ATMOSPHERIC CARBONYL-COMPOUNDS AT A RURAL SOUTHEASTERN UNITED-STATES SITE

Atmospheric concentrations of a series of carbonyl compounds known as formaldehyde (FA), acetaldehyde (AC), acetone (AN), glycolaldehyde (GA ), glyoxal (GL), methylglyoxal (MG), glyoxylic acid (GD), and pyruvic acid (PD) were measured at a rural site in Georgia in summers of 1991 and 1992. The midafternoon median concentrations, in parts per billion , determined for 1991-1992 were FA, 3.6/3.1; AC, 0.58/0.74; AN, 1.7/1. 8; GA, 0.21/0.26; GL, 0.02/0.09; MG, 0.03/0.08; GD, 0.46; and PD, 0.11 , the latter two for 1992 only. All of the carbonyls except AC and AN exhibited a strong diurnal dependence, with maxima in the midafternoon and minima during the night, consistent with a rapid in situ photoche mical production in the daytime and a loss by dry deposition in a shal low inversion during the night. FA correlated well with O-3, GA and MG , consistent with their photochemical production near the surface at t he measurement site. GL and MG showed the strongest correlation among all species, suggesting common origins as well as similar atmospheric lifetimes. The presence of GA, MG, and GL along with FA at the observe d relative concentrations are consistent with laboratory developed iso prene oxidation mechanism and the expectation that isoprene represents a major reactive hydrocarbon in this rural region. At the concentrati ons observed, these carbonyls serve as important radical sources. The contribution of FA accounts for half of that by O-3 and the higher car bonyls approximates half of that by FA. With respect to production of peroxyacetyl nitrate, isoprene contributes as much as acetaldehyde. Th ese results lend further credence to the notion that isoprene plays a pivotal role in photochemical processes, especially in rural environme nts.