NO+NO 2) NO x monitor TEI model 42S Chemiluminescence 0.1 ppb CO on heated gold catalyst NOy

NO x monitor TEI model 42S Chemiluminescence 0.1 ppb CO on heated gold catalyst plus Nylon filter Particle NO 3+HNO 3 (measured as NOy NOy * ) NO x monitor TEI model 42S Chemiluminescence 0.1 ppb Operated in reversed mode for fast response O 3

O 3 monitor TEI model 49 Light absorption 0.5 ppb Standard configuration O 3 a SO 2 monitor TEI model 43S Pulsed fluorescence 0.1 ppb Standard configuration SO 2

CO monitor Aero-Laser GmbH Model AL 5002 Laser-induced UV fluorescence 0.2 ppb Standard configuration CO

3 l—Nephelometer TSI Model 3560 Optical scatteringof particles Standard configuration Light scattering coefficient

Temperature and humidity Custom built 0.1 1 C, 1% T and RH

Location GPS 50 m Longitude, latitude HeadingMagnetic compass 2 1 Wind speed and direction Aventech CAN Model AIMMS20 GPS and compass 1 m/s, 10 1 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 67673.4. Ozone

Peak O 3 levels observed duringthis study were in fact higher than those observed at any of the ground monitoringstations. This is expected since O 3 is removed by dry deposition near the surface (Van Valin et al., 1994). In most cases the highest O 3 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 O 3 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 -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 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. M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6768(see Table 3 ). The differentiation between local and transported sources is based on the wind direction, the location of the O 3 cloud plume, and the relationship between the NO Y and O 3 plumes. Fig. 2f shows a typical case (Flight 7) where the entire area of excess O 3 is within the domain of the study. Table 3 summarizes the 

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0 2 4 6 40 60 80 100

6.00E-006 1.40E-005 2.20E-005 3.00E-005 3.80E-005 4.60E-005 5.40E-005 0 2 4 6

0 5 10 15 20 90 130 170 210 250 290 330

-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 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 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 0 0.6 1.2 1.8 2.4 3 3.6 4.2

0 0.6 1.2 1.8 2.4 3 3.6 4.2 

### NO CO

NO 2 

### NO

### Y

O 3 

### SO 2



### Nitrate Visibility (m-1)

Fig. 2. Image plots of NO, CO, NO Y , Ozone, SO 2 and visibility. All plots are taken from Flight 7 (9 July). M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6769O 3 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, SO 2 emissions in California are very low. It was assumed that the SO 2 traces from offshore sources could be used to identify them. Indeed, duringseveral flights SO 2 emissions, which originated in the vicinity of the San Diego Harbor, were detected as the air mass drifted eastward. A typical SO 2 image is shown in Fig. 2e for Flight 7. Similar SO 2 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 SO 2 images were quite different. During half of the flights, high levels of SO 2 were measured over a wide area at the eastern edge of the domain as shown in Fig. 2e . These, relatively high levels of SO 2 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

o 1  10 5 m 1 , i.e., o 10Mm 1 ). 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 SO 2 at the eastern edge of the domain suggests that the SO 2 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 NO 2 and NO Y plumes ( Fig. 3b and c). The CO trace (not shown) is again very similar to that of the NO Y. The conversion of reactive NO Y species (NO X 1⁄4 NO+NO 2) 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 (NO Y , Fig. 3c). The conversion of NO X to NO Z (NO Z 1⁄4 NO Y –NO X ) is illustrated in Fig. 3e , which shows the so-called ‘chemical age’ (NO Z/NO Y ) of the nitrogen plume (Trainer et al., 1993 ). Low chemical ages ( o 0.4) are indicative of fresh emissions, while high values ( 4 0.7) indicate that the photochemical processes producing NO z are nearly complete, and little or no new additional O 3 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. O 3 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 SO 2 -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 1 C W longitude) SO 2 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 SO 2 levels were observed is defined by very sharp borders, as determined not only from the SO 2 image but also by three other independent measurements—wind direction, O 3 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 O 3 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 O 3 from the north Ft 9 135 Local Ft 10 133 Local and transport of O 3 from the north Ft 11 88 Local Ft 12 105 Local Ft 13 112 Local Ft 14 104 Local and transport of O 3 from the north Ft 15 104 Local M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6770(CO and O 3 ) showed similar behavior, with almost identical geographical coverage. Within the ‘island’, O 3 concentrations dropped to below 60 ppb and CO levels were almost as low as remote troposphere background levels of approximately 100 ppb.

