S. LaDochy, D. Fuentes€¦ · S. LaDochy, D. Fuentes Department of Geography & Urban Analysis, California State University, Los Angeles, CA. 90032, USA EM ail: [email protected]

Visibility trends in the Los Angeles Basin,

1933-present

S. LaDochy, D. FuentesDepartment of Geography & Urban Analysis, California StateUniversity, Los Angeles, CA. 90032, USAEM ail: [email protected]

Abstract

This study looks at (1) the historical and recent trends in Los Angeles Basinvisibility; (2) the temporal and spatial variability of visibility and air pollutionlevels; and (3) the relationships between visibility, paniculate levels andmeteorological variables. Visibility has improved in the downtown L.A. areaover the last two decades, after a decline from the 1930's through the early1970's. However, in the inland valleys, both visibility and pollution levels fail toshow similar improvements. A marked shift in the center of maximum frequencyof low visibility occured. In the early 1950's, lowest visibility centered overdowntown Los Angeles, while in the 7CTs, it resided over western SanBernardino County. Paniculate levels show a similar shift. More recently,several air pollution control strategies have led to improved air quality.Visibilities have shown some improvement, but not consistently in time or space.Multi- variate regression models using paniculate and meteorological variablesexplain these variations in visibility. Coastal visibilities correlate higher withmeteorological parameters, especially humidity, while inland values correlatemore with nitrates and sulfates. Warmer temperatures increase pollution anddecrease visibility, though the relationship is not strong.

1 Introduction

Visitors to Los Angeles often comment about the poor air quality of the region.They often base their perception on the low visibility, particularly in the summermonths, when the surrounding mountains often disappear from view. This

Transactions on Ecology and the Environment vol 29 © 1999 WIT Press, www.witpress.com, ISSN 1743-3541

1024 Air Pollution

lowered visibility during the "smog season" is often in great contrast to theclearer air found during the winter or when brisk Santa Ana winds from theinterior sweep the haze out to sea.

As visibility is often associated with air quality, reductions in air pollutantsshould lead to clearer skies and greater visibility. But the relationship betweenthe amounts and types of air pollutants and observed visibility is not linear norsimple. It is not surprizing that impressive reductions in pollutants have notresulted in similar visual improvements. This paper investigates the variedrelationships between visibility values and air pollution levels. The authors alsoexamine the effects of weather variables on visibility, since they also impactpollution amounts and distribution. Spatial and temporal variations for the L.A.Basin are also addressed as relationships differ from coastal to inland valleylocations. They have also changed over time and with various air pollutioncontrol strategies implimented over recent decades.

Historically, the Los Angeles Basin has always been known for its hazyconditions. A combination of geography and climatology lead to high airpollution potential for the Basin, as sea breezes and lofty mountains minimizehorizontal dispersion of pollutants, while prevalent subsidence inversions limitvertical mixing. With an average of only 26 rainy days a year for downtown L.A.and an abundance of warm, sunny weather, photochemical smog is rarely absent.Then, with a spread of some 14 million Angelenos, most of whom rely onprivately-owned automobiles for transportation, you have the makings of themost polluted city in the country.

In studies of visibility trends, two indicators are most often used, the visualrange and extinction coefficient. The latter measures the percentage of light-absorbing and light-scattering caused by atmospheric constituents. Although thismeasures atmospheric clarity and relates to pollution levels, there is a relativelyshort record of measurements in the Basin and not good spatial coverage.Therefore extinction coefficients were not used in the study's analyses.

Visual range is a measure of how far an observer can see landmark objectsclearly, usually in miles, over at least one-half of the horizon. Fortunately, thereis a long record of visual range observations available from Basin airports anddowntown L.A., giving both an adequate spatial and temporal coverage for thisstudy. There are drawbacks to using visual range [1]. Values are not directlyrelated to pollution concentrations, as scattering and absorption differs frompollutant to pollutant, from vapors to particulates. Observers are also influencedby color and contrast, daylight and night, and there tends to be bias towardsreporting numbers divisible by 5. Objects between 7.1 and 10 miles are recordedas 10 [10]. More importantly, visibility is effected by high humidities, especiallyfrom fog and rain. For visibility trend studies to evaluate anthropogenicinfluences, only data with relative humidities below 70% (V70) are used. Otherways to eliminate year-to-year weather abnormalities is to use 5-year runningmeans to evaluate trends. In this study, visibility data with less than 70%relative humidities and for all humidities are evaluated, to examine differencesbetween moist coastal regions and drier interior sites.


