Particulate organic nitrates: sampling and night/day variation

Atmospheric day and night concentrations of particulate organic nitrates (PON) and several other air pollutants were measured in the summer 1995 over an open-land area in Denmark. The sampling of PON was evaluated comparing 24 h samples with two sets of 12 h samples. These results indicate that the observed low contribution of PON to NOy is real and not the result of an extensive loss during the sampling. Empirical relationships between the vapour pressure and chemical formula of organic compounds were established in order to evaluate the gas/particle distribution of organic nitrates. A positive correlation between the PON to NOz ratio and the inverse temperature may indicate that the organic nitrates associated with particles also may be present in the gas phase. The observed day and nighttime levels of PON were of the same magnitude, although the day to night ratio varied from 0.4 to 4.4.