Inter-storm comparisons from the OSCAR high density network experiment

Abstract

The high density network component of the Oxidation and Scavenging Characteristics of April Rains (OSCAR) experiment combined aircraft, surface and sequential precipitation chemistry measurements to characterize the physicochemical and dynamic features of four storms sampled during an April 1981 field investigation. A surface network of 47 precipitation sampling stations, covering a region roughly 110 km by 110 km, was established in the area surrounding Fort Wayne, Indiana. The network provided temporal and spatial resolution of rainfall chemistry via the use of specially designed automatic sequential bulk precipitation collectors, while aircraft and surface sampling provided measurements of the major aerosols and trace gases in the boundary-layer inflow region. Composite concentration and ion ratio profiles for the events were analyzed to investigate potential pollutant scavenging pathways. This analysis led to the following observations: 1. (i) dryfall deposition during pre-rainfall exposure periods influenced initial sampler stage chemistry; 2. (ii) relative precipitation acidity increased throughout the events; SO 4 2− and NO 3 − were the major contributors to this acidity; 3. (iii) evidence exists for the in-cloud oxidation of SO 2 during Events 3 and 4, while scavenging of HNO 3 and aerosol NO 3 − probably produced precipitation NO 3 −; 4. (iv) the non-frontal meteorology of Event 3 influenced the precipitation chemistry associated with this storm and led to distinct concentration profiles; 5. (v) an anomalous pattern of NH 4 + concentrations observed during Event 1 cannot be explained by known NH 4 + scavenging behavior or by non-scavenging related influences, such as local source contamination or NH 3 volatilization; 6. (vi) Event 4 is more suitable for analysis by one- and two-dimensional diagnostic wet removal models. Analysis of the other events is complicated by more complex meteorological behavior and, in some cases, a less complete chemistry data set. This paper enlarges on these observations with comparisons of the major meteorological and chemical characteristics of the four events.