The occurrence of sulfuric acid-water nucleation in plumes: urban environment

The production of new sulfuric acid-water particles via homogeneous nucleation 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 thereby identifying the meteorological, source-related, and environmental variables governing the behavior of this system. The development of the plume nucleation probability was shown to be dictated by a complicated and nonlinear interaction between the plume dispersion, chemistry, and condensation of sulfuric acid from the gas phase. For all the simulated plumes, those originating from stronger sources and dispersing at slower rates, or having higher SO₂ to fine particulate matter ratios, or higher SO₂ to NO_x concentration ratios, showed greater nucleation probabilities indicative of a higher capability for producing new particles. Nucleation was also favored by cool and humid ambient conditions, greater actinic flux, and higher background hydrocarbon concentrations. The most effective way to decrease the plume nucleation probability was to decrease SO₂ emissions alone. Reducing NO_x or particle emissions may decrease the total particulate mass but leads to higher plume nucleation probabilities, which is likely to increase particle number concentrations farther downwind of the source.