Effect of Emissions ControlPrograms on Visibility in SouthernCaliforniaM I C H A E L J . K L E E M A N , * , †

A N N M A R I E E L D E R I N G , ‡ , §

J E F F R E Y R . H A L L , § A N D G L E N R . C A S S | , ⊥

Department of Civil and Environmental Engineering,University of California, Davis, California 95616,Department of Atmospheric Science, University of California,Los Angeles, California 90095-1565, Jet PropulsionLaboratory, California Institute of Technology,Pasadena, California 91109, and School of Earth andAtmospheric Sciences, Georgia Institute of Technology,Atlanta, Georgia 30332-0340

A new method for the analysis and display of the effectof emissions controls on visibility is applied to conditions insouthern California. An advanced mechanistic air qualitymodel that represents airborne particles as a source-orientedexternal mixture first is used to track emissions sourcecontributions to the size distribution and chemical compositionof airborne particles at Claremont, CA, under heavysmog conditions. The resulting description of the aerosolis used in a Mie scattering calculation to determinethe magnitude and particle size dependence of lightscattering and absorption in the atmosphere. The resultinglight scattering and absorption coefficient values aresupplied to an image processing-based visibility modelthat creates full color representations of the appearanceof the local terrain in the presence of the specified level ofair pollution based on satellite-generated landscapeimages. By linking these models, a direct connection isestablished between source emissions and resulting visualair quality. The composite modeling system then is usedto study the effect that different emissions control strategieswould have on visibility in southern California. Anaggressive program of 92 specific emissions controlmeasures that include primary particle controls plus controlson reactive gases that act as secondary aerosol precursorswould more than double visual range at Claremontunder the 1987 historical conditions studied. Syntheticlandscape images show that the mountains to the northof Claremont that are not visible at a range of 10 km underbase-case conditions would be visible if the emissionscontrols described above were applied.

IntroductionThe visibility reduction that occurs in the Los Angeles areais one of the most widely known examples of a severe air

pollution problem. Through examination of historical dataon standard visual range observations made at local airports,Neiburger (1) traced the early phases of the Los Angelesvisibility problem to the industrialization and populationgrowth that occurred in the Los Angeles area during theSecond World War. Visual range data taken between 1932and 1949 showed lower visibilities in the 1940s than in the1930s, with the lowest visibilities in the period 1944-1947.One of the most obvious features of the early phases of localvisibility deterioration was the nearly complete loss of dayswith extremely good visibility. During the period 1932-1937,very clear afternoons with visual range in excess of 35 mioccurred on 21% of the days of the summer season. By 1943-1947, the number of such very clear afternoons dropped to0.2% of the summer days.

Continued surveillance of the growing visibility problemover the period 1950-1961 showed a zone of very lowvisibilities stretching eastward from downtown Los Angelesalong a corridor adjacent to the foothills of the San Gabrieland San Bernardino Mountains (2). In the area nearestdowntown Los Angeles, a minimum visual range of 3 mi orless was recorded on more than 140 days per year at timeswhen the relative humidity was less than 70%. In the plumedownwind of the city that extended past Ontario and SanBernardino, CA (see Figure 1), a minimum visual range ofless than 3 mi at relative humidities less than 70% occurredon more than 110 days per year over the years 1950-1961.At that time, Riverside, CA, displayed among the best visualconditions in the Los Angeles area with fewer than 50 lowvisibility days per year.

By the early 1970s, visibility improvement can be seen atdowntown Los Angeles. Over the period 1971-1973, ap-proximately 25% of the days of the year showed minimumvisibilities below 3 mi at relative humidities below 70% atcentral Los Angeles. However, in the eastern end of the airbasin near Ontario, visual range declined relative to the 1950s.At Ontario, there were 139 days per year with visual rangeless than 3 mi at low relative humidity on average over theperiod 1962-1973, with an average of 144 such days per yearin the years 1971-1973 (3) as compared to approximately110 days per year in the 1950s (2). Since that time, visibilityhas continued to improve in the western areas of the LosAngeles Basin, with less improvement in the eastern areasof the basin near Claremont, Ontario, Riverside, and SanBernardino. The 3-year average number of days per yearthat visibility was greater than 10 mi in Ontario was 99 in the1976-1978 period and had improved to 113 days per yearin 1988-1990 (4).

