Secondary organic aerosol characterisation at field sites across the United States during the spring–summer period
Sources of secondary organic carbon at 15 field sites across the United States (U.S.) during the years 2003-2010 have been examined. Filter samples have been taken for 24-h at a site in Research Triangle Park, NC; at SEARCH sites in southeastern U.S. during May and August 2005; at LADCO sites from Mar 2004-Feb 2005; Riverside, CA during SOAR in 2005; Cleveland, OH during CMAPS; and Pasadena and Bakersfield, CA during CalNex (see text for acronyms.) Samples were extracted, derivatised, and analysed for organic tracers by GC-MS. The mass fraction method described by Kleindienst et al. was used to determine the contributions of the tracers to secondary organic carbon mass. Secondary organic aerosol masses were determined using laboratory-derived values for the organic mass-organic carbon (OM/OC) ratio. Results from the analysis show secondary organic carbon in the eastern and midwestern U.S. to be consistently dominated by SOA from biogenic emissions during the spring-summer period. SOA from biogenic emissions are far less important in the western U.S. during the same period with isoprene emissions being particularly weak. These sites in the western U.S. are in more densely populated, polluted regions of California and are probably not representative of sites in the rural western U.S. The ratio of tracers from monoterpenes can also provide information regarding presumed sources. Similarly, the ratio of isoprene tracers can provide information on reaction pathways (NOX vs. non-NOX) leading to the formation of SOA in the atmosphere. Updated tables for the identity and fragmentation of SOA molecular tracers and for mass fractions of four biogenic class types (isoprene, monoterpenes, sesquiterpenes, 2-methyl-3-buten-2-ol) and two anthropogenic class types (aromatic hydrocarbons and 2-ring PAHs) are given.
Available from: Katherine von Stackelberg
- "Given our objective to estimate the public health impact of aromatic SOA, CMAQv5.0 model results must be adjusted to reflect any biases specific to this PM2.5 component. Monthly-averaged model results are compared against empirical estimates of aromatic SOA concentrations derived from ambient measurements of 2,3-dihydroxy-4-oxopentanoic acid collected at twelve locations across the U.S. ([20-23,40]. We develop region-specific regression relationships between modeled CMAQ values and measured concentrations in μg of carbon per m3 and use these to adjust the model results prior to estimating health effects. "
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Aromatic hydrocarbons emitted from gasoline-powered vehicles contribute to the formation of secondary organic aerosol (SOA), which increases the atmospheric mass concentration of fine particles (PM2.5). Here we estimate the public health burden associated with exposures to the subset of PM2.5 that originates from vehicle emissions of aromatics under business as usual conditions.
The PM2.5 contribution from gasoline aromatics is estimated using the Community Multiscale Air Quality (CMAQ) modeling system and the results are compared to ambient measurements from the literature. Marginal PM2.5 annualized concentration changes are used to calculate premature mortalities using concentration-response functions, with a value of mortality reduction approach used to monetize the social cost of mortality impacts. Morbidity impacts are qualitatively discussed.
Modeled aromatic SOA concentrations from CMAQ fall short of ambient measurements by approximately a factor of two nationwide, with strong regional differences. After accounting for this model bias, the estimated public health impacts from exposure to PM2.5 originating from aromatic hydrocarbons in gasoline lead to a central estimate of approximately 3800 predicted premature mortalities nationwide, with estimates ranging from 1800 to over 4700 depending on the specific concentration-response function used. These impacts are associated with total social costs of $28.2B, and range from $13.6B to $34.9B in 2006$.
These preliminary quantitative estimates indicate particulates from vehicular emissions of aromatic hydrocarbons demonstrate a nontrivial public health burden. The results provide a baseline from which to evaluate potential public health impacts of changes in gasoline composition.
Available from: Robert Pinder
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ABSTRACT: Isoprene significantly contributes to organic aerosol in the southeastern United States where biogenic hydrocarbons mix with anthropogenic emissions. In this work, the Community Multiscale Air Quality model is updated to predict isoprene aerosol from epoxides produced under both high- and low-NOx conditions. The new aqueous aerosol pathways allow for explicit predictions of two key isoprene-derived species, 2-methyltetrols and 2-methylglyceric acid, that are more consistent with observations than estimates based on semivolatile partitioning. The new mechanism represents a significant source of organic carbon in the lower 2 km of the atmosphere and captures the abundance of 2-methyltetrols relative to organosulfates during the simulation period. For the parametrization considered here, a 25% reduction in SOx emissions effectively reduces isoprene aerosol, while a similar reduction in NOx leads to small increases in isoprene aerosol.
Available from: Yuyan Cui
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ABSTRACT: The Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics & Nitrogen (BEACHON) project seeks to understand the feedbacks and inter-relationships between hydrology, biogenic emissions, carbon assimilation, aerosol properties, clouds and associated feedbacks within water-limited ecosystems. The Manitou Experimental Forest Observatory (MEFO) was established in 2008 by the National Center for Atmospheric Research to address many of the BEACHON research objectives, and it now provides a fixed field site with significant infrastructure. MEFO is a mountainous, semi-arid ponderosa pine-dominated forest site that is normally dominated by clean continental air but is periodically influenced by anthropogenic sources from Colorado Front Range cities. This article summarizes the past and ongoing research activities at the site, and highlights some of the significant findings that have resulted from these measurements. These activities include - soil property measurements; - hydrological studies; - measurements of high-frequency turbulence parameters; - eddy covariance flux measurements of water, energy, aerosols and carbon dioxide through the canopy; - determination of biogenic and anthropogenic volatile organic compound emissions and their influence on regional atmospheric chemistry; - aerosol number and mass distributions; - chemical speciation of aerosol particles; - characterization of ice and cloud condensation nuclei; - trace gas measurements; and - model simulations using coupled chemistry and meteorology. In addition to various long-term continuous measurements, three focused measurement campaigns with state-of-the-art instrumentation have taken place since the site was established, and two of these studies are the subjects of this special issue: BEACHON-ROCS (Rocky Mountain Organic Carbon Study, 2010) and BEACHON-RoMBAS (Rocky Mountain Biogenic Aerosol Study, 2011).
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