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2. Data

Visual range data were gathered from six airports for the years, 1970-1995,including two coastal locations, Los Angeles International Airport (LAX) andLong Beach Airport (LGB). The remaining interior locations in order ofincreasing distance from the coast include Burbank Airport (BUR), OntarioAirport (ONT), Riverside (RIV-March Air Force Base) and San Bernardino(SBD-Norton Air Force Base). 1988, 1989 data was missing for BUR, ONT andRIV, however.

Paniculate air pollution data used in this study includes TSP (total suspendedparticulates) from 1970-1995, PM10 (paniculate matter having mean diameter of10 microns or less) and PM10 Nitrates and Sulfates, 1985-1995. Meteorologicalvariables include relative humidity, surface and upper air temperatures, windspeed and direction, height of inversion base and top, and solar radiation, for the1970-95 period. Upper air temperatures and inversion heights were obtainedfrom radiosonde measurements taken at Loyola Marymount University, near

Figure 1: Visibility study area showing relevant data locations.


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LAX. In earlier years, radiosondes were released from LAX, and then brieflyfrom UCLA, before moving to Loyola.

Figure 1 shows the location of the six airports whose visibilities areused in this study along with current air monitoring stations within the SouthCoast Air Basin and the radiosonde launching site at Loyola MarymountUniverity (LMU).

3 Early visibility studies

In the early years of the L.A. County Air Pollution Control District, Ralph Keith[11] examined the trends in visibility for the L.A. downtown location at noon,when relative humidities and visibilities were generally lowest. Keith foundthat visibility had decreased from 1933 to 1969, with mean values dropping from12.6 miles for the 1933-41 period to 8.3 miles for 1960-68, and still lower to 7.9for 1969. He noted that excellent visibility, over 35 miles, also fell from 25 daysper year for the earliest period to 4 days, 1960-68, with only 1 day beingexcellent in 1969 (figure 2). A later study by Keith [12] looked at visibilitytrends in the L.A. Basin from 1950 through 1977. Keith evaluated seven stations'trends using the number of days with daily minimum visibility less than threemiles with relative humidities under 70%. He found that the four westernstations (LAX, LGB, PLX, downtown, and BUR) showed improved visibilityaccording to days with low visibility, while the three eastern locations (ONT,RIV, SBD) indicated either no improvement or increased low visibility days.LAX showed the most improvement, while RIV had the most deterioration overthe period. The study also found that the center of maximum frequency of lowvisibility days had shifted. In the early 1950's, the center was over central LosAngeles, but by the 1970's, it resided over western San Bernardino county.Davidson, et al, [6] explained the worsening visibility levels at Riverside from1960-1978 as being associated with the explosive growth in Orange Countyduring this period. Emission controls since the 1960% had also led to slowerreactivity of photochemical pollutants present in typical early morning pollutionbuildups in the heavily traveled coastal and central portions of the Basin.Prevailing sea breezes could then move this smog cloud inland past coastal andcentral locations before photochemical reactions proceeded too far. Davidson [5]used 3-year moving averages of the per cent days with minimum visibility(V70), less than 3 miles, for the 1960-1982 period. He also found LAX to havestrong improvement from 1960 through 1977, but then found small degradationafter that to the 1980-82 period. LGB showed little change in the 1960's, butimproved afterwards. Davidson also ran multiple variable regression equationsto explain variations in visibility using TSP (total suspended particulates),sulfates and nitrates, for the years 1978-81. He found the relationships to bestatistically significant at 95%, except for TSP at RIV. His correlation coefficientfor the Basin was r=.823.


Air Pollution 1027

Horie [10] and Farber, et al [7,8] studied visibility trends for the years 1970-1987. While other studies proposed sulfates as a main culprit in visibilityreduction, due to its efficient light-scattering properties [2,4], Horie showed thatother factors, such as nitrates and carbonaceous particles are also partlyresponsible. Horie and Farber, et al, found that even with a reduction of 75% inSO2 emissions between 1973 and 1987, sulfates were not as dramaticallyreduced, and visibility trends for the six airports did not show any obvious trend,but rather only year-to-year fluctuations. Horie did find that temporal patterns ofbasinwide visibility resembled that of the basinwide sul fate concentrations.Farber, et al [8] found that during the past twenty years, visibility was slowlyimproving throughout the Basin due to the reduction in the components of haze.Lewis and Davidson [14] reported that there had been a noticeable overallimprovement in Basin visibilities over the period 1960-1989, using median dailylow visibilities. In the 1980's, these improvements become quite small, withsmallest improvements for inland locations.

