Gaseous and particulate oxidized and reduced nitrogen species in the atmospheric boundary layer in Scandinavia in spring

Observations of the concentration of several nitrogen containing compounds at five rural Scandinavian sites during March-June 1993 are reported. Total nitrate NO3- + HNO3) and total ammonium (NH4+ + NH3) were measured by denuder and filter pack. In general the methods agree well. At all sites the particulate fraction dominated, with the largest fraction of NO3- and the lowest of NH4+ at the sites which were closest to the emission sources. The fraction of NO3- of total nitrate increased with increasing NO2 concentrations, indicating that the nighttime conversion of NO2 to NO3- is an important route of formation for NO3-. A positive correlation was found between HNO3 and O-3 in June at all sites, while no correlation was found early in the spring. Model calculations were made with a lagrangian boundary layer photooxidant model for the whole period, and compared to the measured concentrations. The calculated ratio between mean observed and modelled daily maximum concentrations of ozone over the measurement period were within +/-10% at all sites. The models ability to describe the daily ozone maximum concentration was satisfactory with an average deviation of 19-22% from the observed concentrations. HNO3 was underestimated by over 50% at all sites except the one closest to the emission sources. The correlation between modelled and observed concentrations was generally best for the sites with shortest transport distance from the sources of emission.