The ratio SO 2 /NO Y over the ‘island’ is greater than 4.0—which is very unusual for major sources within theUSA. Even in the plume of a power plant that uses highsulfur coal, such as the TVA Cumberland Plant in Tennessee, a typical ratio of SO 2 to NO Y is less than 0.1. This ratio was approximately 2 before the installation of scrubbers that removed  95% of the SO 2 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 (NO z/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 SO 2 levels prevail further east. In addition, as stated earlier, the low 

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NO NO 2



### NOY 

### Nitrate

O 3 

### NO Z

### /NOY -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 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 32.6 32.8

33 33.2 33.4

30 45 60 75

0.2 0.4 0.6 0.8 1 1 2 3 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

0 0.6 1.2 1.8 2.4 3 3.6

0 2 4 6 8 10 12 14

Fig. 3. Image plots of NO, NO 2 , NO Y , nitrate NO Z/NO Y and O 3 calculated from the measurements performed on 19 July (Flight 11). M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6771light scattering in the ‘island’ provides another indication that the SO 2 emissions are fresh, and that only limited conversion to particulate sulfate could have occurred.

The hypothesis of transport of pollutants from thesouth is supported by atmospheric mesoscale simulations for 11 July (Kahyaog  ̆ lu-Korac ˇ in, 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 Korac ˇ in and Dorman (2001) and Korac ˇ in et al. (2004) . The model grid consisted of a coarse grid (centered at 33.4 1 N, 118.4 1 W) 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 modelsphysics 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 Etaplanetary 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 atapproximately 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 ‘SO 2 island’ that penetrated deeper into the middle of the domain as shown in Fig. 4

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0 0.5 1 1.5 2 2.5

40 60 80 100 SO 2 CO

O 3 60 110 160 210 260 310 360

WDD

90 180 270 -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 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

Fig. 4. Image plots of SO 2 , CO, O 3 and wind direction calculated from the measurements performed on 11 July (Flight 9). M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6772The 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 6773In 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 O 3 peak with high chemical age (NO Z/NO Y 4 0.8). 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 NO Y image ( Fig. 6b) and that the ratio (NO Z/NO Y , Fig. 6c) is rather high ( 4 0.8) 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 (Korac ˇ in 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 33.5 1 N and 34.2 1 N and in longitude by 117.0 1 W and 118.5 1 W. 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 O 3 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. M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6774concentrations, which resulted from transport of vehicular emissions from the LA area.

5. Conclusions



##  Offshore sources (commercial and military vessels) can be detected through their SO 2 plume which is very easily identified due to the low-SO 2 regional background


##  The majority of SO 2 in the region is transported from sources south-southeast of San Diego, most likely from Mexico. The ratio of SO 2/NO Y 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 wasobserved that the entire process of daytime conversion of the fresh NO emissions leadingto the formation of ozone and NO z (HNO 3 , nitrate and PANs) occurred over the metropolitan area within the domain of this study. 

##  In several cases, there is evidence of transport of O 3 precursors and its precursors O 3 from sources in the Los Angeles area.

Acknowledgment

The authors wish to acknowledge Vince Van Pelt andDavid Branscomb for their assistance with the measurements, Dr. John Lewis for providingthe meteorological forecast before the flights, Dr. Julide Kahyaog  ̆ luKorac ˇ in, 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.

