The Southeastern Aerosol Research and Characterization Study: Part II. Filter-based measurements of fine and coarse particulate matter mass and composition

The Southeastern Aerosol Research and Characterization Study (SEARCH) was implemented in 1998-1999 to provide data and analyses for the investigation of the sources, chemical speciation, and long-term trends of fine particulate matter (PM2.5) and coarse particulate matter (PM10-2.5) in the Southeastern United States. This work is an initial analysis of 5 years (1999-2003) of filter-based PM2.5 and PM10-2.5 data from SEARCH. We find that annual PM2.5 design values were consistently above the National Ambient Air Quality Standards (NAAQS) 15 microg/m3 annual standard only at monitoring sites in the two largest urban areas (Atlanta, GA, and North Birmingham, AL). Other sites in the network had annual design values below the standard, and no site had daily design values above the NAAQS 65 microg/m3 daily standard. Using a particle composition monitor designed specifically for SEARCH, we found that volatilization losses of nitrate, ammonium, and organic carbon must be accounted for to accurately characterize atmospheric particulate matter. In particular, the federal reference method for PM2.5 underestimates mass by 3-7% as a result of these volatilization losses. Organic matter (OM) and sulfate account for approximately 60% of PM2.5 mass at SEARCH sites, whereas major metal oxides (MMO) and unidentified components ("other") account for > or = 80% of PM10-2.5 mass. Limited data suggest that much of the unidentified mass in PM10-2.5 may be OM. For paired comparisons of urban-rural sites, differences in PM2.5 mass are explained, in large part, by higher OM and black carbon at the urban site. For PM10, higher urban concentrations are explained by higher MMO and "other." Annual means for PM2.5 and PM10-2.5 mass and major components demonstrate substantial declines at all of the SEARCH sites over the 1999-2003 period (10-20% in the case of PM2.5, dominated by 14-20% declines in sulfate and 11-26% declines in OM, and 14-25% in the case of PM10-2.5, dominated by 17-30% declines in MMO and 14-31% declines in "other"). Although declining national emissions of sulfur dioxide and anthropogenic carbon may account for a portion of the observed declines, additional investigation will be necessary to establish a quantitative assessment, especially regarding trends in local and regional emissions, primary carbon emissions, and meteorology.     

The Southeastern Aerosol Research and Characterization Study: part 1--Overview

This paper presents an overview of a major, long-term program for tropospheric gas and aerosol research in the southeastern United States. Building on three existing ozone (O3)-focused research sites begun in mid-1992, the Southeastern Aerosol Research and Characterization Study (SEARCH) was initiated in mid-1998 as a 7-year observation and research program with a broader focus including aerosols and an expanded geographical coverage in the Southeast. The monitoring network comprises four urban-rural (or urban-suburban) site pairs at locations along the coast of the Gulf of Mexico and inland, including two moderately sized and two major urban areas (Pensacola, FL; Gulfport, MS; Atlanta, GA; and Birmingham, AL). The sites are equipped with an extensive suite of instruments for measuring particulate matter (PM), gases relevant to secondary O3 and the production of secondary aerosol particles, and surface meteorology. The measurements taken to date have added substantially to the knowledge about the temporal behavior and geographic variability of tropospheric aerosols in the Southeast. Details are presented in four papers to follow.     

The midlatitude North American background aerosol and global aerosol variation

Protocols for the particulate matter (PM) National Ambient Air Quality Standards (NAAQS), and the Regional Haze Rule (RHR) give two complementary definitions for "natural" background airborne particle concentrations in the United States. The definition for the NAAQS derives largely from reported annual averages, whereas the definition for the RHR takes into account the frequency of occurrence of a range of visibility conditions estimated using fine particle composition. These definitions are simple, static representations of background or "unmanageable" aerosol conditions in the United States. An accumulation of data from rural-remote sites representing global conditions indicates that the airborne particle concentrations are highly variable. Observational campaigns show weather-related variations, including incidents of regional or intercontinental transport of pollution that influence background aerosol levels over midlatitude North America. Defining a background in North America based on long-term observations relies mainly on the remote-rural IMPROVE network in the United States, with a few additional measurements from Canada. Examination of the frequency of occurrence of mass concentrations and particle components provides insight not only about annual median conditions but also the variability of apparent background conditions. The results of this analysis suggest that a more elaborate approach to defining an unmanageable background could improve the present approach taken for information input into the U.S. regulatory process. An approach interpreting the continental gradients in fine PM (PM2.5) concentrations and composition may be warranted.     

Organizing For Risk Oriented Climate Alteration Research

The ubiquity in effects and complexity of modern environmental issues requires careful consideration of focused research to insure that answers to key policy oriented questions are obtained. Broad-based, unstructured research programs have proven to be inefficient instruments for characterizing risks from environmental stress. Both the expense and the importance of timeliness of information preclude a traditional “bottom-up” approach to research. Instead, a more “top-down” organized approach that links the natural and socioeconomic sciences has advantages to support environmental risk assessment and research prioritization of climate alteration. Early examples of analysis using an integrating framework for risk assessment focus on the need for research on human interactions and the environmental damage function in addition to the basic earth sciences. Nonlinearities in environmental impact of climate change, and uncertainties in the extent of growth of energy efficiencies, are seen to be key unknowns in the risk assessment of climate alternatives. To date, earth science research has not been structured well for environmental risk assessment of the climate change question. The U.S. national research program aimed at risk assessment of climate alteration is examined as an example. The examination suggests that the present conceptual plan falls short of an optimum structure derived from exploitation of an integrating framework, even though it is rich in scientific strength and diversity. To strengthen its public value and accessibility, the research program could account in its planning for prioritized needs defined by an integrating analysis for risk assessment and management.     

Design Scenario for the Radioisotopic Estimation of the Biogenic Component of Airborne Particles

Abstract

An experimental design is described to estimate the fraction of secondary fine particle from the biogenic component of volatile organic compounds (VOCs) in the atmosphere using radiocarbon isotopic abundance ratios. The method distinguishes between “modern” carbon (C), and “old” C of primary and secondary origins based on three components, condensed-phase organic carbon (OC), semi-volatile particulate compounds (SVOCs), and VOCs. The method depends on interpretation of diurnal and seasonal variation in OC, SVOC, and VOC concentrations. Sampling employs a filter-denuder unit, which collects the three C components for isotopic analysis. The samples are collected repetitively for a daily sequence of the same hourly intervals covering diurnal periods with similar meteorological conditions. Collected C is thermally treated to separate OC from black carbon on filters and VOCs or SVOCs from adsorbents, with all four fractions individually oxidized to carbon dioxide to determine the radiocarbon content by accelerator mass spectrometry. Using C isotope abundance, the data are interpreted for fractions of primary modern C and secondary modern C as estimated from averaging diurnal and seasonal variations in the concentration data. As support for interpretation, samples of OC, SVOCs, and VOCs would be analyzed for speciation to identify source indicator species present.