ALBERT

Description: Secondary organic aerosol (SOA) constitutes a major fraction of submicrometer atmospheric particulate matter. Quantitative simulation of SOA within air-quality and climate models—and its resulting impacts—depends on the translation of SOA formation observed in laboratory chambers into robust parameterizations. Worldwide data have been accumulating indicating that model predictions of SOA are...

Description: The heterogeneous replacement of chloride by nitrate in individual sea-salt particles was monitored continuously over time in the troposphere with the use of aerosol time-of-flight mass spectrometry. Modeling calculations show that the observed chloride displacement process is consistent with a heterogeneous chemical reaction between sea-salt particles and gas-phase nitric acid, leading to sodium nitrate production in the particle phase accompanied by liberation of gaseous HCl from the particles. Such single-particle measurements, combined with a single-particle model, make it possible to monitor and explain heterogeneous gas/particle chemistry as it occurs in the atmosphere.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Gard -- Kleeman -- Gross -- Hughes -- Allen -- Morrical -- Fergenson -- Dienes -- E Galli M -- Johnson -- Cass -- Prather -- New York, N.Y. -- Science. 1998 Feb 20;279(5354):1184-7.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉E. E. Gard, D. S. Gross, B. D. Morrical, D. P. Fergenson, T. Dienes, M. E. Galli, K. A. Prather, Department of Chemistry, University of California, Riverside, CA 92521, USA. M. J. Kleeman, L. S. Hughes, J. O. Allen, R. J. Johnson, G. R. Cass, Departme.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/9469803" target="_blank"〉PubMed〈/a〉

Description: Air quality models that generate the concentrations of semi-volatile and other condensable organic compounds using an explicit reaction mechanism require estimates of the physical and thermodynamic properties of the compounds that affect gas/aerosol partitioning: vapour pressure (as a subcooled liquid), and activity coefficients in the aerosol phase. The model of Griffin, Kleeman and co-workers (e.g., Griffin et al., 1999; Kleeman et al., 1999) assumes that aerosol particles consist of an aqueous phase, containing inorganic electrolytes and soluble organic compounds, and a hydrophobic phase containing mainly primary hydrocarbon material. Thirty eight semi-volatile reaction products are grouped into ten surrogate species which partition between the gas phase and both phases in the aerosol. Activity coefficients of the organic compounds are calculated using UNIFAC. In a companion paper (Clegg et al., 2007) we examine the likely uncertainties in the vapour pressures of the semi-volatile compounds and their effects on partitioning over a range of atmospheric relative humidities. In this work a simulation for the South Coast Air Basin surrounding Los Angeles, using lower vapour pressures of the semi-volatile surrogate compounds consistent with estimated uncertainties in the boiling points on which they are based, yields a doubling of the predicted 24-h average secondary organic aerosol concentrations. The dependency of organic compound partitioning on the treatment of inorganic electrolytes in the air quality model, and the performance of this component of the model, are determined by analysing the results of a trajectory calculation using an extended version of the Aerosol Inorganics Model of Wexler and Clegg (2002). Simplifications are identified where substantial efficiency gains can be made, principally: the omission of dissociation of the organic acid surrogates; restriction of aerosol organic compounds to one of the two phases (aqueous or hydrophobic) where equilibrium calculations suggest partitioning strongly in either direction; a single calculation of activity coefficients of the organic compounds for simulations where they are determined by the presence of one component at high concentration in either phase (i.e., water in the aqueous phase, or a hydrocarbon surrogate compound P8 in the hydrophobic phase) and are therefore almost invariant. The implications of the results for the development of aerosol models are discussed.

