On the acidity of dew

Abstract

Nightly dew samples were collected on Allegheny Mountain in southwest Pennsylvania in August 1983. The dew was found to be acidic, with meanpH∼4.0. Free H+comprised >80% of the total acidity, and the composition approximated an H2SO4/HNO3mixture with average SO42−/NO3−mol ratio ∼1.3. The chemistry of the dew resembled that of rain at Allegheny, though the dew was generally more dilute. Atmospheric HNO3(g) was chiefly responsible for the dew NO3−. SO2was the source of ∼80% of the dew SO42−. The allocation of dew H+among atmospheric precursors was HNO3∼30%, SO2≥ 60%, aerosol H+≤ 10%. The most acidic dews were associated with wind from the west. Average deposition velocities (stated in centimeters per second) to dew were HNO3, 0.24; NO2, <0.02; SO2, ∼0.04; aerosol SO42−, ∼0.02; aerosol NH4+, 0.024; and aerosol species, 0.02–1. Comparison of the HNO3deposition velocity with ten‐fold higher summer daytime values reveals a pronounced diurnal variation. The 0.24 cm/s value for HNO3may represent a maximum for any gas or submicron aerosol species whenever the atmosphere is strongly stable. Not all of the S(IV) in dew had been converted to SO42−by morning; thus the rate of oxidation must have been on a time scale of hours at nighttime ambient temperatures. In contrast, S(IV) in rain and fog samples was not evident. Deposition fluxes and cumulative deposition totals involving dew, rain, and settled fog water were compared. Rain proved by far the most important for deposition of all solute species, including acid.