Case studies of aerosol size distribution and chemistry during passages of a cold and a warm front

Abstract

Surface-based measurements of particle size distributions, mass, water content, and the ammonium, sulfate and nitrate ion concentrations of atmospheric aerosols, centered around precipitation events associated with cold and warm frontal passages, are reported. In both cases precipitation scavenges coarse aerosols (diameters > 1.5 μm) considerably more efficiently than it removes fine aerosols (diameters < 1.5 μm). The concentration of the coarse mode recovers to near pre-rain levels 3–4 h after the rain for both cases. The rates of precipitation scavenging on the coarse mode are found to be about 48 and 67% h −1 for the cold and warm front cases, respectively. The corresponding value for the fine mode is about 32% h −1 for the warm front case, which had little wind shift and thus no change of source region, but no estimate is made for the cold front case because of the marked wind shift. An estimate of the water content of the ambient aerosol indicates that, at times, as much as 95% of the aerosol mass may be water. With few exceptions, significant increases in the concentrations of aerosol mass and the above chemical components have been observed at the onset of rain, with the nitrate ion increase being most dramatic. Ammonium sulfate is found almost exclusively in the accumulation mode while the ammonium nitrate content varies between the accumulation mode and the larger particle mode.