The interhemispheric distribution and the budget of acetonitrile in the troposphere

Abstract

Gas chromatography in conjunction with a thermionic nitrogen‐specific detector was used to determine mixing ratios of acetonitrile in air samples collected in Europe and over the Atlantic Ocean. In the city of Mainz, values of the order of 340 pptv were observed with large variations indicating the vicinity of sources. In the rural community of Deuselbach the average mixing ratio was 147±28 pptv; over the North Sea the range was 65–196 pptv depending on wind direction, with the lowest values occurring for northerly winds from the open ocean. The distribution of CH3 CN with geographic latitude over the Atlantic Ocean was explored between 30°S and 50°N on board R/V Polarstern during the cruise ANT V/5 in March–April 1987. Over the open ocean, maximum mixing ratios were observed near 4°S with values of 175 pptv. At latitudes near 30°S the mixing ratio averaged 90.4 pptv, whereas at 30°N the average was 52.1 pptv. The lowest mixing ratios of 21 pptv were found near 50°N. The tropical maximum is attributed to the advection with the trade winds of continental air from Africa, enriched with acetonitrile from biomass burning. The mixing ratios north and south of the maximum correlate well with the surface temperature of seawater, indicating a gas‐liquid equilibrium for CH3 CN dissolved in seawater. From the observations and with the further assumption that CH3 CN is vertically well mixed, its total mass content in the troposphere was estimated as 0.37–0.57 Tg. Global emission rates for various sources were estimated as follows: automobiles 0.27 Tg/year, oil‐fired power stations 0.0035 Tg/year, and biomass burning 0.80 Tg/year. The total estimated source strength is 1.1±0.5 Tg/year. The tropospheric residence time of acetonitrile was calculated from these data as 0.23–0.90 year with a probable value of 0.45 year. Wet precipitation and reaction with OH radicals are known sinks for tropospheric CH3 CN, but they can take up only 30% of the global emission rate. We suggest absorption in the ocean to be the major sink.