Description: Air quality models that generate the concentrations of semi-volatile and other condensable organic compounds using an explicit reaction mechanism require estimates of the physical and thermodynamic properties of the compounds that affect gas/aerosol partitioning: vapour pressure (as a subcooled liquid), and activity coefficients in the aerosol phase. The model of Griffin, Kleeman and co-workers (e.g., Griffin et al., 2003; Kleeman et al., 1999) assumes that aerosol particles consist of an aqueous phase, containing inorganic electrolytes and soluble organic compounds, and a hydrophobic phase containing mainly primary hydrocarbon material. Thirty eight semi-volatile reaction products are grouped into ten surrogate species which partition between the gas phase and both phases in the aerosol. Activity coefficients of the organic compounds are calculated using UNIFAC. In a companion paper (Clegg et al., 2008) we examine the likely uncertainties in the vapour pressures of the semi-volatile compounds and their effects on partitioning over a range of atmospheric relative humidities. In this work a simulation for the South Coast Air Basin surrounding Los Angeles, using lower vapour pressures of the semi-volatile surrogate compounds consistent with estimated uncertainties in the boiling points on which they are based, yields a doubling of the predicted 24-h average secondary organic aerosol concentrations. The dependency of organic compound partitioning on the treatment of inorganic electrolytes in the air quality model, and the performance of this component of the model, are determined by analysing the results of a trajectory calculation using an extended version of the Aerosol Inorganics Model of Wexler and Clegg (2002). Simplifications are identified where substantial efficiency gains can be made, principally: the omission of dissociation of the organic acid surrogates; restriction of aerosol organic compounds to one of the two phases (aqueous or hydrophobic) where equilibrium calculations suggest partitioning strongly in either direction; a single calculation of activity coefficients of the organic compounds for simulations where they are determined by the presence of one component at high concentration in either phase (i.e., water in the aqueous phase, or a hydrocarbon surrogate compound P8 in the hydrophobic phase) and are therefore almost invariant. The implications of the results for the development of aerosol models are discussed.

Description: Air quality models that generate the concentrations of semi-volatile and other condensable organic compounds using an explicit reaction mechanism require estimates of the vapour pressures of the organic compounds that partition between the aerosol and gas phases. The model of Griffin, Kleeman and co-workers (e.g., Griffin et al., 2005) assumes that aerosol particles consist of an aqueous phase, containing inorganic electrolytes and soluble organic compounds, and a hydrophobic phase containing mainly primary hydrocarbon material. Thirty eight semi-volatile reaction products are grouped into ten surrogate species. In Part 1 of this work (Clegg et al., 2008) the thermodynamic elements of the gas/aerosol partitioning calculation are examined, and the effects of uncertainties and approximations assessed, using a simulation for the South Coast Air Basin around Los Angeles as an example. Here we compare several different methods of predicting vapour pressures of organic compounds, and use the results to determine the likely uncertainties in the vapour pressures of the semi-volatile surrogate species in the model. These are typically an order of magnitude or greater, and are further increased when the fact that each compound represents a range of reaction products (for which vapour pressures can be independently estimated) is taken into account. The effects of the vapour pressure uncertainties associated with the water-soluble semi-volatile species are determined over a wide range of atmospheric liquid water contents. The vapour pressures of the eight primary hydrocarbon surrogate species present in the model, which are normally assumed to be involatile, are also predicted. The results suggest that they have vapour pressures high enough to exist in both the aerosol and gas phases under typical atmospheric conditions.

