ATMOSPHERIC CHEMICAL-TRANSFORMATIONS OF NITROGEN-COMPOUNDS MEASURED IN THE NORTH-SEA EXPERIMENT, SEPTEMBER 1991

Airborne concentrations of nitrogen dioxide, nitric acid, ammonia and particulate nitrate, ammonium and sulphate have been measured on two s hips located 200 km apart on the North Sea such that one was always di rectly downwind of the other. The concentrations diminish sharply betw een the ships, but the major factor appears to be entrainment into the free troposphere. After allowance for loss by entrainment, the change s in concentration during advection of the air mass are used to estima te the rates of atmospheric chemical conversions. Daytime loss of nitr ogen dioxide is consistent with the reaction with the hydroxyl radical at a concentration of 1-2 x 10(6) cm-3. During nighttime, NO2 is lost at a similar rate. consistent with the rate of the NO2 + 03 reaction, without subsequent amplification by reaction of NO3 with further NO2. Loss rates for the sum of nitric acid and aerosol nitrate indicate th at formation approximately balances loss due to dry deposition, whilst in the case of the sum of ammonia and particulate ammonium, there is a rapid net loss of these species.