DEW AND FROST CHEMISTRY AT A MIDCONTINENT SITE, UNITED-STATES

From July 1989 to July 1990 a total of 98 dew and 9 frost samples were collected at the University of Arkansas Agricultural Experiment Station, Fayetteville. The total water flux from dews -d frosts p- year was less than 2% of that from rains. Acid and nutrient fluxes were also much lower in dew. In the following series of ions the number in parentheses gives the percent of the yearly flux of the ion in dew compared to rain for the same time period: H+ (0.06), Ca2+ (25), Mg2+ (11), K+ (21), Na+ (4), NH4+ (10), Cl- (4), NO3- (5), and SO42- (5). In contrast, the concentration of the various ions in dew, except H+ , is generally several times that in rain. Dew is primarily a Ca(HCO3)2 and (NH4)2SO4 solution; whereas rain is primarily a NH4HSO4 solution. The mean pH of dew is 6.37 compared to 4.99 for rain. The average acetate-formate concentration of dew in equivalents was (1) equal to about 53% of the nitrate-sulfate concentration in rain for April-June and (2) 10 times more concentrated than in rain for the year. The steps governing dew composition are indicated to be (1) formation on dry deposition solids (2) dissolution of the soluble portion of the dry deposition by dew water, and (3) sorption of gaseous NH3, acetic, and formic acids into the dew solution.