Physical and chemical influences on PAN concentrations at a rural site

The atmospheric concentration of peroxyacetyl nitrate was measured at Weybourne, Norfolk during October/November 1994 and June 1995 to investigate the factors controlling PAN concentration. Higher background and lower peak concentrations were found in winter mainly because of differences in the balance between photochemical production and thermal destruction but air mass origin was also important. The highest concentrations observed in winter occurred in air from Europe while those in summer occurred in plumes of polluted UK air. The overnight decay rates of PAN and ozone were calculated for a number of nights when the concentrations appeared to follow first-order decay kinetics. Observed first-order decay rates were generally consistent with dry deposition of PAN and ozone in a stable nocturnal boundary layer over land. Similar decay rates occurred for PAN and ozone concentrations in marine air which could not be attributed to dry deposition. A simple photochemical model was developed which illustrated that the temporal variations of PAN and ozone concentrations at a fixed site could be the result of varying degrees of photochemical processing of a homogeneous air mass along the trajectory leading to the measurement site.