CORRESPONDENCE
SIR: In the April issue of Environmental Science a n d Technology, Appel et al. hypothesize in their article “Sulfate and Nitrate Data from the California Aerosol Characterization Experiment (ACHEX)” [ES&T, 12,418-25 (1978)], that two temporally segregated mechanisms of nitrate formation operate in the Los Angeles atmosphere to form two distinct populations of nitrate particles. One population of particles is characterized as occurring at peak concentration in the early morning and consists of relatively large particles. The other population is formed in the afternoon and consists of very fine particles. It is suggested that the former particulate forms from the reaction of NO:! and water, while the latter is produced photochemically-probably from the reaction of hydroxy radical and NO*. Appel et al. discount the production of particulate nitrate from the reaction of NO2 and 03,showing that the correlation of particulate nitrate to ozone is, if anything, negative. This proof, however, does not consider that the reaction of ozone with nitrogen oxides has very different results depending on insolation. In particular, during the day NO m u s t constitute a significant fraction of NO,. The presence of NO strongly limits the concentration of NO3 (and its reversible reaction . during the day, although 0 3 conproduct N P O ~ )Therefore, centrations are a t their diurnal peak, there can be no great production of “ 0 3 from Nz05 and water. At night, however, the reaction of 03 and NO, will proceed to a net formation of N2Oj as a major percentage of total NO,. This species is then capable of slow but complete combination with water vapor to form HNO:+ This mechanism of formation of “ 0 3 during the night can only occur where NO emissions have not completely depleted 0 3 , Le., aloft above an inversion layer. The nitrate aerosol product of this series of reactions could age and agglomerate through the night. Further, the aerosol would be most strongly noted by ground samplers during the fumigation consequent with the breakup of the nocturnal inversion. Appel et al. report the occurrence of a population of relatively large nitrate particles in the early morning and in the
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presence of low ozone levels. The mechanism above suggests that these observations can be explained by the reaction through the night of 0 3 , NO,, and water.
Mark R. Antell U S . Environmental Protection Agency Washington, D.C. 20460
SIR: In response to Mr. Antell’s letter concerning our paper “Sulfate and Nitrate Data from the California Aerosol Characterization Experiment (ACHEX)” ( I ), his suggested mechanism is one which should be given consideration. If nitric acid formation aloft (by any mechanism) is significant, then fumigation during mid-morning could yield substantial particulate nitrate, in part because of the increasing ammonia level near ground level (2). I t should be noted that Orel and Seinfeld (3) concluded that nitrate levels and size distributions predicted for the Los Angeles Basin by chemical equilibria in the system NOzNO-H20(1iquid)-SO2-NH3-CO2-NaC1 were similar to those observed in the ACHEX (Le., reactions involving ozone or other oxidants need not be invoked).
Literature Cited (1) Appel, B. R., Kothny, E. L., Hoffer, E. M., Hidy, G. M., Wesolowski, J. J., Enuiron. Sci. Technol., 12,418 (1978). (2) Okita, T.. Ohta, S., “Measurement of Nitrogen Compounds in the
Atmosphere and in Cloud Water in Relation to Their Transformation in the Atmosphere”, 175th National ACS Meeting, Anaheim, Calif., Mar. 1978. ( 3 ) Orel, A., Seinfeld, J. H., Enuiron. Sei. Technol., 11, 1000 (1977).
Bruce R. Appel Air and Industrial Hygiene Laboratory California Department of Health Services 2 15 1 Berkeley Way Berkeley, Calif. 94704
0013-936X/78/0912-1438$01.00/0
@ 1978 American Chemical Society
