LAGRANGIAN ANALYSIS OF THE TOTAL AMMONIA BUDGET DURING ATLANTIC STRATOCUMULUS TRANSITION EXPERIMENT MARINE AEROSOL AND GAS-EXCHANGE

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 laun ched from the R/V Oceanus, the National Center for Atmospheric Researc h (NCAR) Electra followed a tagged parcel of this air mass for about 4 2 hours in four successive flights. A different part of this air mass passed over Santa Maria Island, where we measured its aerosol size dis tribution with a micro-orifice uniform deposit impactor (MOUDI) impact or and used diffusion denuders to determine that virtually all availab le ammonia vapor had reacted with sulfuric acid to form ammonium aeros ol. We have used a continuity equation to evaluate the budget of total ammonia in this air mass, and conclude that during this experiment th e North Atlantic was emitting ammonia vapor. The aircraft observations show that ammonium concentrations stayed relatively constant, in spit e of dilution by low-ammonia free tropospheric air. Balancing that bud get requires a surface ammonia source of 26 +/- 20 mu mol m(-2) d(-1). By contrast, non-sea-salt sulfate (nss SO4=) concentrations decreased with time, causing the neutralization ratio (NH4+/nss SO4=) to increa se 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 incr easing ratio is further evidence of a substantial sea-to-air ammonia f lux. Previous estimates of marine ammonia emissions have been in the r ange of 1.8 to 15 mu mol 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 atmo sphere may redistribute marine ammonia over hundreds or thousands of k ilometers by vapor emission, conversion to aerosols, and deposition in rainfall.