Multi-year measurements of radiation fog composition at a rural site in Pennsylvania

Radiation fog samples have been collected on the campus of Susquehanna University in central Pennsylvania since 2008. This location experiences about 20 dense fog events per year and is influenced by local agricultural emissions and transport from the Midwest. Fog samples have been collected with an automated Caltech Heated Rod Cloudwater Collector (CHRCC) that has a 50% cut diameter of approximately 9 micrometers. The collection system is activated when visibility drops below 500 m and a single bulk sample is collected during each fog episode. Sample analyses initially focused on pH and inorganic ions but more recently includes organic acids, total organic carbon (TOC), and total nitrogen (TN). These measurements provide a unique, long term record of fog composition in this area and may be used to better understand aqueous phase processing mechanisms and enhanced deposition of atmospheric trace species.

Reflecting the agricultural nature of the region, ammonium was the most abundant ion measured throughout the study (mean = 270 microEq/l through 2013). Due in part to high ammonium concentrations, the median pH value was 6.2. In order of decreasing average concentrations, ammonium was followed by sulfate (135 microEq/l), calcium (68 microEq/l), and nitrate (55 microEq/l). Organic acids were also found to contribute significantly to total ion loading, accounting for approximately 30% of anions in solution. On average, formate was present at concentrations similar to nitrate while acetate averaged about half that amount. Propionate and maleate varied significantly from sample to sample while oxalate concentrations remained relatively constant. Seven additional organic acids were measured, but concentrations of these were often below detection limits.

The average TOC concentration was 6.8 mgC/l. Assuming an organic matter to organic carbon ratio of 1.8, organic matter accounted for 42% of total solute mass on average. The measured organic acids represent approximately one quarter of organic matter. For samples in which total nitrogen was measured, the average concentration was 5.2 mgN/l. The measured inorganic nitrogen species accounted for approximately 75% of total nitrogen on a molar basis; the remaining 25% of total nitrogen is assumed to be associated with organic species. Inorganic and organic nitrogen were significantly correlated.

To place these measurements in context, results from this study will be compared with local dew and precipitation measurements and with measurements from other cloud and fog studies. Inorganic ion concentrations at the study site are lower than values reported for most other fog studies conducted in the U.S. However, organic acid concentrations appear to be similar to other fog studies. TOC concentrations are lower than those observed in California radiation fogs, but similar to measurements at other locations.