Lagrangian analysis of the total ammonia budget during Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange

Abstract

During the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) program, we made repeated measurements of ammonium aerosol in a European air mass as it passed over the North Atlantic Ocean near the Azores. After tracking balloons had been launched from the R/V Oceanus, the National Center for Atmospheric Research (NCAR) Electra followed a tagged parcel of this air mass for about 42 hours in four successive flights. A different part of this air mass passed over Santa Maria Island, where we measured its aerosol size distribution with a micro‐orifice uniform deposit impactor (MOUDI) impactor and used diffusion denuders to determine that virtually all available ammonia vapor had reacted with sulfuric acid to form ammonium aerosol. We have used a continuity equation to evaluate the budget of total ammonia in this air mass, and conclude that during this experiment the North Atlantic was emitting ammonia vapor. The aircraft observations show that ammonium concentrations stayed relatively constant, in spite of dilution by low‐ammonia free tropospheric air. Balancing that budget requires a surface ammonia source of 26 ± 20 μmol m−2 d−1. By contrast, non‐sea‐salt sulfate (nss SO4=) concentrations decreased with time, causing the neutralization ratio (NH4+/nss SO4=) to increase from about 0.8 to 1.3. Ships at the beginning and end of the track recorded near‐surface ratios of 0.73 and 1.65, respectively. This increasing ratio is further evidence of a substantial sea‐to‐air ammonia flux. Previous estimates of marine ammonia emissions have been in the range of 1.8 to 15 μmol m−2 d−1. Since our flux estimate was made at the end of the spring productivity cycle when the demand for fixed nitrogen may have been at a minimum, it may not be representative of global average ammonia fluxes. It does suggest, however, that the atmosphere may redistribute marine ammonia over hundreds or thousands of kilometers by vapor emission, conversion to aerosols, and deposition in rainfall.