Description: The effect of global climate change on the annual average concentration of fine particulate matter (PM2.5) in California was studied using a climate-air quality modeling system composed of global through regional models. Output from the NCAR/DOE Parallel Climate Model (PCM) generated under the "business as usual" global emissions scenario was downscaled using the Weather Research and Forecasting (WRF) model followed by air quality simulations using the UCD/CIT airshed model. The system represents major atmospheric processes acting on gas and particle phase species including meteorological effects on emissions, advection, dispersion, chemical reaction rates, gas-particle conversion, and dry/wet deposition. The air quality simulations were carried out for the entire state of California with a resolution of 8-km for the years 2000–2006 (present climate with present emissions) and 2047–2053 (future climate with present emissions). Each of these 7-year analysis periods was analyzed using a total of 1008 simulated days to span a climatologically relevant time period with a practical computational burden. The 7-year windows were chosen to properly account for annual variability with the added benefit that the air quality predictions under the present climate could be compared to actual measurements. The climate-air quality modeling system successfully predicted the spatial pattern of present climate PM2.5 concentrations in California but the absolute magnitude of the annual average PM2.5 concentrations were under-predicted by ~4–39% in the major air basins. The majority of this under-prediction was caused by excess ventilation predicted by PCM-WRF that should be present to the same degree in the current and future time periods so that the net bias introduced into the comparison is minimized. Surface temperature, relative humidity (RH), rain rate, and wind speed were predicted to increase in the future climate while the ultra violet (UV) radiation was predicted to decrease in major urban areas in the San Joaquin Valley (SJV) and South Coast Air Basin (SoCAB). These changes lead to a predicted decrease in PM2.5 mass concentrations of ~0.3–0.7 μg m−3 in the southern portion of the SJV and ~0.3–1.1 μg m−3 along coastal regions of California including the heavily populated San Francisco Bay Area and the SoCAB surrounding Los Angeles. Annual average PM2.5 concentrations were predicted to increase at certain locations within the SJV and the Sacramento Valley (SV) due to the effects of climate change, but a corresponding analysis of the annual variability showed that these predictions are not statistically significant (i.e. the choice of a different 7-year period could produce a different outcome for these regions). Overall, virtually no region in California outside of coastal + central Los Angeles, and a small region around the port of Oakland in the San Francisco Bay Area experienced a statistically significant change in annual average PM2.5 concentrations due to the effects of climate change in the present~study. The present study employs the highest spatial resolution (8 km) and the longest analysis windows (7 years) of any climate-air quality analysis conducted for California to date, but the results still have some degree of uncertainty. Most significantly, GCM calculations have inherent uncertainty that is not fully represented in the current study since a single GCM was used as the starting point for all calculations. The PCM results used in the current study predicted greater wintertime increases in air temperature over the Pacific Ocean than over land, further motivating comparison to other GCM results. Ensembles of GCM results are usually employed to build confidence in climate calculations. The current results provide a first data-point for the climate-air quality analysis that simultaneously employ the fine spatial resolution and long time scales needed to capture the behavior of climate-PM2.5 interactions in California. Future downscaling studies should follow up with a full ensemble of GCMs as their starting point, and include aerosol feedback effects on local meteorology.

Description: Molecular markers are organic compounds used to represent known sources of particulate matter (PM) in statistical source apportionment studies. The utility of molecular markers depends on, among other things, their ability to represent PM volatility under realistic atmospheric conditions. We measured the particle-phase concentrations and temperature-induced volatility of commonly-used molecular markers in California's heavily polluted San Joaqin Valley. Concentrations of elemental carbon, organic carbon, levoglucosan, and polycyclic aromatic hydrocarbons were not reduced by mild (~10 K) heating. In contrast, both hopane/sterane and n-alkane concentrations were reduced, especially during the summer sampling events at the urban site. These results suggest that hopanes and steranes have effective saturation concentrations ~1 μg m−3, and therefore can be considered semi-volatile. The volatility of an individual compound depends both on its inherent properties (primarily vapour pressure) and the interactions between itself and any potential absorbing phase. The volatility behavior of n-alkanes during the urban summer is consistent with that predicted for absorption by suberic acid (a C8 diacid) using a group contribution modelling method. Observations can also be matched by an absorbent whose composition is based on recently-obtained high-resolution aerosol mass spectrometer factors (approximately 33% "hydrocarbon-like" and 67% oxygenated organic aerosol). The reduced evaporation of the n-alkanes, hopanes, and steranes with mild heating during rural and/or winter experiments could be explained by a more oxygenated absorbing phase along with a non-absorptive partitioning mechanism, such as adsorption to soot. This suggests that the temperature-induced volatility of large hydrocarbons in PM is most important if a relatively non-polar absorbing organic phase exists. While the activity coefficients of most organic aerosol compounds may be close to unity, the assumption of ideality for large hydrocarbons (e.g., hopanes) may result in large errors in partitioning calculations.

