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ARE CARS OR TREES MORE IMPORTANT TO PARTICULATE MATTER AIR POLLUTION? WHAT RADIOCARBON MEASUREMENTS HAVE TO SAY
Citation:
Lewis, C W., G. A. Klouda, AND W. D. Ellenson. ARE CARS OR TREES MORE IMPORTANT TO PARTICULATE MATTER AIR POLLUTION? WHAT RADIOCARBON MEASUREMENTS HAVE TO SAY. To be Presented at Science Forum 2003, Washington, DC, May 5-7, 2003.
Impact/Purpose:
The analysis of ambient air samples by X-ray fluorescence is a critical component in the interpretation of complex air quality studies. The data products generated by the XRF laboratory are therefore critical to protecting human health and the environment. Analysis of particulate matter by XRF contributes to the following outcomes:
1) By characterizing emissions from different source types EPA is able to ensure timely attainment of the national ambient air quality regulations (NAAQS).
2) These analyses assist in the development of models for predicting pollutant concentrations from source emissions, apportioning sources from air concentration data, and for predicting exposures from different source types. The goal of the aforementioned models is to accurately understand risk and the ways to reduce risks to human health and the environment.
3) Datasets generated by the X-ray fluorescence laboratory cross-cut several laboratories (NERL, NHEERL, NRMRL, etc.) and represent an important collaboration effort. The XRF laboratory products are therefore crucial in the integration of varied, and sometimes wide ranging, agency goals. It is these cross-cutting relationships that enable ORD to collectively assess pollutants which pose the greatest risk to global, regional, state and local populations.
Description:
Air pollution in the form of particulate matter (PM) originates from both human activities and "natural" phenomena. Setting and achieving National Ambient Air Quality Standards (NAAQS) for PM has to take into account the latter since they are in general less controllable than the former. One of the elusive contributors to "natural" PM, particularly during summertime, is biogenic secondary organic aerosol (SOA), which results in the formation of PM from atmospheric gas-to-particle conversion of volatile organic compounds (VOC) emitted by vegetation. Theoretical estimates of the importance of biogenic SOA as a component of summertime PM differ greatly, ranging from negligible to dominant, because of uncertainties in both the conversion mechanism, and the amount and characteristics of biogenic VOC emissions. However, quite direct experimental estimates can be obtained by measuring the amount of the naturally-occurring radioactive isotope 14C (radiocarbon) in a PM sample. The method depends on the tiny but nearly constant fraction of 14C relative to ordinary carbon (12C) in all living and recently living material, and its absence in fossil fuels. Beginning a few years ago a series of fine PM (PM-2.5) summertime samples are being collected at various sites in the Southeastern U.S. for subsequent radiocarbon measurement, including collaboration with the State of Texas in Houston and the State of Florida in Tampa. Initial results indicate a surprisingly large carbonaceous biogenic fraction -- approximately one half -- presumably in large part from SOA. If the initial results are representative this finding has serious implications for PM-2.5 control and standard setting.