Meteorological indicators of summer precipitation chemistry in central Pennsylvania

Daily precipitation chemistry data from nine consecutive summers (1993-2001) at a central Pennsylvania site were analyzed by objective and subjective methods and classified into key synoptic-scale circulation types. Significant differences in acidity and analyte concentrations were found among the seven storm types. The Convective type had the highest mean concentrations for all major ions compared to the Warm-Front (WF) type, which featured the lowest concentrations. The highest sulfate-to-nitrate ratio (2.36), assigned to the Warm-Sector (WS) type, indicates that acidity within air-masses transported to central Pennsylvania from the southwest is primarily in the form of H2SO4. About 20% of the overall acidity in precipitation during summer is neutralized by base cations. Both the Low-to-South and WF types have the lowest magnitudes of neutralization due to a lack of base cations of crustal origin. Both circulation systems are featured by an onshore flow from the Atlantic Ocean. Meteorological indicators for each weather type were developed based on values and geographical locations of anomalies of synoptic-scale variables, expressed as significant departures from their long-term means (as extracted from the Reanalysis NCEP/NCAR Model). The results suggest distinctive patterns for the regional position of the anomalies for the differing circulation systems. The two warm-weather types (High-to-South and Convective and WS) are featured by negative anomalies of sea-level pressure (SLP) and 850 hPa geopotential height (GPH) located northwest of the site and positive anomalies of the 850 hPa temperature to the east. Both are also associated with deep convection. The Cold-Front and Low-to-East types manifest an opposite symmetrical position of the anomalies, i.e., negative anomalies of SLP and 850 hPa GPH to the northeast and negative 850hPa temperature anomalies northwest of the site. Both types are characterized by lower lifted indices, indicating weak atmospheric instability. (C) 2003 Elsevier Science Ltd. All rights reserved.