Health Benefits in California of Strengthening the Fine Particulate Matter Standards.

The Clean Air Act requires the United States Environmental Protection Agency to review routinely the National Ambient Air Quality Standards, including fine particulate matter (PM2.5). A non-governmental Independent Particulate Matter Review Panel recently concluded that the current PM2.5 standards do not protect public health adequately and recommended revising the daily standard from 35 to 25-30 µg/m3 and the annual standard from 12 to 8-10 µg/m3. To assess the public health implications of adopting the PM2.5 standards proposed by the panel, the health benefits are quantified from their implementation based on both current (observed) and future (simulated) air quality data for California. The findings indicate that strengthening the standards would provide significant public health benefits valued at $42-$149 billion. Additionally, the stronger standards are shown to benefit environmental justice via health savings that are allocated more within environmentally and socioeconomically disadvantaged communities.

Benefits of near-zero freight: The air quality and health impacts of low-NOx compressed natural gas trucks.

The use of low-NOx compressed natural gas (CNG) medium-duty vehicles (MDVs) and heavy-duty vehicles (HDVs) has the potential to significantly reduce NOx emissions and yield improvements in regional air quality. However, the extent of air quality improvement depends on many factors including future levels of vehicle deployment, the evolution of emissions from other sources, and meteorology. An analysis of the impacts requires modeling the atmosphere to account for both primary and secondary air pollutants, and the use of health impact assessment tools to map air quality changes into quantifiable metrics of human health. Here, we quantify and compare the air quality and health impacts associated with the deployment of low-NOx CNG engines to power future MDV and HDV fleets in California relative to both a business-as-usual and a more advanced fleet composition. The results project that reductions in summer ground-level ozone could reach 13 ppb when compared to a baseline fleet of diesel and gasoline HDV and MDV and could reach 6 ppb when compared to a cleaner fleet that includes some zero-emission vehicles and fuels. Similarly, for all CNG cases considered reductions in PM2.5 are predicted to range from 1.2 ug/m3 to 2.7 ug/m3 for a summer episode and from 3.1 ug/m3 to approximately 7.8 ug/m3 for a winter episode. These improvements yield short-term health benefits equivalent to $47 to $56 million in summer and $38 to $43 million in winter during episodes conducive to poor air quality. Additionally, the use of zero emission vehicle options such as battery electric and hydrogen fuel cell trucks could achieve approximately 25% to 31% higher benefits for an equivalent fleet penetration level due to the additional emission reductions achieved.Implications: The paper provides a quantitative estimate of the air quality and human health benefits that can be achieved through the use of novel compressed natural gas engines (i.e., low-NOx CNG) in medium- and heavy-duty vehicles and provide a comparison with zero emission vehicles. Thus, our findings will provide support for policy development seeking to transform the trucking sector to meet clean air and climate goals given the current struggle policymakers have with selecting between alternative truck technologies due to variance in factors like cost and technical maturity.

An Episodic Assessment of Vehicle Emission Regulations on Saving Lives in California.

Historically, California has been a world leader in the development and application of environmental regulations. Policies to address air pollution have reduced criteria pollutant emissions, improved regional air quality, and benefited public health. To this end, California has imposed strict regulations on light-duty, medium-duty, and heavy-duty vehicles to reduce ambient concentrations of health-damaging pollutants such as ozone and fine particulate matter (PM2.5). Here, we compare the impact on air quality in California should California not have adopted on-road vehicle regulations (No Regulations Case) with the air quality associated with current regulations (Regulated Case). Simulations of atmospheric chemistry and transport are conducted to evaluate the impact of emissions on ambient levels of ozone and PM2.5, and a health impact assessment tool is used to quantify and monetize societal impairment. Compared with the "Regulated Case," the "No Regulations Case" results in a maximum peak 8 h ozone level of 162 ppb and 24 h PM2.5 of 42.7 µg/m3 in summer, and 107 µg/m3 and 24 h PM2.5 in winter. The associated increases in the daily incidence of human health outcomes are $66 million per day and $116 million per day during peak pollutant formation periods in summer and winter, respectively. Overall, the findings quantitatively establish the role and importance of on-road vehicle regulations in protecting societal well-being.

Comprehensively assessing the drivers of future air quality in California.

