Continuous ammonia (NH3) measurements with a temporal resolution of 5min were implemented at selected SEARCH sites in the southeastern U. S. during 2007. The SEARCH continuous NH3 instrument uses a citric acid denuder difference technique employing a dual-channel nitric oxide-ozone chemiluminescence analyzer. Data from two SEARCH sites are presented, Jefferson Street, Atlanta (JST) (urban), and Yorkville, Georgia (YRK) (rural), for the period July–December, 2007. Highest NHx (total ammonia=gaseous NH3+PM2.5 NH4+) values were observed in August and September at both JST and YRK. Highest NH3 values occurred in August and September at JST, but in August through October at YRK. Lowest NH3 and NHx values occurred in December at both sites. YRK is significantly impacted by nearby poultry sources, routinely experiencing hourly average NH3 mixing ratios above 20ppbv. Wind sector analysis clearly implicates the nearby poultry operations as the source of the high NH3 values. Weekday versus weekend differences in composite hourly mean diurnal profiles of NH3 at JST indicate that mobile sources have a measurable but relatively small impact on NH3 observed at that site, and little or no impact on NH3 observed at YRK. A distinctive composite mean hourly diurnal variation was observed at both JST and YRK, exhibiting maxima in the morning and evening with a broad minimum during midday. Analysis of observed NH3 diurnal variations from the literature suggests a hypothesized mechanism for the observed behavior based on interaction of local emissions and dry deposition with the formation and collapse of the dynamically mixed atmospheric boundary layer during the day and shallow nocturnal layer at night. Simple mixed layer concentration box model simulations confirm the plausibility of the suggested mechanism.

Particulate matter (PM) with an aerodynamic diameter <= 2.5 mu m (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 mu g/m(3), 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 organic carbon (OC) and elemental carbon (EC) content of filter-based, 24-hr integrated particulate matter with aerodynamic diameters between 2.5 and 10 microm (PM10-2.5) was measured at two urban and two rural locations in the southeastern United States. On average, total carbon (OC + EC) comprised approximately 30% of PM10-2.5 mass at these four sites. Carbonate carbon was measured on a subset of samples from three sites and was found to be undetectable at a rural site in central Alabama, less than 2% of PM10-2.5 at an urban site in Georgia, and less than 10% of PM10-2.5 at an urban-industrial site in Alabama. Manual scanning electron microscopy (SEM) and computer-controlled SEM (CCSEM) along with energy dispersive X-ray spectroscopy (EDS) were used to identify individual carbonaceous particles in a selected subset of samples collected at one rural site and one urban-industrial site in Alabama. CCSEM results showed that biological material (e.g., fungal spores, pollen, and vegetative detritus) accounted for 60-70% of the carbonaceous mass in PM10-2.5 samples with concentrations in the range of 2-16 microg/m3. Samples with higher PM10-2.5 concentrations (25-42 microg/m3) at the urban-industrial site were found by manual SEM to have significant amounts of unidentified carbonaceous material, likely originating from local industrial activities. Both filter-based OC and EC concentrations and SEM-identified biological material tended to have higher concentrations during warmer months. Upper limits for organic mass (OM) to OC ratios (OM/OC) are estimated for PM10-2.5 samples at 2.1 for urban sites and 2.6-2.7 for rural sites.

Twenty-four-hour integrated gaseous NH3 and fine particulate (PM2.5)NH4+ were measured during 2004 at eight sites in the southeastern U.S. Mean NH3 concentrations for 2004 ranged from 2.44 ppbv at an urban-industrial site in North Birmingham, AL, to 0.23 ppbv at a rural-forested site near Centreville, AL. NH3 mixing ratios were found to be higher at urban sites than at nearby rural (or suburban sites) except for sites which were directly influenced by local sources. Only weak correlations with temperature were observed for NH3 at the sites; slightly greater correlations were observed for total ammonia (NHx=NH3+NH4+) vs. temperature. A weak seasonal variation was observed for NH3 mixing ratios at all sites, with all but one site exhibiting biannual maxima in spring and late summer/fall. Mean PM2.5 NH4+ concentrations ranged from 1.78μgm-3 in Atlanta, GA, to 1.02μgm-3 at Oak Grove, MS, and were more uniform across the network than NH3 mixing ratios, with only slightly larger values at urban sites as compared to nearby rural (or suburban) sites. All sites exhibited highest NHx between July and September and lowest NHx in November and December. The gaseous NH3 fraction (NH3/(NH3+NH4+)) was observed to decrease with increasing values of PM2.5 SO42- at all sites. At two rural-forested sites and two sites near the Gulf of Mexico, the NH3 gaseous fraction approached a relatively constant value of 5–10% as PM2.5SO42- increased beyond 5–7μgm-3, suggesting that NH3 availability at these locations limits aerosol neutralization.

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.

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 degrees C tapered element oscillating microbalance (TEOM). TEOM measurements are complemented by observations of light scattering by nephelometry. Measurements of major constituents include: (1) SO42- 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 CO,, 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.