Abstract
Airborne concentrations of nitrogen dioxide, nitric acid, ammonia and particulate nitrate, ammonium and sulphate have been measured on two ships located 200 km apart on the North Sea such that one was always directly downwind of the other. The concentrations diminish sharply between the ships, but the major factor appears to be entrainment into the free troposphere. After allowance for loss by entrainment, the changes in concentration during advection of the air mass are used to estimate the rates of atmospheric chemical conversions. Daytime loss of nitrogen dioxide is consistent with the reaction with the hydroxyl radical at a concentration of 1–2 × 10
6 cm
−3. During nightime, NO
2 is lost at a similar rate, consistent with the rate of the NO
2 + O
3 reaction, without subsequent amplification by reaction of NO
3 with further NO
2. Loss rates for the sum of nitric acid and aerosol nitrate indicate that 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.