Deliberate abatement of regional visibility problems suchas the one that exists in Los Angeles requires that the causeand effect relationships between air pollutant source emis-sions and visual conditions be understood quantitatively.Visibility reduction is caused by light scattering and absorp-tion by airborne particles and to a lesser extent by lightabsorption by NO2. Recent developments in air qualitymodeling now make it possible to track the size distribution,chemical composition, and source origin of regional par-ticulate air quality problems in sufficient detail that the effectof emissions controls on the airborne particle size andcomposition distribution can be represented accurately (5-9). Mie scattering calculations can be used to connect thesedetailed predictions of changes in particle size and composi-tion to changes in the level of atmospheric light scatteringand absorption (5, 10). The purpose of the present paper isto use these tools to determine how air pollution sourcescontribute to the Los Angeles area visibility problem and to

* Corresponding author e-mail: mjkleeman@ucdavis.edu; phone:(530)752-7872; fax: (530)752-7872.

† University of California, Davis.‡ University of California, Los Angeles.§ California Institute of Technology.| Georgia Institute of Technology.⊥ Deceased.

Environ. Sci. Technol. 2001, 35, 4668-4674

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investigate the effect that available emissions control strate-gies could have on creating improved local visibility. Theresults of these calculations will be communicated throughthe use of a visibility model that produces synthetic pho-tographs of the appearance of the mountains along thenorthern edge of the Los Angeles Basin at various levels ofemissions control (11, 12).

BackgroundElevated concentrations of atmospheric pollutants in manyurban areas commonly lead to visibility impairment rangingfrom a brown discoloration at the horizon to the formationof a nearly opaque white haze. This visibility impairmentcan be quantified through determination of the light extinc-tion coefficient, which is inversely proportional to theobserved visual range. The total light extinction coefficient(bext) is the sum of four contributions: light scattering byatmospheric gases (Rayleigh scattering; bsg), light absorptionby gases (NO2 in urban atmosphere; bag), light scattering byparticles (bsp), and light absorption by particles (bap):

The extinction coefficient described by eq 1 then can berelated to an estimate of visual range (LV) using Koschmieder’sequation:

where the factor of 0.02 represents the minimum contrastthreshold for detection by a typical human observer that anobject is just barely visible for the case of a large black objectsilhouetted against the horizon sky (13). Visual range providesa simple method to compare visibility under differentatmospheric conditions even though it does not conveyinformation about the coloration and light intensity of thesky and nearby objects.

Each component of the atmospheric light extinctioncoefficient can be calculated theoretically when there is

sufficient information describing the concentration of NO2

and the airborne particle size and chemical compositiondistribution. Light scattering caused by atmospheric gasesis a simple function of the wavelength of light and atmo-spheric pressure (14). Similarly, the absorption of light byNO2 is well characterized (15, 16). Light scattering caused byparticles suspended in the atmosphere has been successfullyexplained in many studies of pollutant effects on visibility(5, 10, 17-21) through the use of Mie light scatteringcalculations (22, 23) along with the assumption that theparticles are spherical. To support a Mie theory light scatteringcalculation, the refractive index and diameter of the particlesmust be known. In the present study, an air quality modelis used that provides explicit information on the numberconcentrations, diameter, and chemical composition of theairborne particles (more than 50 species including sulfateand nitrate) at essentially the single particle level (7). Therefractive index of each particle can be estimated from itschemical composition using the volume-weighted averageof the refractive indices of the substances in the particles(19, 20, 24) or a partial molar refractive index method (25).

In the present analysis, each particle is assumed to be ahomogeneous mixture of the chemical components presentin the particle rather than having a core and shell withdiffering composition. Previous work has shown that theparticle scattering coefficient may vary by 5-10% dependingon the assumptions made about whether the particle has acentral core that differs from the outer shell (20, 24, 26, 27).Light absorption by particles is primarily a function of theamount of elemental carbon and mineral matter present inthe particles and can be calculated once the complexcomponent of the refractive index of the particle has beenestimated by the methods referenced above.