In this study we analyze the impact of major drivers of future air quality, both separately and simultaneously, for the year 2035 in three major California air basins: the South Coast Air Basin (SoCAB), the San Francisco Bay Area (SFBA), and the San Joaquin Valley (SJV). A variety of scenarios are considered based on changes in climate-driven meteorological conditions and both biogenic and anthropogenic emissions. Anthropogenic emissions are based on (1) the California Air Resources Board (CARB) California Emissions Projection Analysis Model (CEPAM), (2) increases in electric sector emissions due to climate change, and (3) aggressive adoption of alternative energy technologies electrification of end-use technologies, and energy efficiency measures. Results indicate that climate-driven changes in meteorological conditions will significantly alter day-to-day variations in future ozone and PM2.5 concentrations, likely increasing the frequency and severity of pollution periods in regions that already experience poor air quality and increasing health risks from pollutant exposure. Increases in biogenic and anthropogenic emissions due to climate change are important during the summer seasons, but have little effect on pollutant concentrations during the winter. Results also indicate that controlling anthropogenic emissions will play a critical role in mitigating climate-driven increases in both ozone and PM2.5 concentrations in the most populated areas of California. In the absence of anthropogenic emissions controls, climate change will worsen ozone air quality throughout the state, increasing exceedances of ambient air quality standards. If planned reductions in anthropogenic emissions are implemented, ozone air quality throughout the less urban areas of the state may be improved in the year 2035, but regions such as the SoCAB and the east SFBA will likely continue to experience high ozone concentrations throughout the summer season. Climate change and anthropogenic emissions controls are both found to decrease wintertime PM2.5 concentrations in the SJV, eliminating nearly all exceedances of PM2.5 National Ambient Air Quality Standards (NAAQS) in the year 2035. However, reductions in anthropogenic emissions are unable to fully mitigate the impact of climate change on PM2.5 concentrations in the SoCAB and east SFBA. Thus, while future air quality in the SJV is projected to be improved in the year 2035, air quality in the SoCAB and east SFBA will remain similar or marginally worsen compared to present day levels. Conversely, we find that aggressive adoption of alternative energy technologies including renewable resources, electrification of end-use technologies, and energy efficiency measures can offset the impacts of climate change. Overall, the two main drivers for air quality in 2035 are changes in meteorological conditions due to climate change and reductions in anthropogenic emissions.

Assessment of the emissions and air quality impacts of biomass and biogas use in California.

UNLABELLED: It is estimated that there is sufficient in-state "technically" recoverable biomass to support nearly 4000 MW of bioelectricity generation capacity. This study assesses the emissions of greenhouse gases and air pollutants and resulting air quality impacts of new and existing bioenergy capacity throughout the state of California, focusing on feedstocks and advanced technologies utilizing biomass resources predominant in each region. The options for bioresources include the production of bioelectricity and renewable natural gas (NG). Emissions of criteria pollutants and greenhouse gases are quantified for a set of scenarios that span the emission factors for power generation and the use of renewable natural gas for vehicle fueling. Emissions are input to the Community Multiscale Air Quality (CMAQ) model to predict regional and statewide temporal air quality impacts from the biopower scenarios. With current technology and at the emission levels of current installations, maximum bioelectricity production could increase nitrogen oxide (NOx) emissions by 10% in 2020, which would cause increases in ozone and particulate matter concentrations in large areas of California. Technology upgrades would achieve the lowest criteria pollutant emissions. Conversion of biomass to compressed NG (CNG) for vehicles would achieve comparable emission reductions of criteria pollutants and minimize emissions of greenhouse gases (GHG). Air quality modeling of biomass scenarios suggest that applying technological changes and emission controls would minimize the air quality impacts of bioelectricity generation. And a shift from bioelectricity production to CNG production for vehicles would reduce air quality impacts further. From a co-benefits standpoint, CNG production for vehicles appears to provide the best benefits in terms of GHG emissions and air quality. IMPLICATIONS: This investigation provides a consistent analysis of air quality impacts and greenhouse gas emissions for scenarios examining increased biomass use. Further work involving economic assessment, seasonal or annual emissions and air quality modeling, and potential exposure analysis would help inform policy makers and industry with respect to further development and direction of biomass policy and bioenergy technology alternatives needed to meet energy and environmental goals in California.

The future of airborne sulfur-containing particles in the absence of fossil fuel sulfur dioxide emissions.

