EPISODIC NATURE OF WET DEPOSITION OF ACIDIC MATERIAL AT A SITE IN SOUTH-EAST ENGLAND

The causal factors for episodic deposition of acidic material in rainf all cannot be fully understood from conventional daily network data. A brief review of the meteorological conditions leading to episodes is given. A definition of 'episodicity' was considered and applied to a 6 month data set collected at high temporal resolution using a micropro cessor based acid rain monitor at a site in south-east England. The de position was highly episodic for all of the measured variables when da ta from individual rain events were considered. Combining the data int o daily averages resulted in changes of episodicity classification for several chemical species. A large percentage of the total deposition recorded during the field experiment occurred in a 5 day period when t here was an independent report of ecological damage at other locations in England. Nitrate deposition showed the highest degree of episodici ty with 51% of the 6 month total occurring during the 5 day episode. M eteorological details of the transport and wet deposition processes du ring this period were examined. Back trajectory analysis indicated tha t the episode was the result of pollutant loading in eastern and centr al Europe of the air masses reaching the site together with an absence of upwind precipitation scavenging.