Rainwater formaldehyde: concentration, deposition and photochemical formation

Formaldehyde (HCHO) concentrations were measured in 116 rain samples in Wil mington, NC from June 1996 to February 1998. Concentrations ranged from bel ow the detection limit of 10 nM, to 13 mu M, in the range of HCHO levels re ported at other locations worldwide. The volume-weighted annual average rai nwater formaldehyde concentration was 3.3 +/- 0.3 mu M and comprised approx imately 3% of the measured dissolved organic carbon. Using the volume weigh ted average HCHO concentration and annual precipitation of 1.4 m, an annual formaldehyde deposition of 4.6 mmol m(-2) yr(-1) was determined. Rainwater is a significant source of formaldehyde to surface waters and may contribu te as much as 30 times the resident amount found in natural waters of south eastern North Carolina during the summer. Formaldehyde concentrations did n ot correlate with precipitation volume suggesting continuous supply during rain events. Evidence is presented which indicates part of this supply may be from direct photochemical production in the aqueous phase. Formaldehyde levels exhibited a distinct seasonal oscillation, with higher concentration s during the summer. This pattern is similar to that observed with other ra inwater parameters at this site including pH, nitrate, and ammonium, and is most likely the result of increased photochemical production, as well as b iogenic and anthropogenic emissions during summer months. The concentration of formaldehyde in both winter El Nine rains and summer tropical rains was less than half its concentration in non-El Nine or non-tropical events, su ggesting significant terrestrial input. Formaldehyde was correlated with hy drogen peroxide and non-sea-salt sulfate deposition suggesting a relationsh ip between HCHO, H2O2, S(VI) within the troposphere. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.