Sulfuric acid (H2SO4), formed from oxidation of sulfur dioxide (SO2) emitted during fossil fuel combustion, is a major precursor of new airborne particles, which have well-documented detrimental effects on health, air quality, and climate. Another precursor is methanesulfonic acid (MSA), produced simultaneously with SO2 during the atmospheric oxidation of organosulfur compounds (OSCs), such as dimethyl sulfide. In the present work, a multidisciplinary approach is used to examine how contributions of H2SO4 and MSA to particle formation will change in a large coastal urban area as anthropogenic fossil fuel emissions of SO2 decline. The 3-dimensional University of California Irvine-California Institute of Technology airshed model is used to compare atmospheric concentrations of gas phase MSA, H2SO4, and SO2 under current emissions of fossil fuel-associated SO2 and a best-case futuristic scenario with zero fossil fuel sulfur emissions. Model additions include results from (i) quantum chemical calculations that clarify the previously uncertain gas phase mechanism of formation of MSA and (ii) a combination of published and experimental estimates of OSC emissions, such as those from marine, agricultural, and urban processes, which include pet waste and human breath. Results show that in the zero anthropogenic SO2 emissions case, particle formation potential from H2SO4 will drop by about two orders of magnitude compared with the current situation. However, particles will continue to be generated from the oxidation of natural and anthropogenic sources of OSCs, with contributions from MSA and H2SO4 of a similar order of magnitude. This could be particularly important in agricultural areas where there are significant sources of OSCs.

Images reveal that atmospheric particles can undergo liquid-liquid phase separations.

A large fraction of submicron atmospheric aerosol particles contains both organic material and inorganic salts. As the relative humidity cycles in the atmosphere and the water content of the particles correspondingly changes, these mixed particles can undergo a range of phase transitions, possibly including liquid-liquid phase separation. If liquid-liquid phase separation occurs, the gas-particle partitioning of atmospheric semivolatile organic compounds, the scattering and absorption of solar radiation, and the reactive uptake of gas species on atmospheric particles may be affected, with important implications for climate predictions. The actual occurrence of liquid-liquid phase separation within individual atmospheric particles has been considered uncertain, in large part because of the absence of observations for real-world samples. Here, using optical and fluorescence microscopy, we present images that show the coexistence of two noncrystalline phases for real-world samples collected on multiple days in Atlanta, GA as well as for laboratory-generated samples under simulated atmospheric conditions. These results reveal that atmospheric particles can undergo liquid-liquid phase separations. To explore the implications of these findings, we carried out simulations of the Atlanta urban environment and found that liquid-liquid phase separation can result in increased concentrations of gas-phase NO(3) and N(2)O(5) due to decreased particle uptake of N(2)O(5).

Nonequilibrium atmospheric secondary organic aerosol formation and growth.

Airborne particles play critical roles in air quality, health effects, visibility, and climate. Secondary organic aerosols (SOA) formed from oxidation of organic gases such as α-pinene account for a significant portion of total airborne particle mass. Current atmospheric models typically incorporate the assumption that SOA mass is a liquid into which semivolatile organic compounds undergo instantaneous equilibrium partitioning to grow the particles into the size range important for light scattering and cloud condensation nuclei activity. We report studies of particles from the oxidation of α-pinene by ozone and NO(3) radicals at room temperature. SOA is primarily formed from low-volatility ozonolysis products, with a small contribution from higher volatility organic nitrates from the NO(3) reaction. Contrary to expectations, the particulate nitrate concentration is not consistent with equilibrium partitioning between the gas phase and a liquid particle. Rather the fraction of organic nitrates in the particles is only explained by irreversible, kinetically determined uptake of the nitrates on existing particles, with an uptake coefficient that is 1.6% of that for the ozonolysis products. If the nonequilibrium particle formation and growth observed in this atmospherically important system is a general phenomenon in the atmosphere, aerosol models may need to be reformulated. The reformulation of aerosol models could impact the predicted evolution of SOA in the atmosphere both outdoors and indoors, its role in heterogeneous chemistry, its projected impacts on air quality, visibility, and climate, and hence the development of reliable control strategies.

Future impacts of distributed power generation on ambient ozone and particulate matter concentrations in the San Joaquin Valley of California.

