Sulfate and Nitrate Data from the California Aerosol Characterization Experiment (ACHEX) Bruce R. Appel”, Evaldo L. Kothny, Emanuel M. Hoffer, George M. Hidy’, and Jerome J. Wesolowski Air and Industrial Hygiene Laboratory, Laboratory Services Branch, California State Department of Health, 2151 Berkeley Way, Berkeley, Calif. 94704
Sulfate and nitrate data obtained in the summers of 1972 and 1973 with five fixed and one mobile sampling laboratory within California’s South Coast Air Basin are presented. Included are 24-h average concentrations, diurnal variations, and size distributions obtained using a Lundgren cascade impactor. In addition, results for ammonia, ammonium, and other chemical species are given. Two-hour average concentrations of nitrate and sulfate up to 149 and 70 pg/m3, respectively, are observed. The mass median diameters for sulfate are 0.3-0.4 ym, and for nitrate 0.3-1.6 pm, the latter varying with locations in the Basin. Ratios of ionic constituents and ambient ammonia levels suggest that the sulfate and nitrate are present principally as ammonium salts. The data are consistent with mechanisms for sulfate and nitrate formation from SO2 and NO, involving photochemical oxidants, oxidation in water droplets, and for SOz, oxidation on carbon particles. The chemical analyses of aerosol samples collected in the Aerosol Characterization Experiment (ACHEX) have provided data on many of their constituents ( I ) . Of special importance have been nitrate and sulfate determinations. Whereas such data were obtained for aerosols collected throughout California, this paper focuses on data obtained in the South Coast Air Basin (SCAB). Although both sulfate and nitrate levels are elevated in most urban areas, California’s SCAB exhibits particularly high values of nitrate as compared with other parts of the United States. Thus, caution must be exercised in extrapolating the following discussion to other areas. Sulfate and nitrate are of concern because of possible adverse health effects as well as visibility reduction. The possible harmful effects of these materials are enhanced because they are present in particles predominantly in the respirable size range. Light scattering, with the attendant reduction in visibility, is especially important for particles in the 0.1-1-pm size range. Sulfate and nitrate are among the most important contributors to atmospheric aerosols in this size range. This paper includes the observed 24-h average concentrations of suspended sulfate and nitrate in the Basin, diurnal patterns for sulfate and nitrate as compared to other pollutants, and size distributions of sulfate and nitrate on a 24-h average basis. Mechanisms for sulfate and nitrate formation consistent with these data are discussed. Sampling in the SCAB was done during the periods September-October 1972 and July-October 1973. In 1972, sulfate and nitrate analyses were obtained only on 24-h hi-volume samples, whereas in 1973, short-term, low-volume samples were also analyzed for these species. Accordingly, this paper will emphasize the 1973 results. Experimental Aerosol Collection and Analysis Strategy. Particles less than about 20 pm were sampled isokinetically in the aerosol manifold system of the ACHEX mobile laboratory by three Present address, Environmental Research and Technology, Inc., 2625 Townsgate Road, Westlake Village, Calif. 91361. 418
Environmental Science & Technology
Mobile Laboratory O S a t e l l l t e statrons
Figure 1. Sampling sites and typical summer daytime winds 1972-1973 ACHEX
udits: a five-stage, Lundgren cascade impactor and two filters collecting all particles Ica. 20 pm (“total filters”). In addition, aerosols were sampled on a 24-h basis with a conventional hi-volume sampler using Whatman 41 (cellulose) filters. The inlet for all samplers was from about 5 to 13 m above ground level. The total filters were 47 mm Gelman GA-1 cellulose acetate and 47 mm Gelman A glass fiber filters sampling a t 3.0 cfm and providing >99% removal of particles (measured with room air dust with a condensation nuclei counter) ( 2 , s ) . The cellulose ester and cellulose filters were used for determining inorganic aerosol constituents, and the glass fiber filters, carbonaceous material. Cellulose acetate filters were also employed as the fifth stage (“after filter”) of the Lundgren impactors. Total and after filters were changed with the same frequency, usually eight times per 24-h period, with 2-h changes during most daylight hours. In addition to the mobile laboratory, total filter samples with the same time resolution were collected at five satellite stations during the 1973summer sampling periods. Figure 1 shows the locations sampled. The Lundgren impactor has been previously described ( 4 ) . The present units were modified by addition of a second stage 1 to remove more completely dry soil particles >8.0 pm in aerodynamic diameter ( 5 )and were calibrated using laboratory-generated aerosols. The 50% cutoff points for unit density spheres, based on design criteria, agreed well with experimental calibrations and were 8.0, 4.0, 1.5, and 0.5 pm. The impaction surfaces were strips of polyethylene coated with a sticky hydrocarbon resin. Both laboratory and field evaluations of this surface indicate that substantial particle bounce-off should be expected with dry particles (5-7). This effect should be of lesser importance with sulfate and nitrate particles, however, which are expected to be small liquid or sticky solid particles. The impaction strips for stages 1-4, representing a 24-h period, were extracted in water to obtain 24-h average size distributions for sulfate and nitrate. For determining detailed size distributions, the 24-h average sulfate and nitrate
