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SEASONAL AND REGIONAL VARIATIONS OF PRIMARY AND SECONDARY ORGANIC AEROSOLS OVER THE CONTINENTAL UNITED STATES: SEMI-EMPIRICAL ESTIMATES AND MODEL EVALUATION
Citation:
YU, S., P. BHAVE, R. L. DENNIS, AND R. MATHUR. SEASONAL AND REGIONAL VARIATIONS OF PRIMARY AND SECONDARY ORGANIC AEROSOLS OVER THE CONTINENTAL UNITED STATES: SEMI-EMPIRICAL ESTIMATES AND MODEL EVALUATION. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 41(13):4690-4697, (2007).
Impact/Purpose:
The objectives of this task are to continue development and improvement of EPA's mesoscale (regional through urban scale) air quality modeling systems, such as the Community Multiscale Air Quality (CMAQ) model, as air quality management and NAAQS implementation tools. This task focuses on needed research and development of air quality models targeted for a major CMAQ model release in FY08. Model development for a broad scope of application is envisioned. For example, CMAQ will need to be able to simulate air quality feedbacks to meteorology and climate as well as intercontinental transport. The 2008 release of CMAQ is timed to coincide with EPA/OAR's and the states' needs for an improved model for assessments of progress (mid-course corrections) in the post-SIP submittal timeframe.
Description:
Seasonal and regional variations of primary (OCpri) and secondary (OCsec) organic carbon aerosols across the continental U.S. for the year 2001 were examined by a semi-empirical technique using observed OC and elemental carbon (EC) data from 142 routine monitoring sites in mostly rural locations across the U.S, coupled with the primary OC/EC ratios, (OC/EC)pri,obtained from a chemical transport model (i.e., Community Multiscale Air Quality (CMAQ)model). This application yields the first non-mechanistic estimates of the spatial and temporal variations in OCpri and OCsec over an entire year on a continental scale. There is significant seasonal and regional variability in the relative contributions of Cpri and OCsec to OC. Over the continental U.S., the median OCsec concentrations are 0.13, 0.36, 0.63, 0.44 and 0.42 µg C m-3 in winter (DJF), spring (MAM), summer (JJA), fall (SON) and annual, respectively, making 21, 44,51, 42 and 43% contributions to OC respectively. OCpri exceeds OCsec in all seasons except summer. Regional analysis shows that the southeastern region has the highest concentration of OCpri (annual median = 1.35 µg C m-3), whereas the central region has the highest concentration of OCsec (annual median = 0.76 µg C m-3). The mechanistic OCsec estimates from the CMAQ model were compared against the independently-derived semi-empirical OCsec estimates. The results indicate that the mechanistic model reproduced the monthly medians of the semiempirical OCsec estimates well over the northeast, southeast, midwest, and central regions in all months except the summer months (June, July and August), during which the modeled regional monthly medians were consistently lower than the semi-empirical estimates. This indicates that the CMAQ model is missing OCsec formation pathways that are important in the summer.