D. Alan Hansen

The Southeastern Aerosol Research and Characterization Study: Part 1—Overview

Abstract 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.

Uncertainties in predicted ozone concentrations due to input uncertainties for the UAM-V photochemical grid model applied to the July 1995 OTAG domain

The photochemical grid model, UAM-V, has been used by regulatory agencies to make decisions concerning emissions controls, based on studies of the July 1995 ozone episode in the eastern US. The current research concerns the effect of the uncertainties in UAM-V input variables (emissions, initial and boundary conditions, meteorological variables, and chemical reactions) on the uncertainties in UAM-V ozone predictions. Uncertainties of 128 input variables have been estimated and most range from about 20% to a factor of two. 100 Monte Carlo runs, each with new resampled values of each of the 128 input variables, have been made for given sets of median emissions assumptions. Emphasis is on the maximum hourly-averaged ozone concentration during the 12–14 July 1995 period. The distribution function of the 100 Monte Carlo predicted domain-wide maximum ozone concentrations is consistently close to log-normal with a 95% uncertainty range extending over plus and minus a factor of about 1.6 from the median. Uncertainties in ozone predictions are found to be most strongly correlated with uncertainties in the NO2 photolysis rate. Also important are wind speed and direction, relative humidity, cloud cover, and biogenic VOC emissions. Differences in median predicted maximum ozone concentrations for three alternate emissions control assumptions were investigated, with the result that (1) the suggested year-2007 emissions changes would likely be effective in reducing concentrations from those for the year-1995 actual emissions, that (2) an additional 50% NOx emissions reductions would likely be effective in further reducing concentrations, and that (3) an additional 50% VOC emission reductions may not be effective in further reducing concentrations.

The Southeastern Aerosol Research and Characterization Study: Part II. Filter-Based Measurements of Fine and Coarse Particulate Matter Mass and Composition

Abstract 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 µg/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 µg/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 ≥60% of PM2.5 mass at SEARCH sites, whereas major metal oxides (MMO) and unidentified components (“other”) account for ≥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 3: Continuous Measurements of Fine Particulate Matter Mass and Composition

Abstract Deployment of continuous analyzers in the Southeastern Aerosol Research and Characterization Study (SEARCH) network began in 1998 and continues today as new technologies are developed. Measurement of fine particulate matter (PM2.5) mass is performed using a dried, 30 °C tapered element oscillating microbalance (TEOM). TEOM measurements are complemented by observations of light scattering by nephelometry. Measurements of major constituents include: (1) SO4 2− via reduction to SO2; (2) NH4 + and NO3 − via respective catalytic oxidation and reduction to NO, (3) black carbon (BC) by optical absorption, (4) total carbon by combustion to CO2, and (5) organic carbon by difference between the latter two measurements. Several illustrative examples of continuous data from the SEARCH network are presented. A distinctive composite annual average diurnal pattern is observed for PM2.5 mass, nitrate, and BC, likely indicating the influence of traffic-related emissions, growth, and break up of the boundary layer and formation of ammonium nitrate. Examination of PM2.5 components indicates the need to better understand the continuous composition of the unmeasured “other” category, because it contributes a significant fraction to total mass during periods of high PM2.5 loading. Selected episodes are presented to illustrate applications of SEARCH data. An SO2 conversion rate of 0.2%/hr is derived from an observation of a plume from a coal-fired power plant during early spring, and the importance of local, rural sources of NH3 to the formation of ammonium nitrate in particulate matter (PM) is demonstrated.

Air Quality Measurements for the Aerosol Research and Inhalation Epidemiology Study

Abstract Measurements of pollutant gases, airborne particulate matter mass and composition, and meteorology have been made at a core site near downtown Atlanta, GA, since August 1998 in support of the Aerosol Research and Inhalation Epidemiology Study (ARIES). This site is one of eight in the Southeastern Aerosol Research and Characterization network. The measurement objective is to provide a long-term, multivariate dataset suitable for investigating statistical associations of respiratory and cardiovascular disease with airborne particulate matter composition, meteorology, and copollutant gases through epidemiologic modeling. Measurements are expected to continue through 2010. Ancillary multiyear measurements at additional sites in the Atlanta metropolitan area and in short-term exposure assessments have been used to estimate the exposure/measurement error associated with using data from a central site to approximate human exposures for the entire area. To date, 13-, 25-, and 53-month air quality datasets have been used in epidemiologic analyses.

