PNNL

Abstract

Overnight transport and chemical processing of urban and petrochemical industrial plumes from Houston, Texas, were investigated during July 2005 as part of the Southeast Texas Transport Study (SETTS). We report here on the July 26 episode in which the photochemically aged urban + industrial plume observed just north of the Houston metropolitan area at sunset was tagged with a pair of Controlled-Meteorological (CMET) balloons and tracked for 8 hours as the plume drifted ~300 km further north. An instrumented aircraft sampled in the plume at sunset and twice again at increasing downwind distances/processing times as guided by the positions of the CMET balloons. Analysis of the aircraft and CMET balloon data showed that while the plume experienced appreciable shearing due to the development of a low-altitude nocturnal jet, the pollutant concentrations remained undiluted even after 8 hours of transport due to lack of turbulent mixing. Analysis of aircraft data provided evidence for significant conversion of NOx to semi-volatile nitrates (i.e., gaseous HNO3 + particulate inorganic and organic nitrates) over this period. A constrained plume modeling analysis of the quasi-Lagrangian aircraft observations showed that NOx was likely converted to HNO3 and semi-volatile organic nitrates via reactions of industrial olefins and aldehydes with NO3 radicals. Heterogeneous hydrolysis of N2O5 to HNO3 appears to have played a negligible role. The overnight transport of such concentrated and aged urban/industrial plumes could potentially affect the air quality of regions several hundred kilometers downwind the next day.