Distributed power generation-electricity generation that is produced by many small stationary power generators distributed throughout an urban air basin-has the potential to supply a significant portion of electricity in future years. As a result, distributed generation may lead to increased pollutant emissions within an urban air basin, which could adversely affect air quality. However, the use of combined heating and power with distributed generation may reduce the energy consumption for space heating and air conditioning, resulting in a net decrease of pollutant and greenhouse gas emissions. This work used a systematic approach based on land-use geographical information system data to determine the spatial and temporal distribution of distributed generation emissions in the San Joaquin Valley Air Basin of California and simulated the potential air quality impacts using state-of-the-art three-dimensional computer models. The evaluation of the potential market penetration of distributed generation focuses on the year 2023. In general, the air quality impacts of distributed generation were found to be small due to the restrictive 2007 California Air Resources Board air emission standards applied to all distributed generation units and due to the use of combined heating and power. Results suggest that if distributed generation units were allowed to emit at the current Best Available Control Technology standards (which are less restrictive than the 2007 California Air Resources Board standards), air quality impacts of distributed generation could compromise compliance with the federal 8-hr average ozone standard in the region.

Partitioning phase preference for secondary organic aerosol in an urban atmosphere.

Secondary organic aerosol (SOA) comprises a significant portion of atmospheric particular matter. The impact of particular matter on both human health and global climate has long been recognized. Despite its importance, there are still many unanswered questions regarding the formation and evolution of SOA in the atmosphere. This study uses a modeling approach to understand the preferred partitioning behavior of SOA species into aqueous or organic condensed phases. More specifically, this work uses statistical analyses of approximately 24,000 data values for each variable from a state of the art 3D airshed model. Spatial and temporal distributions of fractions of SOA residing in the aqueous phase (fAQ) in the South Coast Air Basin of California are presented. Typical values of fAQ within the basin near the surface range from 5 to 80%. Results show that the likelihood of large fAQ values is inversely proportional to the total SOA loading. Analysis of various meteorological parameters indicates that large fAQ values are predicted because modeled aqueous-phase SOA formation is less sensitive than that of organic-phase SOA to atmospheric conditions that are not conducive to SOA formation. There is a diurnal variation of fAQ near the surface: It tends to be larger during daytime hours than during nighttime hours. Results also indicate that the largest fAQ values are simulated in layers above ground level at night. In summary, one must consider SOA in both organic and aqueous phases for proper regional and global SOA budget estimation.

Potential for atmospheric-driven lead paint degradation in the South Coast Air Basin of California.

Exposure to lead in paint or lead residues in house dust and soil is one of the leading environmental risks to the health of children in the United States. Components of photochemical smog can increase the degradation of binders in lead paint, leading to increased release of lead pigment granules to hands in surface contact or for deposition in house dust and soil. This study uses photochemical air quality modeling to map areas susceptible to increased lead paint degradation as a result of photochemical atmospheric pollutants to prioritize areas of concern. Typical air quality episodes in the South Coast Air Basin of California (SoCAB) are modeled for the 1970s, 1980s, and 1990s. Results indicate that large areas of the SoCAB were susceptible to atmospheric-driven accelerated lead paint degradation. Inner city urban areas from central Los Angeles to Azusa and most of Orange County had the highest susceptibility to accelerated lead paint degradation, followed by inland locations near the San Bernardino Mountains. This study identifies photochemical oxidant gases as contributors to greater lead release from indoor painted surfaces in urban areas.

Determining air quality and greenhouse gas impacts of hydrogen infrastructure and fuel cell vehicles.

Adoption of hydrogen infrastructure and hydrogen fuel cell vehicles (HFCVs) to replace gasoline internal combustion engine (ICE) vehicles has been proposed as a strategy to reduce criteria pollutant and greenhouse gas (GHG) emissions from the transportation sector and transition to fuel independence. However, it is uncertain (1) to what degree the reduction in criteria pollutants will impact urban air quality, and (2) how the reductions in pollutant emissions and concomitant urban air quality impacts compare to ultralow emission gasoline-powered vehicles projected for a future year (e.g., 2060). To address these questions, the present study introduces a "spatially and temporally resolved energy and environment tool" (STREET) to characterize the pollutant and GHG emissions associated with a comprehensive hydrogen supply infrastructure and HFCVs at a high level of geographic and temporal resolution. To demonstrate the utility of STREET, two spatially and temporally resolved scenarios for hydrogen infrastructure are evaluated in a prototypical urban airshed (the South Coast Air Basin of California) using geographic information systems (GIS) data. The well-to-wheels (WTW) GHG emissions are quantified and the air quality is established using a detailed atmospheric chemistry and transport model followed by a comparison to a future gasoline scenario comprised of advanced ICE vehicles. One hydrogen scenario includes more renewable primary energy sources for hydrogen generation and the other includes more fossil fuel sources. The two scenarios encompass a variety of hydrogen generation, distribution, and fueling strategies. GHG emissions reductions range from 61 to 68% for both hydrogen scenarios in parallel with substantial improvements in urban air quality (e.g., reductions of 10 ppb in peak 8-h-averaged ozone and 6 mug/m(3) in 24-h-averaged particulate matter concentrations, particularly in regions of the airshed where concentrations are highest for the gasoline scenario).

