Emergence of a New Chemical Regime: Growing Abundance of Water Soluble Organics in Cloud Water Associated with a Growing Ion Imbalance

Long-term reductions in anthropogenic sulfate in the eastern U.S. have been captured by observations spanning several decades across New York State, including cloud water composition measurements at Whiteface Mountain (WFM). These observations reflect one of the major goals of the Clean Air Act: dramatic reductions in sulfuric acid deposition. Concomitant long-term reductions in nitrate and ammonium have also been recorded, associated with reduced particulate mass loadings. Altogether, these outcomes resulting from national regulatory interventions are considered one of the greatest success stories in the history of atmospheric science. Measurements at WFM began with inorganic ion speciation, which has continued largely unchanged to the present day. Water Soluble Organic Carbon (WSOC) measurements were added in 2009. In the past two decades cloud water samples have become increasingly anion deficient, leading to a growing number of samples being considered “invalid” due to ion imbalance. While most published literature on cloud water composition at WFM does not include “invalid” samples, in the present day “invalid” samples outnumber “valid” samples at WFM. The growing ion imbalance suggests that there is a growing abundance of analytes that are not being observed with the traditional suite of measurements. When choosing not to apply these ion balance criteria to the data analysis, a striking increase in WSOC emerges over the past decade. A growing abundance of ammonium in cloud water has also reached a new milestone, surpassing both sulfate and nitrate combined. Thus, while the acidity of cloud water has decreased dramatically, the growing concentration of soluble material in cloud water suggests that we are entering a new chemical regime in the northeastern U.S., which is not necessarily “clean”. We describe our re-evaluation of long-term cloud water trends at WFM, and report on an important recent addition to our cloud water monitoring efforts: low molecular weight organic acids.