What controls gas phase acid exchange over forests? Investigating fluxes of HNCO and other organic acids over a pine forest

Isocyanic acid (HNCO) is a water-soluble organic acid of interest to the atmospheric chemistry community due to its potential toxicity and its production mechanisms from oxidation of reduced organic nitrogen molecules. One key uncertainty in the isocyanic acid literature has been its dry deposition rate. We measured isocyanic acid fluxes using an acetate chemical ionization mass spectrometer coupled to the eddy covariance technique over a rural Colorado pine forest during multiple seasons. HNCO mixing ratios ranged from 0 - 130 ppt, consistent with other clean continental sites in the United States. However, while HNCO deposited from the atmosphere to the surface most evenings and during daytime during some seasons, many observed daytime HNCO flux periods showed upward fluxes. Quality-filtered HNCO fluxes ranged from -69 - 71 nmol m^-2 h^-1 with observed seasonal medians that are upward in winter (0.63 nmol m^-2 h^-1) and fall (2.5 nmol m^-2 h^-1), but downward in spring (-0.29 nmol m^-2 h^-1) and summer (-5.3 nmol m^-2 h^-1). Of course, an observed flux is the sum of all ecosystem sources and sinks. Here, we explore these initially puzzling results by exploring ecosystem interactions with gas phase acids, with particular emphasis on the role of water films in controlling exchange velocities of isocyanic acids. We extend the analysis to observed fluxes of other volatile organic acids to investigate persistent ecosystem sources over this forest site.