Air quality study of high ozone levels in South
California
Amela Jeričević1, Darko Koračin2, Jinhua Jiang2, Judith Chow2, John Watson2,
Eric Fujita2 and Hiroaki Minoura3
1
Croatian Meteorological and Hydrological Service, Gric 3, 10000
Zgreb, Croatia.
2
Desert Research Institute, University and Community College System
of Nevada, P.O. Box 60220, Reno, NV 89506, U.S.A.
3
Toyota Motor Engineering & Manufacturing North America, Inc., 1555
Woodridge Ave, Ann Arbor, MI 4810, U.S.A.
Abstract
Physical and chemical characteristics within the two distinct meteorological regimes, i.e., convective and stable atmospheric conditions in a
complex highly urbanized terrain of the California South Coast Air Basin
(CSCAB; the Los Angeles area) were investigated. The Community Multiscale Air Quality (CMAQ) model was used with a horizontal resolution of
5 km x 5 km to produce the 3D fields of pollutant concentrations. Input
meteorological fields were obtained by the MM5 numerical weather prediction model while the input emissions were provided by the Californian
Environmental Protection Agency. Modeled meteorological surface parameters and their vertical profiles as well as modeled planetary boundary
layer heights (PBL) were compared to the corresponding measurements.
The CMAQ simulations of ozone concentrations were compared against
the relatively large number of measurements from the CSCAB area. The
main goal of the research was to identify the governing atmospheric processes and sources in the coastal area that contributed to the high levels of
pollution and to investigate the air quality model's capabilities to simulate
the air quality in the complex topography.
Introduction
The California South Coast Air Basin (CSCAB; the Los Angeles area)
is known for its air pollution problems, especially for photochemical smog
episodes. There has been a considerable amount of studies of air pollution
2
in California (e.g., Blumenthal et al. 1977; Farber et al., 1982; Roth et al.,
1984; Luria et a. 2005). Frequently observed high levels of pollutants occur due to elevated anthropogenic emissions as well as due to special air
circulation patterns which drive the pollutants complex temporal and spatial distribution. In this work an episode of high ozone concentrations that
occurred in July 2005 was analyzed. CMAQ model was applied in order to
investigate spatial and temporal distribution of pollutants and advantages
and limitations as well as recommendations for future work are presented
in this paper.
Results
Ozone observations in California South Coast Air Basin
Measured hourly ozone concentrations from 21 stations in the domain
during July 2005 are used for investigation. Due to specific meteorological
conditions spatial and temporal distributions in surface ozone concentrations were not uniform especially during the episode of high ozone concentrations between 15 and 20 July 2005. Areas with different observed
ozone levels were identified. Daily distributions of measured and modeled
hourly ozone and NO2 concentrations at different stations in CSCAB domain during July 2005 are shown in Fig.1.Generally the highest ozone
concentrations are observed on the stations inland in the eastern part of the
Los Angeles (e.g., station 2077 in Fig. 1.) while the lowest levels are present at coastal stations on the west part (e.g., station 2937 in Fig. 1.). Medium levels with characteristic urban patterns are visible on 4 stations situated in the domain centre (e.g., station 2898 and 2160 in Fig. 1.). For most
of the stations measured ozone concentrations are characteristic for urban
areas being the highest during the daytime when chemical production of
ozone occurs in presence of NOx and sunlight and low nighttime ozone
levels resulting from titration with NOx. The ozone July episode was most
intensive in the inland stations on the eastern part of the Los Angeles while
at coastal stations on the west part ozone levels were low and without high
daily variations.
3
2077
2898
20
20
15
15
10
10
5
5
ppm
0.135
0.12
0.105
0
5
10
15
20
25
30
0
5
10
2160
15
20
25
30
2937
20
20
15
15
10
10
5
5
0.09
0.075
0.06
0.045
0.03
0.015
0
0
5
10
15
20
25
30
0
5
10
15
20
25
30
Figure 1. Daily distribution of measured hourly ozone concentrations at
different stations in CSCAB (California South Coast Air Basin) domain
during July 2005.
CMAQ simulations
Surface high pressure system with subsiding air resulting in stable atmospheric conditions was present over the sea and in the coastal area
while low pressure areas in the central and eastern part of the domain,
where high ozone concentrations were measured, was prevailing during
the episode (not shown). Surface wind speeds were generally low, especially during the night, with prevailing west direction. Modeled planetary
boundary layer heights (PBL) were uniformly low during the nighttime
while during the daytime in the southwest and central part of the domain
low PBL values were present and northeast and east parts of the domain
had significantly higher PBL values (not shown). Areas with higher daytime PBL hight corresponds to the low pressure systems and high measured ozone levels were characterized with unstable, turbulent processes
and intensive and mixing in the atmosphere.
4
Intercomparison of the measured hourly ozone concentrations to the
modeled ozone and NO2 values for different stations in the CSCAB (California South Coast Air Basin) domain are shown in Fig. 2. Generally model underestimates measured ozone values but model showed an ability to
reproduce daily variations and high correlation factors ~ 0.7 are found for
most of the stations. Also boundary conditions used in CMAQ simulations
were provided using profile option. However, a test with different boundary conditions (not shown) using the nesting option into a lower resolution
CMAQ run on the larger domain showed a much better agreement with the
measurements indicating that significant contribution to the high ozone
levels was originating from the larger Californian area .
Conclusions
An application of CMAQ air pollution model was conducted on an
ozone episode occurred in July 2005. It is found that model was able to reproduce complex physical and chemical phenomena within the two distinct
meteorological regimes, i.e., convective and stable atmospheric conditions
in a complex highly urbanized terrain of the California South Coast Air
Basin (CSCAB; the Los Angeles area). Due to the fact that the highest
ozone levels were found to be related with unstable conditions and high
mixing in further work special attention should be given to the investigation of the relationship between the pollutant concentrations and turbulence characteristics in PBL including the investigation of existing vertical
diffusion schemes performance in the CMAQ model.
References
Blumenthal D.L., White, W.H. and T.B. Smith, 1977.:’Anatomy of a Los Angeles smog episode:
Pollutant transport in the daytime sea breeze regime’, Atm. Environ. Vol. 12, pp. 893-907.
Farber R.J., Huang, A.A., Bregman, L.D., Mahoney R.L., Eatough D.J., Hansen L.D., Blumenthal D.L., Keifer, W.S. and D.W. Allard, 1982.:’The third in the Los Angeles Basin’, The Sci.
of the total Environ. 23, 345-360.
Roth, P.M., Reynolds, S.D., Tesche, T.W., Gutfreund, P.D. and C. Seigneur:’An appraisal of
emissions control requirements in the California South Coast Air Basin’, Environ. Int., Vol.
9, 549-571.
Luria, M., tanner, R.L., Valente, R.J., Bairai, S.T., Koracin, D. and A.W. Gertler, 2005.:’Local
and transported pollution over San Diego, California’, Atm. Environ., Vol. 39, 6765-6776.
5
0.18
2898
Pomona
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concentrations
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0
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hours
0.18
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2937
Costa Mesa
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ozone_obs
concentrations
ozone_model
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0.12
NO2_model
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12
18
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42
48
54
60
66
72
78
84
90
96
hours
Figure 2. Daily course of measured hourly ozone concentrations (column) and NO2 (black dot) values and modeled ozone (thick black line) and
NO2 (thin black line) values on different stations in the CSCAB (California
South Coast Air Basin) domain during 15 and 16 July 2005.