Citation:

Yu, S, R L. Dennis, P. Bhave, AND R Mathur. SEASONAL AND REGIONAL VARIATIONS OF PRIMARY AND SECONDARY ORGANIC AEROSOLS OVER THE CONTINENTAL UNITED STATES: OBSERVATION-BASED ESTIMATES AND MODEL EVALUATION. Presented at 2004 Models-3 Conference, Chapel Hill, NC, October 18-20, 2004.

Impact/Purpose:

The goal of this task is to thoroughly characterize the performance of the emissions, meteorological and chemical/transport modeling components of the Models-3 system, with an emphasis on the chemical/transport model, CMAQ. Emissions-based models are composed of highly complex scientific hypotheses concerning natural processes that can be evaluated through comparison with observations, but not validated. Both performance and diagnostic evaluation together with sensitivity analyses are needed to establish credibility and build confidence within the client and scientific community in the simulations results for policy and scientific applications. The characterization of the performance of Models-3/CMAQ is also a tool for the model developers to identify aspects of the modeling system that require further improvement.

Description:

Due to the lack of an analytical technique for directly quantifying the atmospheric concentrations of primary (OC_pri) and secondary (OC_sec) organic carbon aerosols, different indirect methods have been developed to estimate their concentrations. In this study, seasonal and regional variations of OC_pri and OC_sec over the continental U.S. for the year 2001 were estimated by using observed OC and elemental carbon (EC) data from Interagency Monitoring of Protected Visual Environments (IMPROVE) and Southeastern Aerosol Research and Characterization (SEARCH) networks, coupled with the primary OC/EC ratios, (OC/EC)_pri, obtained from an emission/transport-model (US EPA Models-3/Community Multiscale Air Quality (CMAQ) model) (i.e., emission/transport of primary OC/EC ratio method). These observation-based estimates show that the yearly mean OC_pri concentrations vary greatly by location from 0.27 µg C m^-3 to 3.72 µg C m^-3 (0.71 +/- 0.82 µg C m^-3, mean +/- standard deviation), while the yearly mean OC_sec concentrations vary from 0.16 µg C m^-3 to 1.76 µg C m^-3 (0.61 +/- 1.04 µg C m^-3). Seasonal analysis shows that OC_sec concentrations are 0.34 +/- 0.62, 0.47 +/- 0.86, 0.80 +/- 1.03, and 0.72 +/- 1.30 µg C m^-3 in winter, spring, summer, and fall, respectively, making 30, 41, 51, and 42% contributions to OC respectively. Regional analysis indicates that both OC_pri and OC_sec concentrations are the highest over the southeast area (yearly mean OC_pri: 1.00 +/- 1.23 µg C m^-3 and OC_sec: 1.25 +/- 0.96 µg C m^-3) in all seasons. It is found that OC_pri accounts for a large fraction of OC over the Northeast, Southeast, Midwest, West and Midwest (>56%) in all seasons except summer, during which OC_pri and OC_sec concentrations make approximately equal contributions to OC. On the basis of comparison of OC_pri and OC_sec at the nearest paired urban/rural sites from SEARCH, the yearly mean OC_pri concentrations at the urban sites contribute 71 to 80% to OC, larger than the OC_pri contributions at the corresponding paired rural sites (54 to 74%). In contrast, the yearly mean OC_sec concentrations at rural sites frequently exceed those at the nearby urban sites. The higher OC_sec concentrations during the summer at the three rural sites result in the higher yearly contributions of OC_sec to OC (41 to 46%) than at the corresponding paired urban sites (20 to 29%).

The comparisons of the model and observation-based estimates show that over the continental United States, the modeled OCsec concentrations are 0.49 +/- 0.45, 0.67 +/- 0.59, 0.96 +/- 0.99, 1.06 +/- 1.03, and 0.84 +/- 0.87 µg C m^-3 in winter, spring, summer, fall and year, respectively, slightly higher than the observation-based concentration estimates listed above. The modeled OC_pri concentrations are 0.74 +/- 0.96, 0.58 +/- 0.81, 0.86 +/- 1.02, 0.69 +/- 0.63, and 0.72 +/- 0.86 µg C m^-3 in winter, spring, summer, fall and year, respectively, very close to those of the observations which are 0.70 +/- 0.82, 0.60 +/- 0.58, 0.73 +/- 1.00, 0.78 +/- 0.80, and 0.71 +/- 0.82 µg C m^-3.

The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.

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