Analytical Determination of the Aerosol Organic Mass-to-Organic Carbon Ratio

Abstract Particulate matter (PM) with an aerodynamic diameter ≤2.5 μm (PM2.5) was collected daily (mid-July 1998 to the end of December 1999) over a 24-hr sampling period in a mixed light industrial-residential area in Atlanta, GA, to provide a subset of data for the Aerosol Research and Inhalation Epidemiology Study (ARIES). This study included the measurement of organic carbon (OC), elemental carbon (EC), and individual organic compounds. OC and EC average mean concentrations were 4.50 ± 0.33 and 2.08 ± 0.19 μg/m3, respectively. The ratio of organic matter mass (OM) to OC in PM2.5 aerosols in Atlanta was measured using three different approaches: (1) solvent extract residue gravimetric masses to individual OC concentrations of sequential apolar to polar solvent extracts (dichloromethane, acetone, and water); (2) mass balance of the PM2.5 measured gravimetric mass minus the mass concentrations of the inorganic/elemental constituents to the total OC concentration; and (3) polar organic compound speciation with the concentration weighted ratio to the total OC concentration. We found very good agreement between approach 1 and 2. The average OM/OC ratio calculated from the extract residue mass was 2.14 ± 0.17. The average OM/OC ratio determined by mass balance was 2.16 ± 0.43 for the whole period. The concentration weighted ratio calculated from the concentrations of polar organic compounds ranged between 1.55 and 1.72, which was likely a lower limit for the ratio because of the limited number of the polar organic compounds that can be quantified using gas chromatographic methods. We found seasonal differences with an OM/OC range of 1.77 in December 1999 to 2.39 in July 1999. These results suggest that the previously accepted value of 1.4 for the OM/OC ratio was too low even for urban locations during the winter months. Molecular-level speciation of the PM2.5-associated organic compounds showed that the concentrations of the molecular markers for wood smoke represented approximately 12–15% of the total polar organic compound concentrations during the winter months

The Quest for an Advanced Regional Air Quality Model

This article describes an international initiative that addresses regional and smaller scale air pollution. It should provide an advanced set of tools for air quality managers to use in making scientifically based policy decisions. The authors refer to these tools as comprehensive modeling systems (CMSs) because of the depth and breadth of their formulations and applications. The initiative is being carried out by the Consortium for Advanced Modeling of Region Air Quality (CAMRAQ). This article explains why the consortium feels the time for a CMS is opportune, what CAMRAQs objectives are, how it is organized, what its roots are, how we are approaching coordinated research, where we currently are collaborating on projects, and what opportunities we see for future collaboration. 21 refs., 1 fig., 1 tab.

A North American Field Study to Evaluate Eulerian Models

The largest environmental measurement program in North American history is underway. It is geared toward providing a two-year data set of air quality, emissions, precipitation composition, and meteorological variables with which the performance of Eulerian acid deposition models can be evaluated. It is sponsored by or includes the participation of over a dozen organizations in governmental and private sectors from Canada, the United States, and West Germany (Table 1). What has led up to this massive undertaking?

Integrating Observations and Modeling in Ozone Management Efforts

Many urban areas in the Eastern United States have been classified to be in non-attainment for ozone, placing a high priority on finding cost-effective emission control measures for improving ambient ozone air quality. Recognizing the complexities associated with the nation’s ozone non-attainment problem, the 1990 Clean Air Amendments mandated the use of grid-based photochemical models for evaluating emission control strategies in urban areas having a serious or higher designation. Given the influx of elevated concentrations of ozone and its precursors into the urban-scale modeling domains (regional-scale transport), many states in the Eastern U.S. were unable to demonstrate ozone attainment for urban areas in their 1994 State Implementation Plans (SIPs) submittal using the urban-scale models. The 1994 SIPs were based on the UAM-IV photochemical model (Morris et al., 1990), which is an urban-scale model that reflects the state-of-science of the late 1980’s. Systems Applications International (SAI) recently developed the UAM-V, a regional-scale ozone air quality model, which contains some new features over the UAM-IV such as multi-scale modeling capability, grid nesting, plume-in-grid (PiG) treatment for point sources, etc. (SAI, 1995). Of particular interest is this model’s treatment of subgrid-scale processes relating to the transport, transformation, and interaction of elevated plumes with the ground-level plume.

An Integrated Modeling and Observational Approach for Designing Ozone Control Strategies for the Eastern U.S.

Despite vigorous attempts to control the ozone problem during the past three decades, ozone levels in many areas over the Eastern United States continue to exceed the National Ambient Air Quality Standards. Until recently, photochemical models were applied to simulate historical ozone episodic events to examine the future ozone non-attainment problem. When the model’s ability to reproduce the observed ozone air quality was deemed acceptable, control measures needed to meet and maintain the ozone standards were evaluated using projected emissions inventories and historical episodic meteorological conditions. Since the episodic meteorological events under which the model has performed best may never occur in the future, there is an inherent uncertainty in the controls identified as required to comply with the ozone standards.