During the Southern Oxidants Study 1999 and the Texas 2000 Air Quality Study, airborne measurements of particle size distributions were made in a variety of power plant, petrochemical, and urban plumes. Data from a 5-channel condensation condensation counter, a laser optical particle counter, and a white light optical particle counter were combined to determine particle size distributions from 0.004-10 µm diameter with 1 second resolution.
In all plumes from power plant and petrochemical industry sources, substantial enhancements in particle mass were found in only those plumes rich in SO2. Even in the plumes of petrochemical complexes with large emissions of NOx, propene and ethene near Houston, Texas, particle enhancements were not detected in the absence of SO2. Nearby, modest sources of SO2 produced detectable enhancements in particle volume.
In the absence of in-cloud oxidation, particulate sulfate is produced by condensation of the oxidation products of SO2 and OH. Gas-phase HNO3 is produced by the oxidation of NO2 by OH; thus the ratio of HNO3 to total reactive nitrogen, NOy, indicates approximate total parcel exposure to OH. Assuming a composition of 50% (NH4)2SO4, the fraction of plume S found in the particulate phase was calculated as function of HNO3/NOy and compared with a 2-dimensional numerical model of plume chemistry and dynamics. In the plume of the Parish power plant near Houston, the increase in plume particulate S fraction with increasing HNO3/NOy was well simulated by the model. In the merged industry plume from the Houston ship channel, the apparent increase in the fraction of plume S in the particulate phase was underpredicted by the model. This difference may be due to model formulation errors, unaccounted SO2 losses, or to particle growth by nonsulfate species. Similar analyses on other days produce very similar results for both the Parish power plant and the coalesced ship channel plumes. These observations point to the oxidation products of SO2 as the primary compounds controlling particulate mass formation from the studied industrial sources. Condensation of other species, particularly semi-volatile organics from petrochemical sources, is not excluded by this analysis, but does not appear to occur in the absence of substantial SO2 oxidation.