Carboxylic acids: Seasonal variation and relation to chemical and meteorological parameters

Formic and acetic acid measured as daily averages in 1993-1994 show equal and highly correlated concentrations up to 3 ppb in the summer (May-August). In the winter (October-March) the formic acid/acetic acid ratio was 0.6 and the formic acid concentrations were usually below 1 ppb. In winter the carboxylic acids correlate with O-x, NOy, SO2 and particulate sulphur. The main sources are suggested to be ozonolysis of anthropogenic alkenes and reactions between peroxyacetyl radicals and RO2 radicals. In spring-summer the carboxylic acids correlate with O-3, O-x, HNO3, PAN, NOy, SO2, particulate sulphur and temperature. In addition to the sources of the winter a contribution from ozonolysis of biogenic alkenes is likely. Quite similar formic acid/acetic acid ratios for all wind directions suggest that the source(s) are atmospheric oxidation processes distributed over large areas. The highest concentrations occurring for winds from east to south and the correlation with e.g., particulate sulphur indicate chemical production in polluted air masses during long range transport.