A Field Instrument for Measuring Rain Conductivity in Real-time

Alfred M. Moyle
Pennsylvania State University,
Meteorology Department,
University Park, PA USA

NADP data have repeatedly demonstrated that summertime rains in the northeastern United States are characteristically acidic due to high concentrations of sulfate, a chemical signature that can be traced back to the release of sulfur dioxide during the burning of coal in the region. Attempts to model source-receptor relationships and the spatial patterns of acidic deposition depend on the kind of precipitation chemistry data that AIRMoN and NTN provide for validation (ground truth). However, the acquisition of complete chemical data is expensive, especially when ever-higher spatial and temporal resolution is needed to test model results or reveal mechanistic information about cloud processes. The strong statistical links that exist between the concentration of sulfate, pH, and the electrical conductivity of rainwater have allowed us to develop a relatively inexpensive, single-parameter monitor of precipitation quality that meets the scientific needs.

Experience gained during the summer seasons of 2006 and 2007 demonstrated that electrical conductivity is relatively easy to measure in a field setting. The variations in conductivity can be monitored continuously throughout individual storms, indirectly yielding high temporal resolution of rain pH and sulfate concentration. Unique chemical signatures from convective and stratiform rain events were evident from data gathered by sampling on the roof of the Walker Building on the Penn State campus, some 10 km from the nearest NADP site (PA15).

The conductivity monitor has now been collocated with the AIRMoN, NTN, and CAPMoN samplers at the PA15 site, thus allowing unambiguous comparisons to be made between the diverse data sets. The new, continuous measurements of conductivity will supplement the current measurement strategies based on daily and weekly sampling, thereby providing new insights into the chemical evolution of acidic storms and furthering the technology for eventually monitoring rain acidity remotely in real-time across a mesonetwork of stations.