References

California Air Resources Board, 2004. web site: http:// www.arb.ca.gov/adam/cgi-bin/db2www/polltrendsb.d2w/ Branch

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CO NO Y

0 5 10 15

NO Z /NO Y

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 -116.232.6 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, NO Y and NO Z/NO Y calculated from the measurements performed on 11 July (Flight 8) and (d) the results of a particle dispersion model showing transport of pollution from the Los Angeles area to the San Diego region. M. Luria et al. / Atmospheric Environment 39 (2005) 6765–6776 6775Grell, G.A., Dudhia, J., Stauffer, D.R., 1994. A description of the fifth-generation Penn State/NCAR mesoscale model(MM5). National Center for Atmospheric Research, Technical Note TN-398, 122pp. Kahyaog  ̆ lu-Korac ˇ in, J., 2004. The development and applicationof atmospheric modelingsystems to determine the environmental impact of regional scale and local emissions on complex mountain terrain and coastal-urban areas. Ph.D. Dissertation, University of Nevada, Reno, 135pp. Available from Desert Research Institute, 2215 Raggio Pkwy, Reno, NV 89512. Kauper, E.K., Niemann, B.L., 1977. Los Angeles to San Diego three dimensional ozone study. ARB Contract number A-6- 090-30, available on the web at ftp://ftp.arb.ca.gov/carbis/ research/apr/past/a6-090-30.pdf Korac ˇ in, D., Dorman, C., 2001. Marine atmospheric boundary layer divergence and clouds along California in June 1996. Monsoon Weather Review 129, 2040–2055. Korac ˇ in, D., Isakov, V., Frye, J., 1998. A Lagrangian particle dispersion model (LAP) applied to transport and dispersion of chemical tracers in complex terrain. Presented at the Tenth Joint Conference on the Applications of Air Pollution Meteorology, Phoenix, AZ, 11–16 January 1998. Paper 5B.5, pp. 227–230. Korac ˇ in, D., Isakov, V., Podnar, D., Frye, J., 1999. Application of a Lagrangian random particle dispersion model to the short-term impact of mobile emissions. Proceedings of the Transport and Air Pollution conference, Graz, Austria, 31 May–2 June 1999. Korac ˇ in, D., Frye, J., Isakov, V., 2000. A method of evaluating atmospheric models usingtracer measurements. Journal of Applied Meteorology 39, 201–221. Korac ˇ in, D., Dorman, C.E., Dever, E.P., 2004. Coastal perturbations of marine layer winds, wind stress, and wind stress curl alongCalifornia and Baja California in June 1999. Journal of Physical Oceanography 34, 1152–1173. Luria, M., Valente, R.J., Gillani, N.V., Tanner, R.L., Imhoff, R.E., Meagher, J.F., 1999. The evolution of photochemical smogin a power plant plume. Atmospheric Environment 31, 3023–3036. Luria, M., Tanner, R.L., Valente, R.J., Imhoff, R.E., 2000. The influence of natural hydrocarbons on ozone formation in an isolated power plant plume. Journal of Geophysical Research 105, 9177–9188. Luria, M., Imhoff, R.E., Valente, R.J., Tanner, R.L., 2003. Ozone production yield in a large power plant plume. Atmospheric Environment 37, 3593–3603. Olszyna, K.J., Bailey, E.M., Simonaitis, R., Meagher, J.F., 1994. O 3 and NO Y relationship at a rural site in southeastern USA. Journal of Geophysical Research 99, 14,557–14,563. Tanner, R.L., Valente, R.J., Meagher, J.F., 1998. Measuring inorganic nitrate species with short time resolution from an aircraft platform by dual-channel ozone chemiluminescence. Journal of Geophysical Research 103, 22,387–22,395. Trainer, M., et al., 1993. Correlation of ozone with NOy in photochemically aged air. Journal of Geophysical Research 98, 16,745–16,753. Valente, R.L., Imhoff, R.E., Tanner, R.L., Meagher, J.F., Daum, P.H., Hardesty, R.M., Banta, R.V., Alvarez, R.J., McNider, R., Gillani, N.V., 1998. Ozone production during an urban air stagnation episode over Nashville, TN. Journal of Geophysical Research 103, 22,555–22,568. Van Valin, C.C., Boatman, J.F., Luria, M., Ray, J.D., Aneja, V.P., Blake, D.R., Rodgers, M., Sigmon, J.T., 1994. Air quality over the appalachian region in August 1988: acomparison between aircraft and ground based measurements. Journal of Geophysical Research 99, 1043–1057. 

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