Soluble species in the Arctic summer troposphere: Acidic gases, aerosols, and precipitation

We report here the distribution of selected acidic gases and aerosol species in the North American Arctic and sub‐Arctic summer troposphere. The summertime troposphere is an acidic environment, with HCOOH and CH 3 COOH the principal acidic gases and acidic sulfate aerosols dominating the particulate phase. Our data show that the acidic gas and aerosol composition is uniform on a large spatial scale. There appears to be a surface source of NH 4 + over the Arctic Ocean pack ice which may reflect release of NH 3 from decay of dead marine organisms on the ice surface near ice leads, release from rotting sea ice, or an upward flux from surface ocean waters in open ice leads. This NH 3 appears to partially neutralize aerosol acidity in the boundary layer. Over sub‐Arctic tundra in southwestern Alaska inputs of marine biogenic sulfur from the nearby Bering Sea appear to be an important source of boundary layer aerosol SO 4 2− . While there were only minor effects on aerosol chemistry over the tundra from sea salt, the rainwater chemistry showed influence from marine aerosols which were apparently incorporated into air masses during frontal passages moving inland from the Bering Sea. The rainwater acidity over the tundra ( p H 4.69) is typical of remote regions. The principal acidity components are H 2 SO 4 and carboxylic acids, especially HCOOH. The carboxylic acids appear to have a strong continental biogenic source, but hydrocarbons of marine origin and emissions from forest fires may also be important. The wet deposition fluxes of NO 3 − ‐N and SO 4 2− ‐S over sub‐Arctic tundra during July–August 1988 were 2.1 and 2.4 mmol m −2 yr −1 . Wet deposition of NO 3 − was nearly 3 times higher than the average NO y deposition flux, which is believed to represent primarily dry deposition of HNO 3 (Bakwin et al., this issue). Our measurements indicate that the mid‐troposphere in the Arctic is generally contaminated with low levels of anthropogenic pollutants even in summer when direct atmospheric coupling with mid‐latitude source regions was previously believed to be minimal. Stratospheric inputs may also be important as a source of Arctic tropospheric SO 4 2− . On several occasions we sampled directly within plumes or highly contaminated air masses representing various anthropogenic sources. The composition of these pollution sources suggested that they were important in determining the large‐scale distribution of acidic gases and aerosol species in the Arctic summer troposphere. Outside the plumes the anthropogenic influences are chemically diffuse and variable, making it very difficult to quantitatively ascertain the magnitude of the effects. Present‐day “background” air during summertime in the North American Arctic and sub‐Arctic mid‐troposphere appears to have the following average composition (parts per trillion by volume): HCOOH (70), CH 3 COOH (70), HNO 3 (40), NO 3 − (10), SO 4 2− (25), and NH 4 + (55). These concentrations which were observed on only a few isolated days can be compared to the grand average (Arctic and sub‐Arctic) mid‐tropospheric levels during July–August 1988: HCOOH (166 ± 81), CH 3 COOH (215 ± 90), HNO 3 (48 ± 29), NO 3 − (22 ± 17), SO 4 2− (61 ± 30), and NH 4 + (68 ± 30). A “first‐look” comparison of the large‐scale mid‐tropospheric composition in a remote area of the northern hemisphere with that over a remote region of the southern hemisphere, the Amazon Basin, suggests no identifiable difference in the levels of NH 4 + but possibly twofold and fivefold enhancement of NO 3 − and SO 4 2− in “background” air for North America.