[1] Particle size distributions and gas-phase particle precursors and tracer species were measured aboard an aircraft in the plumes downwind from industrial and urban sources in the vicinity of Houston, TX during the daytime in late August and early September 2000. Plumes originating from the Parish gas-fired and coal-fired power plant, petrochemical industries along the Houston ship channel, the petrochemical facilities near the Gulf coast, and the urban center of Houston were studied. Most of the particle mass flux advected downwind of Houston came from the industries and electrical utilities at the periphery of the city rather than from sources in the urban core. In SO 2 -rich plumes that did not contain elevated concentrations of volatile organic compounds (VOCs), particle volume increased with increasing plume oxidation (age) at a rate consistent with condensation and neutralization of the gas-phase oxidation products of SO 2 . In plumes that were rich in both SO 2 and VOCs, observed particle growth greatly exceeded that expected from SO 2 oxidation, indicating the formation of organic particulate mass. In plumes that were enhanced in VOCs but not in SO 2 , and in the plume of the Houston urban center, no particle volume growth with increasing plume oxidation was detected. Since substantial particle volume growth was associated only with SO 2 -rich plumes, these results suggest that photochemical oxidation of SO 2 is the key process regulating particle mass growth in all the studied plumes in this region. However, uptake of organic matter probably contributes substantially to particle mass in petrochemical plumes rich in both SO 2 and VOCs. Quantitative studies of particle formation and growth in photochemical systems containing nitrogen oxides (NO x = NO + NO 2 ), VOCs, and SO 2 are recommended to extend those previously made in NO x -VOC systems.

A low-turbulence, aerosol sampling inlet (LTI) has been developed for use on aircraft. The inlet makes use of boundary layer suction in a porous diffuser to slow the sample flow from aircraft air speeds near 150 m/s to velocities near 5 m/s without generating turbulence. The reduction of turbulence reduces losses of supermicron particles by turbulen. t deposition and permits the use of laminar flow calculations and well-understood drag formulations to accurately predict particle motion. Large particles are enhanced in the sample flow due to inertia. These enhancements are predicted with numerical analysis of fluid flow and integration of the equations of motion for the particles. The diffuser discussed in this article has been used in a number of field experiments, and the enhancement factors have been provided to the experimenters measuring aerosol downstream of the inlet. Some particles are doubtless lost in transport from the LTI to the aircraft fuselage. Estimates of those losses have also been made and provided to experimenters. The enhancement factors (EF) and losses are a function of particle Stokes