Removal by Snowfall of Emissions from a Coal-Fired Power Station: Observations and Modelling

Abstract

A case study to determine wet deposition near a250 MW coal-fired power station unit on the southcoast of Finland was carried out during a wintertimeprecipitation event. In addition to intensivemeteorological observations, the experimental part ofthe study included the collection of 28 depositionsamples within 10 km of the power station andmeasurements of sulphur dioxide in the air at twoplaces, one inside and the other outside the downwindsector. Model estimates of the wet deposition patternwere also made, assuming a Gaussian shape and agradual rise for the plume and taking into accountinclined fall trajectories of snowflakes through the plume.By comparing the model results with the chemicalanalysis outcomes, it was inferred that for sulphuremissions in wet snowfall within the first 10 km fromthe source, the precipitation scavenging coefficientwas probably of the order of 10-6 s-1 orless. This implies that the percentage of emittedsulphur being scavenged in the study area was about0.7% at most. Although the observed decreases ofacidity from the mean pH-value of 4.1 occurred only indeposition samples on the leeward side of the powerstation, no significant effects of alkaline fly ashcould be confirmed. On the other hand, the modelestimates do suggest that the sulphur and particularlythe hydrochloride emissions may have added to theacidity of snow at downwind distances of 1–5 km fromthe source, but chiefly in a narrow sector whichremained between the collectors.