Chlorine activation indoors and outdoors via surface-mediated reactions of nitrogen oxides with hydrogen chloride.

Gaseous HCl generated from a variety of sources is ubiquitous in both outdoor and indoor air. Oxides of nitrogen (NO(y)) are also globally distributed, because NO formed in combustion processes is oxidized to NO(2), HNO(3), N(2)O(5) and a variety of other nitrogen oxides during transport. Deposition of HCl and NO(y) onto surfaces is commonly regarded as providing permanent removal mechanisms. However, we show here a new surface-mediated coupling of nitrogen oxide and halogen activation cycles in which uptake of gaseous NO(2) or N(2)O(5) on solid substrates generates adsorbed intermediates that react with HCl to generate gaseous nitrosyl chloride (ClNO) and nitryl chloride (ClNO(2)), respectively. These are potentially harmful gases that photolyze to form highly reactive chlorine atoms. The reactions are shown both experimentally and theoretically to be enhanced by water, a surprising result given the availability of competing hydrolysis reaction pathways. Airshed modeling incorporating HCl generated from sea salt shows that in coastal urban regions, this heterogeneous chemistry increases surface-level ozone, a criteria air pollutant, greenhouse gas and source of atmospheric oxidants. In addition, it may contribute to recently measured high levels of ClNO(2) in the polluted coastal marine boundary layer. This work also suggests the potential for chlorine atom chemistry to occur indoors where significant concentrations of oxides of nitrogen and HCl coexist.

Air quality impacts of distributed energy resources implemented in the northeastern United States.

Emissions from the potential installation of distributed energy resources (DER) in the place of current utility-scale power generators have been introduced into an emissions inventory of the northeastern United States. A methodology for predicting future market penetration of DER that considers economics and emission factors was used to estimate the most likely implementation of DER. The methodology results in spatially and temporally resolved emission profiles of criteria pollutants that are subsequently introduced into a detailed atmospheric chemistry and transport model of the region. The DER technology determined by the methodology includes 62% reciprocating engines, 34% gas turbines, and 4% fuel cells and other emerging technologies. The introduction of DER leads to retirement of 2625 MW of existing power plants for which emissions are removed from the inventory. The air quality model predicts maximum differences in air pollutant concentrations that are located downwind from the central power plants that were removed from the domain. Maximum decreases in hourly peak ozone concentrations due to DER use are 10 ppb and are located over the state of New Jersey. Maximum decreases in 24-hr average fine particulate matter (PM2.5) concentrations reach 3 microg/m3 and are located off the coast of New Jersey and New York. The main contribution to decreased PM2.5 is the reduction of sulfate levels due to significant reductions in direct emissions of sulfur oxides (SO(x)) from the DER compared with the central power plants removed. The scenario presented here represents an accelerated DER penetration case with aggressive emission reductions due to removal of highly emitting power plants. Such scenario provides an upper bound for air quality benefits of DER implementation scenarios.

Enhanced photolysis in aerosols: evidence for important surface effects.

While there is increasing evidence for unique chemical reactions at interfaces, there are fewer data on photochemistry at liquid-vapor junctions. This paper reports a comparison of the photolysis of molybdenum hexacarbonyl, Mo(CO)(6), in 1-decene either as liquid droplets or in bulk-liquid solutions. Mo(CO)(6) photolysis is faster by at least three orders of magnitude in the aerosols than in bulk-liquids. Two possible sources of this enhancement are considered: (1) increased light intensity due to either Morphology-Dependent Resonances (MDRs) in the spherical aerosol particles and/or to increased pathlengths for light inside the droplet due to refraction, which are termed physical effects in this paper; and (2) interface effects such as an incomplete solvent-cage at the gas-liquid boundary and/or enhanced interfacial concentrations of Mo(CO)(6), which are termed chemical effects. Quantitative calculations of the first possibility were carried out in which the light intensity distribution in the droplets averaged over 215-360 nm was obtained for 1-decene droplets. Calculations show that the average increase in light intensity over the entire droplet is 106%, with an average increase of 51% at the interface. These increases are much smaller than the observed increase in the apparent photolysis rate of droplets compared to the bulk. Thus, chemical effects, i.e., a decreased solvent-cage effect at the interface and/or enhancement in the surface concentration of Mo(CO)(6), are most likely responsible for the dramatic increase in the photolysis rate. Similar calculations were also carried out for broadband (290-600 nm) solar irradiation of water droplets, relevant to atmospheric conditions. These calculations show that, in agreement with previous calculations by Mayer and Madronich [B. Mayer and S. Madronich, Atmos. Chem. Phys., 2004, 4, 2241] MDRs produce only a moderate average intensity enhancement relative to the corresponding bulk-liquid slabs when averaged over a range of wavelengths characteristic of solar radiation at the Earth's surface. However, as in the case of Mo(CO)(6) in 1-decene, chemical effects may play a role in enhanced photochemistry at the aerosol-air interface for airborne particles.

