A Sudden Increase in the Concentration of Dissolved Organic Carbon in Precipitation at a Northeastern US Forest

Abstract The oxidation of organic matter from fuel combustion or vegetation emissions into organic acids is a major source of dissolved organic carbon (DOC) in precipitation. Long‐term measurements of DOC in precipitation are rare, but the existing records mostly show decreases due to reduction in fuel combustion. Here, we show a recent, sudden increase in precipitation DOC concentration in a 27‐year record from the Hubbard Brook Experimental Forest (HBEF) in northern New Hampshire, USA Starting in 2010, where mean annual DOC concentration increased from about 80 to 130 μmol L −1 in 2022. No other solutes in precipitation showed a similar sudden change. The weekly DOC concentration was not clearly related to the 72‐hr air mass trajectory characteristics or changes in trajectories. We assessed the feasibility of multiple possible causes for the DOC increase, including an increase in biogenic volatile organic compound (BVOC) emissions from the forest or from forest fires, changes in oxidation processes in the troposphere, and changes in gas‐phase solubility due to increasing pH in precipitation. Further study of sudden changes in BVOC emissions in the region, possible causes, and air quality effects are warranted.