4 Methodology and results

In order to account for changes in the overall patterns of visibility over the LosAngeles Basin, we need to assess (1) the long-term trends and variabilities invisibility for the six airports, (2) the concurrent concentrations of paniculatematter, and (3) other contributory factors, such as meteorological variabilityduring this period. And finally, the authors apply a predictive model, usingparticulates and meteorological variables, to explain visibility variations.

DOWNTOWN LOS ANGELES

Figure 2: Days of low visibility and average noon visibility


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4.1 Visibility trends

The studies cited above describe the visibility trends in the Basin as related to

the changes in air quality. Hence, many of those studies tended to concentrateon the low-visibility categories, or days in non-compliance with federal or stateguidelines. In order to relate visibility to pollution levels alone, V70 levels wereexclusively used. Keith [13] argued that less than 3 miles in dry air is anappropriate measure for determining visibility trends in the Basin as well asbeing more meaningful as a measure of pollution than the State air qualitystandard for visibility, 10 miles. Using less than 3 miles showed improvementsin the 1970S for the westernmost stations. However, Davidson's [5] study usingthe number of days less than 10 miles, the state standard, showed much lessimprovements.

Figure 3 shows arithmetic average annual visibilities for the period 1970-1995 for the six Basin locations in the study. It is fairly clear that V70 values aregenerally much higher than those for all humidities. However, the differences aremuch greater for coastal stations than for eastern Basin locations. Fog andcoastal low status are more frequent nearest the ocean. For the two coastalstations, LAX and LGB, both show similar range of values for all humidities andV70, though LGB has somewhat lower peaks for V70. However, the inter-annual variations differ in the dates of peaks and troughs, especially for V70.LAX peak years run 1972, 1975, 1979, 1982 and 1986, while LGB peaks are1970, 1973, 1976, 1978, 1981, 1983 and 1985, seemingly out of phase withLAX. One explanation may be in the location of the LGB station, a convergencezone behind the Palos Verdes Hills. Stronger sea breezes may dispersepollutants at LAX, while carrying them into the convergence area of LGB. TheLGB location is more inland and experiences warmer summer temperatures,when onshore flows tend to cool LAX. Burbank differs from all other locationsin its yearly patterns. V70 values decrease from the early 1970% to the early1980's, after which they increase to a 1984 peak, dropping again afterwards.

Overall values for all humidities and V70 run higher than LGB and ONT, similarto LAX in V70, though higher than LAX for all data. ONT shows a generalimprovement in visibilities in both categories throughout the period, althoughboth V70 and all humidities show a drop in values in 1981, as with LAX and

BUR, and a decrease in visibility in the late 1980's. Overall values are lowcompared to other stations. Although Riverside has the highest paniculate levelsin the Basin, V70 values have climbed since the late 1970S, reporting some ofthe highest annual visibilities for the six stations. San Bernardino also showssimilar improvements since the late 1970S, though average V70 values runlower than Riverside.


Air Pollution 1029

LGB VISIBILITY, 1970-95

RIV VISIBILITY, 1976-95 SBD VISIBILITY, 1976-95

BUR VISIBILITY, 1970-95 ONTARIO VISIBILITY, 1970-95

A.A

Figure 3: Visibility trends for the six airports, 1970-1995


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4.2 Particulate trends

Particulate air pollution levels have decreased since the low visibility days of the1960's [6], however, from 1975-1990, the emission levels have risen greatly [9].Since PM10 monitoring began only in 1985, we use total suspended particulates,TSP, to show trends in paniculate levels in the Basin. Table 1 shows that thelarge improvement in coastal visibilities from the 1970's, do not correspond tosimilar improvements in TSP for W. LA (West L.A.), LGB, and LA

STATIONW.LALGBLABURAZUSARIV-RUBSBD

1975-7959-

88-

L9812489

1980-84658188-9412197

1985-8959839694104126114

1990-94526784788410191

199550597168848889

Table 1: 5-yr running averages of TSP, 1975-1994 and 1995, ug/nv*

(downtown). Inland locations also do not show corresponding rises and fallsbetween visibilities and TSP values. Annual PM10 means were also comparedfor corresponding locations. Again, paniculate PMlOs did not account forswings in visibilities.