Description: The effect of climate change on population-weighted concentrations of particulate matter (PM) during extreme pollution events was studied using the Parallel Climate Model (PCM), the Weather Research and Forecasting (WRF) model and the UCD/CIT 3-D photochemical air quality model. A "business as usual" (B06.44) global emissions scenario was dynamically downscaled for the entire state of California between the years 2000–2006 and 2047–2053. Air quality simulations were carried out for 1008 days in each of the present-day and future climate conditions using year-2000 emissions. Population-weighted concentrations of PM0.1, PM2.5, and PM10 total mass, components species, and primary source contributions were calculated for California and three air basins: the Sacramento Valley air basin (SV), the San Joaquin Valley air basin (SJV) and the South Coast Air Basin (SoCAB). Results over annual-average periods were contrasted with extreme events. The current study found that the change in annual-average population-weighted PM2.5 mass concentrations due to climate change between 2000 vs. 2050 within any major sub-region in California was not statistically significant. However, climate change did alter the annual-average composition of the airborne particles in the SoCAB, with notable reductions of elemental carbon (EC; −3%) and organic carbon (OC; −3%) due to increased annual-average wind speeds that diluted primary concentrations from gasoline combustion (−3%) and food cooking (−4%). In contrast, climate change caused significant increases in population-weighted PM2.5 mass concentrations in central California during extreme events. The maximum 24-h average PM2.5 concentration experienced by an average person during a ten-yr period in the SJV increased by 21% due to enhanced production of secondary particulate matter (manifested as NH4NO3). In general, climate change caused increased stagnation during future extreme pollution events, leading to higher exposure to diesel engines particles (+32%) and wood combustion particles (+14%) when averaging across the population of the entire state. Enhanced stagnation also isolated populations from distant sources such as shipping (−61%) during extreme events. The combination of these factors altered the statewide population-averaged composition of particles during extreme events, with EC increasing by 23 %, nitrate increasing by 58%, and sulfate decreasing by 46%.

Description: The effect of global climate change on the annual average concentration of fine particulate matter (PM2.5) in California was studied using a climate – air quality modeling system composed of global through regional models. Output from the NCAR/DOE Parallel Climate Model (PCM) generated under the "business as usual" global emissions scenario was downscaled using the Weather Research and Forecasting (WRF) model followed by air quality simulations using the UCD/CIT airshed model. The air quality simulations were carried out for the entire state of California with a resolution of 8-km for the years 2000–2006 (present climate) and 2047–2053 (future climate). The 7-year windows were chosen to properly account for annual variability with the added benefit that the air quality predictions under the present climate could be compared to actual measurements. The climate – air quality modeling system successfully predicted the spatial pattern of present climate PM2.5 concentrations in California but the absolute magnitude of the annual average PM2.5 concentrations were under-predicted by ~35–40% in the major air basins. The majority of this under-prediction was caused by excess ventilation predicted by PCM-WRF that should be present to the same degree in the current and future time periods so that the net bias introduced into the comparison is minimized. Surface temperature, relative humidity (RH), rain rate, and wind speed were predicted to increase in the future climate while the ultra violet (UV) radiation was predicted to decrease in major urban areas in the San Joaquin Valley (SJV) and South Coast Air Basin (SoCAB). These changes resulted in a ~0.6–1.9 μg m−3 decrease in predicted PM2.5 concentrations in coastal and central Los Angeles. Annual average PM2.5 concentrations were predicted to increase at certain locations within the SJV and the Sacramento Valley due to the effects of climate change, but a corresponding analysis of the annual variability showed that these predictions are not statistically significant (i.e. the choice of a different 7-year period could produce a different outcome for these regions). Overall, virtually no region in California outside of coastal and central Los Angeles experienced a statistically significant change in annual average PM2.5 concentrations due to the effects of climate change in the present study. The present study employs the highest spatial resolution (8 km) and the longest analysis windows (7 years) of any climate-air quality analysis conducted for California to date, but the results still have some degree of uncertainty. Most significantly, GCM calculations have inherent uncertainty that is not fully represented in the current study since a single GCM was used as the starting point for all calculations. Ensembles of GCM results are usually employed to build confidence in climate calculations. The current results provide a first data-point for the climate-air quality analysis that simultaneously employ the fine spatial resolution and long time scales needed to capture the behavior of climate-PM2.5 interactions in California. Future downscaling studies should follow up with a full ensemble of GCMs as their starting point.