Analysis and Modeling of Fine Particulate Matter (PMfine) in the Southeast United States

Abstract

Particulate matter in the atmosphere with aerodynamic diameter 2.5 micrometer or less (PM[subscript 2.5]) has been studied extensively due to its impact on human health, climate, and other environmental effects. PM[subscript 2.5]) in the southeast US as a whole and particularly in North Carolina (NC) are the focus of this research. The main body of this study includes two parts. The first part focuses on regional i.e. Southeast U.S. PM[subscript 2.5]) analysis consisting of two urban inland sites including locations N.Birmingham, AL (BHM) and Jefferson, GA (JST); two urban coastal sites viz., Gulfport, MS (GFP), Pensacola and FL (PNS); and three rural sites including Yorkville, GA (YRK), Centreville, AL (CTR), and Oak Grove, MS (OAK) (Chapter 2). The second part focuses on North Carolina counties consisting of three urban sites and two rural/agriculture sites in NC. The three urban N.C. sites are located in Forsyth County, Mecklenburg County, and Wake County, respectively; and the two rural/agriculture sites are located in Lenoir County and Caswell County, respectively (Chapter 3).

The analysis of data and results indicate that the annual National Ambient Air Quality Standard (NAAQS) for PM[subscript 2.5]) (annual standard: 15 mg/m³) is exceeded in all of urban inland areas and one agriculture site which is surrounded by several power plants, but the annual average of PM[subscript 2.5]) is less than the annual standard at all of the other agricultural sites and urban coastal sites. High annual average concentrations are dominated by elevated values during summer. The 24-hour standard (65 mg/m³) for PM[subscript 2.5]) is not exceeded at any of the urban and/or agriculture sites.

In the first part, analysis of chemical species revealed that yearly average fractions of organic matter (OM) with yearly average 4.26 <font mg/m&#179;, sulfate (SO&#8324;&#178;&#8315;) with yearly average 4.23 mg/m&#179;, ammonium (NH&#8324;&#8314;) with yearly average 1.89 mg/m&#179; and nitrate (NO&#8323;&#8315;) with yearly average 0.67 mg/m&#179;, elemental carbon (EC) with yearly average 1.04 mg/m&#179; to total PM[subscript 2.5] range between 28-33%, 27-37%, 11-19%, and 3-7%, 5-12% respectively. In the second part, OM (47-50%) with yearly average 7.22 mg/m&#179;, SO&#8324;&#178;&#8315; (27-32%) with yearly average 4.63 mg/m&#179; and NH&#8324;&#8314; (9-11%) with yearly average 1.54 mg/m&#179;, NO&#8323;&#8315; (5-7%) with yearly average 1.04 mg/m&#179; and EC (1-4%) with yearly average 0.51 mg/m&#179; dominate the identifiable components at all sites. Difference in measured fractions of organic carbon matter between the two parts may be partly accounted by different carbon measurement technologies used. Total PM[subscript 2.5]) mass concentrations are found higher during summer, suggesting that warm conditions favor the formation of secondary aerosol components such as sulfate and organic matter. The equivalent ratio of NH&#8324;&#8314; to SO&#8324;&#178;&#8315; equals or is less than 1, suggesting that in most areas in southeast US, NH&#8324;&#8314; combines with SO&#8324;&#178;&#8315; and exists as (NH&#8324;)&#8322;SO&#8324; not NH&#8324;NO&#8323;. Which suggests that the particle nitrate is limited by the availability of NH&#8323; at most locations in the southeast.

Analysis of major chemical components also shows that ratios of organic carbon/elemental carbon (OC/EC) at all sites are higher than 2, especially at rural sites, mirroring the significant presence of secondary organic aerosols (up to 90%) formed perhaps through VOCs gas-to-particle conversion processes. Minimum OC/EC ratio method has been used to estimate the secondary organic carbon (OC[subscript sec]) formation using the equation

OC[subscript sec] = OC[subscript tot]- EC *(OC/EC)[subscript min]

where (OC/EC)min is the minimum ratio observed.

The secondary OC is found to consist of 41- 50% of the total OC at the seven sites analyzed in the first part; but the OCsec/OC ratios are higher (47-68%) for the North Carolina sites, due to different measurement technique, and different environmental conditions.

The correlation analysis and regression analysis show PM[subscript 2.5] is well correlated with O&#8323;. Also, PM[subscript 2.5] mass concentration exhibits a negative correlation with wind speed, and precipitation and a positive association with temperature, and solar radiation. Finally, origins of pollutant sources were addressed using the combinations of back trajectory analysis (HYSPLIT4 model) and hierarchical cluster analysis. This reveals that transport from northeast (North Carolina) direction exerts strong influence on PM[subscript 2.5] mass concentrations in the study areas. Back trajectory analysis also reveals air trajectories coming from marine sector (east and south direction) or from the upper troposphere and then transferring down to the surface carry low PM[subscript 2.5]. Comparably, air trajectories from continental direction of lower atmosphere bring air mass with high PM[subscript 2.5] to NC.