HCHO, HCOOH and CH3COOH in air and rain water at a rural tropical site in North Central India

HCHO, HCOOH and CH3COOH were measured in vapour phase and rain water during monsoon period at Gopalpura, Agra a rural site of north central semi-arid tract of India. Mean concentrations of formaldehyde, formic and acetic acids were 1.4, 1.7 and 1.6 ppbv in the vapour phase and were 4.4, 5.4 and 4.8 μmol −1 (volume weighted) in rain water, respectively. Due to cloud cover and rain which occurs intermittently in monsoon these species did not show typical diurnal pattern as reported by others. In the gas phase a good correlation of formic acid with acetic acid (r = 0.78) and poor correlation with formaldehyde (r = 0.19) suggested that formic and acetic acids may also have significant contributions from other than photochemical sources such as biogenic sources. The strong correlation between formate and acetate (r = 0.96) and formate and formaldehyde (r = 0.95) in rain water suggests that their sources are common, if not identical. Regression analysis between concentration vs precipitation volume and deposition amount vs precipitation volume of the three species suggested that concentration of these species are almost independent of rain volume and controlled by a continuous supply of these species by slowly scavenged material or oxidation of aldehyde to organic acid. HCOO−/HCHO ratio (1.4) also suggested that aqueous-phase oxidation of formaldehyde is a major source of formic acid in rain water.