Black acidic snow in the remote Scottish Highlands [Acid precipitation]

Parts of Scotland experience large acidic depositions 1,2 which may result in ecological damage 3 . Some highland areas appear to suffer enhanced acidic input 2 which frequently exhibits pronounced episodicity 4 . Acidic snowfall leads to acidic flushes during snow-pack melting 5,6 . Polluted grey snow layers have been reported in Scandinavia 7–10 , but we present the first documented case remote from major pollution sources of a distinctive black, acid snowfall episode with p H 3.O. The particulate deposit was very large and consisted of ∼29% carbon; the remaining fraction included particles which could be identified as coal fly-ash. The heavily polluted air originated to the south-south east and was probably transported at an altitude of ∼1,000–1,500 m in association with a stable atmospheric layer whereupon particles were efficiently scavenged by snow. Such transport and deposition mechanisms may produce the greater pollutant deposition sometimes observed in mountain areas 2 . Similar events may not be rare and could make an important contribution to the annual pollutant input.