Ammonia sources, transport, transformation, and deposition in coastal New England during summer

Abstract

During summer 2004, NH 3, size-resolved particulate NH 4 +, and associated characteristics of surface air were measured continuously on Appledore Island, off the southern Maine coast as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). NH 3 concentrations ranged from <0.6 to 123 nmol m -3 with maxima around local noon and minima near dawn. Particulate NH 4 + ranged from 10.3 to 191 nmol m -3. The transport of emissions from intensive agricultural activities in the eastern United States was an important source of total NH 3 (NH 3 + NH 4 +) over the Gulf of Maine during summer. Under cleaner northwest flow, total NH 3 concentrations were relatively low (median = 50.0 nmol m -3) and partitioned roughly equally between phases; under the more polluted midwest flow, total NH 3 concentrations were substantially higher (median = 171 nmol m -3 but dominated by particulate NH 3 +. Because particulate NH 4 + was associated primarily with the highly acidic sub-μm size fractions with low deposition velocities (median flux = 1.5 μmol m -2 day -1), dry-deposition fluxes were dominated by the gas phase (median = 6.2 μmol m -2 day -1). Consequently, phase partitioning with pollutant-derived sulfur aerosol substantially increased both the atmospheric lifetime of total NH 3 against dry deposition and the relative importance of removal via wet- versus dry-deposition pathways. Total NH 3 accounted for 32% of the dry-deposition flux of inorganic N to the Gulf of Maine during summer. The combined dry deposition of total NH 3 and wet deposition of NH 4 + via precipitation contributed 40% of the corresponding total atmospheric N flux. Copyright 2007 by the American Geophysical Union.