Source Contributions to Ambient Particle SizeDistributionsRecently, an air quality model has been developed that iscapable of tracking single particles as they are transportedthrough the atmosphere to downwind receptor sites while

FIGURE 1. Map of the South Coast Air Basin showing the location of the Claremont monitoring site for which visibility control calculationsare performed in comparison to the locations cited in the historical discussion of the Los Angeles visibility problem.

bext ) bsg + bag + bsp + bap (1)

LV ) -ln 0.02bext

(2)

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retaining information about the sources from which theparticles were originally released (6-9). In the present study,24 air parcel trajectories which pass over Claremont, CA, ateach hour of the day on August 28, 1987, are followedbackward for 48 h to their starting position over the PacificOcean on August 27, 1987. Meteorological variables used inmodel calculations including wind speed and direction,temperature, and relative humidity are based on pointmeasurements taken at a network of more than 50 meteo-rological monitoring stations that then are interpolated overthe entire study region shown in Figure 1. The trajectorypaths followed by air parcels prior to their arrival at the chosenreceptor site are calculated by integrating backward throughthe interpolated wind fields.

Calculations begin with the emissions inventory for theSouth Coast Air Basin surrounding Los Angeles provided bythe California Air Resource Board. This inventory describesthe spatial distribution and temporal variation of the gas-phase pollutants (NOx, SOx, CO, reactive organic gases) andthe particulate matter mass emitted from sources within theLos Angeles area. Emissions of total reactive organic gasesnext are speciated into estimates of the detailed distributionof gas-phase organic compounds present using source-specific chemical composition profiles (28). Similarly, emis-sions of particulate matter mass are converted to a detaileddescription of particulate matter size and composition usingan emissions processing model (5) and source-specificemissions profiles (29, 30). Ammonia emissions for the LosAngeles area are based on the inventory developed by Ghariband Cass (31). A summary of the basecase emissions inventoryused in the present study may be found in Table 1 of ref 8.Each air parcel is initialized with gas- and particle-phaseconcentrations measured at San Nicolas Island during thestudy period (6). Ambient air quality at the Claremontreceptor site is calculated by following the evolution of thegases and particles that are injected into each air parcel asthe air passes over the city upwind. Particles released fromdifferent sources (or the same source at different times) aretracked separately from one another as they are mixed in theatmosphere, transformed by secondary atmospheric chemi-cal reactions, and removed at the earth’s surface by depositionprocesses.

Figure 2 shows measurements and air quality modelpredictions of the size distribution of airborne particles atClaremont, CA, on August 28, 1987, averaged over the timeperiod 0600-1800 h Pacific Standard Time (PST). These timeswere chosen to correspond to the daylight hours whenvisibility impairment would be noticeable. Figure 2a showsthe results of model calculations along with the ambientparticle size distribution measured using an electrical aerosolanalyzer (TSI model 3030 EAA), an optical particle counter(Particle Measuring Systems model LAS-X OPC), and thedifference between PM10 and PM2.5 filter-based samplers.The minimum in the measured size distribution between 1and 2.5 µm particle diameter is an artifact of the samplingprocedure. Model predictions reproduce the bimodal natureof the submicron atmospheric aerosol and closely match theparticle size and magnitude of the observed peaks in the sizedistribution.

Figure 2, panels b and c, shows the airborne particle sizedistribution plotted according to the sources from whicheach particle was originally released. In Figure 2b, thesecondary formation products such as ammonium sulfate,ammonium nitrate, and secondary organic species are plottedseparately from the primary particle cores on which theyhave accumulated, while in Figure 2c these secondaryatmospheric reaction products remain associated with thesource from which the primary core of each particle wasoriginally emitted. Model calculations show that the smallermode in the submicron aerosol size distribution (located at

FIGURE 2. Individual source contributions to airborne particle sizedistributions averaged over the period 0600-1800 PST on August28, 1987, at Claremont, CA. Panel a shows the agreement betweenmeasured and predicted airborne particle volume distributions. Panelb separately shows the contribution that primary particles andsecondary particulate matter (organics, sulfate, ammonium, andnitrate) make to the airborne particle mass distribution. Panel cshows source contributions to the airborne particle mass distributionwhen secondary aerosol formation products are grouped alongwith the primary particle cores on which they have formed. Paneld shows the contribution that primary particles and accumulatedsecondary reaction products make to light extinction in theatmosphere as a function of particle size.

