Seasonal and spatial trends in primary and secondary organic carbon concentrations in southeast Texas

Abstract

Seasonal and spatial trends in organic carbon (OC) concentrations in fine particulate matter (PM) were examined using data collected through the regulatory fine PM monitoring network in southeast Texas, and data collected during the Gulf Coast Aerosol Research and Characterization Study (GC-ARCH or Houston Supersite). Primary OC concentrations in the aerosol were estimated by establishing a linear relationship between primary OC and elemental carbon (EC) from 24-hour integrated samples. The relationship between primary OC and EC varied by site and season. Secondary OC (SOC) was estimated as the difference between total OC and primary OC and was found to be between 0.65±1.11 and 1.15±1.52 μg m −3 among sites (mass of carbon only), averaged over 2 years. The mean fraction of SOC in PM 2.5 was found to be between 0.05±0.06 and 0.10±0.33. Primary OC concentrations were on average higher than SOC concentrations at all locations, and were between 1.50±0.69 and 2.89±1.03 μg m −3 (mass of carbon only). Both primary and SOC have highest monthly mean concentrations in early fall through late winter throughout the region. Spatially, sites that are closer to urban emissions have, on average, higher primary and SOC concentrations. One exception to this trend is a remote site located in a forested area north of Houston, where primary and SOC tend to be as high as at urban/industrial locations.