Mixing characteristics and water content of submicron aerosols measured in Los Angeles and at the grand canyon

Data that were acquired in Claremont, CA, during the Southern California Air Quality Study (SCAQS, summer 1987) and at the Grand Canyon, AZ, during the 1990 Navajo Generating Station Visibility study are examined for information on variabilities in composition among particles of a given size and as a function of size. At both sites a tandem differential mobility analyser (TDMA, McMurry and Stolzenburg, Atmospheric Environment 23: 497-507, 1989) was used to measure hygroscopic properties for particles in the 0.05-0.5 μm diameter range, and the size-resolved composition for major ionic species, carbon and elements was obtained which cascade impactors. At the Grand Canyon the elemental composition, including carbon, was measured by microscopy for 1100 individual submicrometer particles. The TDMA data for both locations shows that aerosols are, to some extent, externally mixed. It was typically found that particles of a given size have two distinctly different hygroscopic characteristics, which we have termed "more" and "less" hygroscopic. The more hygroscopic particles are typically more abundant than the less hygroscopic particles. By comparing impactor and TDMA data we conclude that a substantial fraction of the less hygroscopic particles is carbon-containing, with the balance consisting of crustal species. The more hygroscopic particles contain sulfates, nitrates and some carbon. On average about 60-70% of the particulate carbon is estimated to be associated with the less hygroscopic particles. We attribute the day-to-day variabilities in water uptake for the more and less hygroscopic particles to variabilities in the ratio of ions to carbon-containing species. The impactor data show that size distributions of sulfates and carbon are distinctly different. In particular, the ratio of sulfate to carbon varies with particle size and relative humidity. At high relative humidities (RH>50-70%) this ration tends to increase with increasing size, while at lower humidities the ration either decreases with size (Grand Canyon) or has no monotonic dependence on size (Los Angeles).