AIR SAMPLING FLIGHTS ROUND THE BRITISH ISLES AT LOW ALTITUDES: SO2 OXIDATION AND REMOVAL RATES

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Abstract

The results are reported of measurements made in the atmospheric boundary layer during aircraft flights in which plumes from major point sources of SO2 in Northern England were intercepted on their way across England and North Wales to the Irish Sea in July 1982. Over this part of Britain the boundary layer was well mixed, and capped by a strong subsidence inversion at about 1200 m. The mean wind speed in the boundary layer was 9 ms−1 from 070°. A layer of quasistationary stratiform cloud was present over Wales during the whole period of daylight.

SO2 and SO2−4 flux measurements made upwind of the cloud system are consistent with an average oxidation rate for SO2 of ˜ 2% h−1 and a deposition velocity of ˜ 2 cm s−1 over ˜ 4 h travel. A much higher SO2 loss rate of 27% h−1 was observed between two sampling runs encompassing the cloud system representing a transit time of ˜ 42/3 h. 5.3% h−1 could be accounted for by the measured sulphate aerosol downwind of the cloud due to oxidation of SO2. It is concluded that the remaining ˜ 22% h−1 loss of SO2 (which would correspond to a deposition velocity of over 6 cm s−1) may have been lost during passage through the cloud system by direct impaction of cloud droplets on the Welsh Mountains or by very light drizzle.

The mean oxidation rate was estimated to be in the range 5.3 ± 1.5% h−1 to 21 ± 8% h−1 over 4 2/3 hours, depending on the proportion of sulphur lost assumed to be in the form of unoxidized SO2.

These results may be contrasted with those of other flights over the North Sea in which no substantial oxidation was observed during plume interaction with cloud over trajectories extending several hundred km. Unlike these previous flights, oxidation on the present occasion may have been facilitated by the higher oxidant availability (due to greater mixing), by the greater insolation in July than in January and possibly by higher atmospheric ammonia levels due to a larger land fetch in cloud.

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