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approximately 0.25 µm particle diameter) is due to secondaryreaction products that have accumulated onto primarycarbon particles released from sources such as diesel enginesand food cooking. The mode in the size distribution locatedat approximately 0.65 µm particle diameter is caused by theaccumulation of secondary atmospheric reaction productsonto marine background particles that were advected intothe study region from over the Pacific Ocean (6-9). Previousstudies (5-9) show that the air quality modeling system usedin the present study accurately predicts the size distributionof major aerosol components such as organic compounds,elemental carbon, sulfate, nitrate, ammonium ion, andsodium at Claremont, CA, on August 28, 1987.

The particle size and composition distribution providedby the air quality model and summarized in Figure 2, panelsb and c, was used to calculate the size distribution of thelight extinction coefficient according to Mie theory. Figure2d shows the size distribution of the light extinction coef-ficient that is associated with particles released from differentsources as they exist in the atmosphere at Claremont, CA, onAugust 28, 1987, between 0600 and 1800 PST. Transformedmarine background particles advected into southern Cali-fornia from over the Pacific Ocean make the single largestcontribution to the light extinction coefficient at this timeand location. Analysis shows that these background particlesare advected into the urban area and then act as a site forthe accumulation of secondary particulate species such asammonium nitrate (6, 7) producing transformed particleswith a size and composition that scatters light very efficiently.Particles originally released from anthropogenic sources suchas diesel engines and food cooking activities also make asignificant contribution to the light extinction coefficient atClaremont, CA, on August 28, 1987. In contrast, particleslarger than 10 µm or smaller than 0.1 µm in diameter do notscatter light efficiently. As a result, crustal particles originallyreleased from activities such as vehicle traffic on paved roadsdo not make a large contribution to the light extinctioncoefficient even though those coarse dust particles are asignificant contributor to PM10 mass concentrations in theurban atmosphere.

Effect of Emissions Control Programs on Visual RangeThe results shown in Figure 2b indicate that primary particlesemitted from anthropogenic sources are not the exclusivecause of visibility reduction in southern California. Back-ground particles and atmospheric chemical reactions leadingto the formation of secondary particulate species also causesignificant visibility reduction. In the interest of exploringemissions control programs that will increase atmosphericvisual range, it is useful to consider how the historicalsituation described in Figure 2 could be affected by changesin pollutant emission rates. Three different emissions controlstrategies were analyzed in detail in the present study: thecomplete control of primary particle emissions, the elimina-tion of background sulfate particles advected into the LosAngeles area from over the ocean, and a realistic programof 92 specific emissions control measures. The first twoemissions control conditions would not be practical toimplement, but they provide valuable insight about thelimiting behavior of the atmospheric system in the SouthCoast Air Basin surrounding Los Angeles. The final emissionscontrol strategy provides an evaluation of visibility improve-ments that could be realized by an aggressive but realisticemissions control program.

Primary Particle Control ProgramThe first emissions control case considered in the presentanalysis specifies the complete elimination of all anthro-pogenic primary particle emissions in the South Coast Air