Air quality modeling in the South Coast Air Basin of California: what do the numbers really mean?

This study evaluates air quality model sensitivity to input and to model components. Simulations are performed using the California Institute of Technology (CIT) airshed model. Results show the impacts on ozone (O3) concentration in the South Coast Air Basin (SCAB) of California because of changes in: (1) input data, including meteorological conditions (temperature, UV radiation, mixing height, and wind speed), boundary conditions, and initial conditions (ICs); and (2) model components, including advection solver and chemical mechanism. O3 concentrations are strongly affected by meteorological conditions and, in particular, by temperature. ICs also affect O3 concentrations, especially in the first 2 days of simulation. On the other hand, boundary conditions do not significantly affect the absolute peak O3 concentration, although they do affect concentrations near the inflow boundaries. Moreover, predicted O3 concentrations are impacted considerably by the chemical mechanism. In addition, dispersion of pollutants is affected by the advection routine used to calculate its transport. Comparison among CIT, California Photochemical Grid Model (CALGRID), and Urban Airshed Model air quality models suggests that differences in O3 predictions are mainly caused by the different chemical mechanisms used. Additionally, advection solvers contribute to the differences observed among model predictions. Uncertainty in predicted peak O3 concentration suggests that air quality evaluation should not be based solely on this single value but also on trends predicted by air quality models using a number of chemical mechanisms and with an advection solver that is mass conservative.

Gas-phase molecular halogen formation from NaCl and NaBr aerosols: when are interface reactions important?

Unique interface reactions at the surface of sea-salt particles have been suggested as an important source of photolyzable gas-phase halogen species in the troposphere. Many factors influence the relative importance of interface chemistry compared to aqueous-phase chemistry. The Model of Aerosol, Gas, and Interfacial Chemistry (MAGIC 2.0) is used to study the influence of interface reactions on gas-phase molecular halogen production from pure NaCl and NaBr aerosols. The main focus is to identify the relative importance of bulk compared to interface chemistry and to determine when interface chemistry dominates. Results show that the interface process involving Cl-(surf) and OH(g) is the main source of Cl2(g). For the analogous oxidation of bromide by OH, gaseous Br2 is formed mainly in the bulk aqueous phase and transferred across the interface. However, the reaction of Br-(surf) with O3(g) at the interface is the primary source of Br2(g) under dark conditions. The effect of aerosol size is also studied. Potential atmospheric implications and effects of interface processes on aerosol pH are discussed.

Calculations of incremental secondary organic aerosol reactivity.

The incremental secondary organic aerosol reactivity (ISOAR) of a species j is defined as the relative incremental change in secondary organic aerosol (SOA) formed per relative incremental change in the amount of species jemitted. The California Institute of Technology three-dimensional air quality model is used in conjunction with the Caltech Atmospheric Chemistry Mechanism (CACM) and the Model to Predict the Multiphase Partitioning of Organics to calculate spatially and temporally averaged ISOAR values for the South Coast Air Basin of California (SoCAB). The base case SOA concentrations are derived for September 9, 1993. The South Coast Air Quality Management District of California provided emission and meteorological data. ISOAR values are calculated for the lumped surrogate compounds considered by CACM: isoprene, low-yield monoterpenes, high-yield monoterpenes, high-yield aromatics, etc. This work presents basin-wide ISOAR values determined through regression analysis. In addition, ISOAR values are reported at individual locations within the SoCAB. Modeled data are compared with ISOAR values calculated using smog chamber data. Results indicate that long-chain alkanes present the highest ISOAR. On the other hand, short-chain organics present the lowest ISOAR.