4.3 Regression models

Since annual averages of particulates do not seem to explain visibility changeswell, the authors developed a predictive multi-variate linear regression modelusing daily particulate values, along with several meteorological parameters.Using 1970-1987 data, predictive equations explain 60-80% of visibilityvariability for the coastal and inland stations, with the exception of Ontario,which only accounts for slightly more than 42%. BUR's r-squared value is thehighest at 78.2%. The models perform well overall, though they overpredictvisibilities at the lowest values (see figures 4a-d). The models differ betweenstations. For LGB and BUR visibilities, relative humidity accounts for the mostvariations, while humidity and sulfates predict summer visibilities and humidityand nitrates predict winter visibilities. Farther inland, ONT visibilities depend onnitrates and relative humidity, summer visibilities on nitrates and sulfates, whilewinter is dominated by nitrates. RIV visibilities are predicted mostly by nitrates,with winter nitrates, all seasons nitrates, followed by winter sulfates having thehighest correlations. Other variables, such as wind speed, pressure gradients,inversion heights and temperatures, solar radiation and surface temperatures,does not add significantly to the models, although temperatures show a persistentnegative relationship, since summer visibilities are the lowest.


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LONG BEACH VISIBILITY BURBANK VISIBILFT

ONTARIO VISIBILITY

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Figure 4: Visibility predictive models for four locations.

5 Conclusions

Visibility trends in the Greater Los Angeles Basin show differences betweencoastal and inland locations. Coastal and downtown visibilities have improvedsince the 1960's, although recently there has been little change. Inland valleylocation visibilities steadily deteriorated as population skyrocketed,photochemical reactions slowed, allowing sea breezes to carry primary andsecondary pollutants inland. Recent pollution controls have reduced emissionssignificantly, however, visibility values do not show much improvements.Recent visibility improvements at some locations may not be sustained in aregion of rapid growth and EPA ultimatums. Regression models explainvisibility variations well, though simple linear relationships may not be the bestway to represent the data. Coastal location visibilities can be explained bestusing meteorological parameters, particularly humidity. Inland valleys show astronger relationship between visibility and nitrates and sulfates, the former inthe easternmost valleys, the latter in locations closer to downtown.

References

1. Allard, D.W. & Tombach, I.H. The effects of non- standard conditionson visibility measurements, Atmospheric Environment, 15, pp. 1847-1857, 1982.


1032 Air Pollution

2. Appel, B.R., Tokiwa,Y., Hsu, J., Kothny, E.L. & Hahn, E. Visibility asrelated to atmospheric aerosol constituents, Atmos. Environ., 19, pp. 1525-1534,1985.

3. California Air Resources Board, California Air Quality Data, AnnualSummary, J991-1994, GARB, Sacramento, 1992- 95.

4. Cass, G.R. On the relationship between sulfate air quality and visibilitywith examples in Los Angeles, Atmos. Environ., 13, p. 1069, 1979.

5. Davidson, A., Visibility in the South Coast Basin: Patterns,relationships to particulate concentrations, trends, presented at theAnnual Meet. APCA, West Coast Chapter, Scottsdale, AZ., 1983.

6. , Hoggan, M.C. & Nazemi, M.A., Air Quality Trends in theSouth Coast Air Basin, South Coast Air Quality Management District(SCAQMD), El Monte, CA., 1979.

7. Farber, R.J., Young, J.R., & Rozzi, C., Long-term trends in Los AngelesBasin visibility and associated air quality, presented at the 85th AnnualMeeting of the Air & Water Management Association, K.C., Mo., 1992.

8. Farber, R.J., Weising, P.R. & Rozzi, C. PM10 and ozone control strategy toimprove visibility in the Los Angeles Basin, Atmos. Environ., 28, pp. 3277-3283, 1994.

9. Hoggan, M., Cohanim, S., Sin, R., Hsu, M. & Tom, S., Air QualityTrends in California's South Coast and Southeast Desert Air Basins,SCAQMD, Diamond Bar, CA., 1991.

lO.Horie, Y., The Relationship Between SO2 Emissions and Air Pollution inSouthern California—A Case Study on Los Angeles 75 Percent SO2Reduction, Valley Research Corp. Document No. 1064-FF, ValleyResearch Corp., Van Nuys, CA., 1991.

11.Keith, R., Downtown Los Angeles noon visibility trends, Proc. of the11th Air Pollution Meteorology Conf., Amer. Meteorol. Soc., Boston, pp. 85-89, 1971.

12. , Low Visibility Trends in the South Coast Air Basin, 1950-1977,SCAQMD, El Monte, CA., 1979.

13. , A Climatological-Air Quality Profile: California SouthCoast Air Basin, SCAQMD, El Monte, CA., 1980.

14.Lewis, R. & Davidson, A., Air Quality Management Plan Final TechReport V-G: Visibility Assessment for the South Coast Air Basin, SCAQMD,Diamond Bar, C A., 1991.


Documents

S. LaDochy, D. Fuentes€¦ · S. LaDochy, D. Fuentes Department of Geography & Urban Analysis, California State University, Los Angeles, CA. 90032, USA EM ail: [email protected]