THE OCCURRENCE OF SULFURIC ACID-WATER NUCLEATION IN PLUMES - URBAN-ENVIRONMENT

The production of new sulfuric acid-water particles via homogeneous nu cleation was examined in plumes originating from urban point sources. The problem was approached by modeling the time development of the gas phase sulfuric acid concentration during the plume transport, and the reby identifying the meteorological, source-related, and environmental variables governing the behavior of this system. The development of t he plume nucleation probability was shown to be dictated by a complica ted and nonlinear interaction between the plume dispersion, chemistry, and condensation of sulfuric acid from the gas phase. For all the sim ulated plumes, those originating From stronger sources and dispersing at slower rates, or having higher SO2 to fine particulate matter ratio s, or higher SO2 to NOx concentration ratios, showed greater nucleatio n probabilities indicative of a higher capability for producing new pa rticles. Nucleation was also favored by cool and humid ambient conditi ons, greater actinic flux, and higher background hydrocarbon concentra tions. The most effective way to decrease the plume nucleation probabi lity was to decrease SO2 emissions alone. Reducing NOx or particle emi ssions may decrease the total particulate mass but leads to higher plu me nucleation probabilities, which is likely to increase particle numb er concentrations farther downwind of the source.