Diurnal variations in major rainwater components at a coastal site in North Carolina

Concentrations of several major rainwater components were determined in rain events occurring during the early morning hours (12:00 midnight to 6:00 a.m.) and during the afternoon (12:00 noon to 6:00 p.m.) to examine possible diurnal variations. Generally, rainwater components with gas phase origins (H+, NO3−, formaldehyde, H2O2, formic acid, acetic acid, pyruvic acid, oxalic acid, and lactic acid) had higher concentrations during p.m. rain events compared to a.m. events. Although source strengths of both biogenic and anthropogenic rainwater components are generally higher during the daytime, nocturnal removal of a wide variety of components in similar proportions (approximately 2–3× less at night) indicates a physical rather than a chemical process affecting diurnal variations. Rainwater components with aerosol origins (Cl−, and SO42−) displayed the opposite diurnal pattern or showed no diurnal variation. Possible reasons for these variations include one or both of the following scenarios: (1) the formation of dew at night removes gas phase atmospheric gasses but not aerosols or (2) during the night, a marine air mass containing lower concentrations of all analytes and higher concentrations of Cl− is advected into the area.