Basin surrounding Los Angeles (the extreme limit of a primaryparticle control program). Emissions of gaseous pollutantsare allowed to continue at their 1987 values so that the effectof the primary particle control in the presence of continuedsecondary aerosol formation by atmospheric chemical reac-tions can be evaluated. The concentration of backgroundparticles advected into the study region from the marineenvironment also is maintained at 1987 base-case values.The results of model predictions for the size distribution ofairborne particles arriving at Claremont, CA, under August28, 1987, meteorological conditions but with zero anthro-pogenic primary particle emissions are shown in Figure 3a-d. Air quality model calculations demonstrate that particulatematter in the size ranges smaller than 0.2 µm and larger than1 µm particle diameter is almost completely eliminated whentotal control of primary particle emissions is specified. Incontrast, the amount of particulate matter found in the largerof the two modes found in the submicron ambient particlesize distribution (0.3-1.0 µm particle diameter) is practicallyunchanged when primary particle emissions are eliminated.This surprising behavior is produced by the continuedaccumulation of secondary particulate matter onto themarine background particles advected into the study region.Figure 3a-c shows that a significant amount of ammoniumnitrate forms on the marine background seed particlesresulting in a major peak in the airborne particle sizedistribution between 0.3 and 1 µm particle diameter. Lightextinction calculations applied to the particle size andcomposition distribution calculated for the case of completeprimary particle removal are summarized in Figure 3d. Thecalculations summarized in Table 1 indicate that completeelimination of primary particle emissions would increasevisual range averaged over the 0600-1800 h PST at Claremont,CA, on August 28, 1987, from 9.5 km under 1987 historicalconditions to 14.5 km under controlled conditions, anincrease in visual range of 53%.

Removal of Background Sulfate ParticlesBackground sulfate particles advected into the South CoastAir Basin from the marine environment clearly have thecapability to shape the size distribution of the Los Angelesaerosol, as seen in Figures 2 and 3. To directly evaluate theeffect that removal of background particles would have onvisibility in Los Angeles, the air quality and visibility modelswere exercised with the background sulfate particles removedbefore they entered the South Coast Air Basin. The emissionrates of anthropogenic particles and gases were maintainedat their 1987 base-case values during these simulations. Figure3e shows that the removal of the background sulfate particlesresults in a particle volume distribution that has only a singlepeak below 1 µm particle diameter. Removal of the back-ground particles reduces the amount of airborne particlesurface area available between 0.4 and 1 µm particle diameterresulting in significantly more ammonium nitrate accumu-lation on particles with diameters between 0.1 and 0.3 µmas shown in Figure 3f. Despite this redistribution of secondaryaerosol mass to smaller particle sizes, light extinctioncalculations shown in Figure 3h and summarized in Table1 indicate that removal of background sulfate particles fromthe atmosphere results in an improvement in visual rangeof only 11% at Claremont, CA, on August 28, 1987, averagedover the hours between 0600 and 1800 PST. While thebackground sulfate particles greatly influence the final shapeof the particle size distribution in the Los Angeles area, theirremoval from the system would not cure the Los Angelesvisibility problem.

Balanced Emissions Control ProgramThe previous two case studies examined the extreme limitsof two approaches to particle control: complete removal ofeither anthropogenic primary particle emissions or back-

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ground sulfate particles. Complete elimination of either ofthese particle sources is impractical, and in both cases thecontinued emissions of reactive gases lead to secondaryaerosol formation sufficient to produce a significant visibilityproblem. The final emissions control case considered in thepresent analysis specifies a balanced control program withreductions in the emission rate of anthropogenic particles

(-41% PM2.5), gaseous oxides of sulfur (-71% SOx), oxidesof nitrogen (-56% NOx), volatile organic compounds (-70%VOC), and ammonia (-49% NH3) that are based on emissionscontrol technologies available in the South Coast Air Basin.The control strategy simulated here is described in Tables2-4 of ref 8 and involves 92 specific control measuresapplicable to local sources. The results of model predictions

FIGURE 3. Effect of emissions changes on particle size distributions and light extinction at Claremont, CA, averaged over the period0600-1800 PST under August 28, 1987, meteorological conditions. Panels a-d show the predicted airborne particle size distributions andlight extinction as a function of particle size when all primary particle emissions are suppressed. Panels e-h show the predicted airborneparticle size distributions and light extinction as a function of particle size when background sulfate particles are removed from the system.Panels i-l show the predicted airborne particle size distributions when a balanced control strategy is adopted that includes reductionsin primary particle emissions and gaseous particle precursors according to the 92 control measures identified in ref 8.

