The budget of oxidised nitrogen species in orographic clouds

The transformation of NO(x) as it passed through a hill-top cap cloud was investigated by measuring (i) NO(x) (NO and NO2) and particulate nitrate concentrations on both sides of the hill, (ii) nitrite and nitrate in cloud, and (iii) nitrous and nitric acid concentrations below cloud base. Results from four periods during April and May 1993 are presented as part of a EUROTRAC GCE (Ground-based Cloud Experiment) experimental campaign at Great Dun Fell, in Cumbria, England. The overall change in NO(x) in all four periods was less than 20%. On 22 April the air flow was from west to east, and NO(x) concentrations were very much larger than particulate nitrate concentrations. There was evidence for loss of NO(x) and production of nitrous acid as air passed through the cloud. During May the air flow was from east to west; NO(x) concentrations were much smaller, and similar to concentrations of particulate nitrate. Differences in NO(x) concentrations before and after cloud sometimes showed a loss of NO(x) which was greater than the combined measurement uncertainty. Changes in particulate nitrate concentrations were close to the combined measurement errors. Concentrations of nitrate in cloud, however, were greater than could be accounted for by particles alone, suggesting a significant input of HNO3. Measurements below cloud showed that as much as 25% of the cloud nitrate could be released as HNO3 as cloud droplets evaporated. These field measurements show that chemical processing of oxidised nitrogen does occur in clouds, involving consumption of NO(x), and production of gaseous HNO3 as cloud droplets evaporate. The methods used to measure particulate nitrate concentrations, however, were not adequate to construct a complete budget of oxidised nitrogen across the hill.