Gas phase nitric acid, ammonia and related particulate matter at a Mediterranean coastal site, Patras, Greece

Measurements of nitric acid, ammonia and related particulate matter were made at a coastal site in Greece to elucidate the local factors that affect the concentrations of each of these species, as well as the atmospheric transport of particulate sulfate. Seasonal variation with higher concentrations in the summer was observed for all measured species. This variation was more pronounced for nitric acid, ammonia and particulate ammonium, in agreement with greater nitric acid production in the summer because of larger hydroxyl radical concentrations and larger ammonia emissions at higher temperatures. The high summer temperatures and low relative humidity of our region favor gaseous nitric acid and ammonia over particulate ammonium nitrate. In fact our data indicate that ammonium nitrate was not predicted to be formed in the summer, whereas it was predicted to be formed in the spring. Mean nitric acid/particulate nitrate ratios ranged from 1.3 to 4.0. Air mass back trajectories showed that the higher concentrations of the long-lived particulate sulfate were long-range transported from northern directions. High sulfate concentrations 7–11 μg m-3 were observed in the summer. On an equivalent basis particulate sulfate were 5–15 times larger than particulate nitrate reflecting higher sulfur over nitrogen emissions, as is observed in most studies. Nitric acid exhibited the more pronounced diurnal variation from all measured species; this was more apparent in the summer in agreement with a short lifetime, estimated here at about half a day. Peak nitric acid concentrations of 4–6 μg m-3 were measured in the afternoon hours in the summer. Local meteorology and photochemistry appear to control the variation of nitric acid and ammonia concentrations.