TABLE 1. Effect of Alternative Emissions Control Strategies on Visual Range at Claremont, CA, under August 28, 1987,Meteorological Conditions

emissions conditionbsg

(km-1)bag

(km-1)bsp

(km-1)bap

(km-1)bext

(km-1)visual range

(km)

base-case; 1987 historical emissions 0.02 0.07 0.29 0.04 0.41 9.5eliminate primary particles 0.02 0.07 0.18 0 0.27 14.5eliminate background sulfate particles 0.02 0.07 0.24 0.04 0.37 10.6adoption of particle and gas control program given in ref 8 0.02 0.03 0.14 0.01 0.2 19.6

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under these conditions are shown in Figure 3i-l. Figure 3i-kshow that significant reductions in both primary andsecondary particulate matter concentrations at all particlesizes can be expected from the combined control strategy.Figure 3l shows that the light extinction coefficient associatedwith atmospheric particles is significantly reduced by thiscontrol program. The final row of Table 1 indicates that thecombined control strategy would more than double visual

range at Claremont, CA, on August 28, 1987, averaged overthe hours of 0600-1800 PST.

Image ProcessingImages produced to illustrate the visibility reducing effectsof the aerosols are included in Figure 4. Visualization modelsare powerful because rather than providing a single value forthe visual range, the effects on the far-, mid-, and near-field

FIGURE 4. Predicted view of the San Gabriel Mountains from Claremont, CA, on August 28, 1987: (a) downward looking Landsat ThematicMapper image of the South Coast Air Basin surrounding Los Angeles; (b) clear-day image of the San Gabriel Mountains; (c) base-caseimage of the San Gabriel Mountains viewed from a distance of 7 km under August 28, 1987, conditions; (d) image of the San GabrielMountains viewed from a distance of 7 km in the presence of the reactive gas and primary particle control measures summarized in ref8; (e) base-case image of the San Gabriel Mountains viewed from a distance of 10 km under base-case August 28, 1987, conditions; (f)image of the San Gabriel Mountains viewed from a distance of 10 km in the presence of all the reactive gas and primary control measuressummarized in ref 8. Base-case images are calculated using the aerosol size distributions shown in Figure 2. Control images are calculatedusing the aerosol size distributions shown in Figure 3i-k.

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of view are illustrated. As described in ref 12, LandsatThematic Mapper (TM) data are used to generate the baseimage (inherent radiance) at the desired viewing location.By using a detailed distance map of the scene, the lightextinction coefficients, a detailed description of the skylightradiance, and an image of the inherent radiance of the scene,the effects of any level of air pollution can be illustrated againsta synthetic landscape generated from the satellite imagery.

Figure 4a shows a downward looking Landsat ThematicMapper image of the South Coast Air Basin surrounding LosAngeles. The Santa Monica Mountains are along the coaston the left side of the image, and the San Gabriel Mountainsrun along the top right-hand side of the image. The clear-day image (panel b) illustrates the appearance of the SanGabriel Mountains as seen when looking north acrossClaremont under conditions that would be consideredextremely clear for Los Angeles, with a visual range of nearly82 km (50 mi). Figure 4, panels c and e, illustrates theappearance of the mountains when viewed from a distanceof 7 and 10 km through a haze layer that contains particleshaving a size distribution and chemical composition like thatpresent under base-case August 28, 1987, conditions shownin Figure 2. The visual range is predicted to be 9.5 km, andindeed the ridge is apparent from a distance of 7 km andfully obscured when viewed from a distance of 10 km. Thecombined primary particle and gaseous particle precursorcontrol case is expected to result in a visual range of about20 km, as documented in the last entry in Table 1. In Figure4, panels d and f, the ridge can be seen much more clearlythan in the precontrol base-case image, with little obscurationat 7 km and a moderate loss of clarity when viewed from 10km, consistent with the predicted visual range.

AcknowledgmentsThis research was supported by funds granted to the CaltechCenter for Air Quality Analysis.

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Received for review November 10, 2000. Revised manuscriptreceived August 30, 2001. Accepted September 4, 2001.

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