High elevation fog and precipitation chemistry in southern Quebec, Canada

Abstract

Results of the seasonal analysis of the Chemistry of High Elevation Fog (CHEF) project data for the years 1985–1991 are presented. The mountain sites are located in southern Quebec, Canada, and range in altitude from 250 to 970 m. Fog, precipitation, and mixed fog and precipitation chemistry data are given for major inorganic ions for all seasons of the year. This paper documents the occurrence of highly acidic fog events; it points out the potential importance of high elevation fog as a major pathway for acidic wet deposition in southern Quebec; and it provides results paralleling those from the American Mountain Cloud Chemistry Project (MCCP), thus supporting the relevance of investigations of high elevation fog to at least latitude 47°N. Data are from 8034 analyses of event-length samples, of which 1140 are mountain fog samples collected as duplicate pairs, making this one of the largest fog/cloud water databases available. The 1985–1991 fog sample mean pH at Roundtop was 3.90 and at Mont Tremblant 3.72. The mean fog pH for all sites and seasons was 3.79. The minimum fog pH was 2.80 and the maximum 7.20. The mean precipitation pH on Roundtop was 4.22 and at Mont Tremblant 4.19. Mixed sample pH values are intermediate between fog and precipitation. There is a small tendency (< 0.1 pH units) for both rain and snow to be more acidic at low elevations, with snow having a higher pH range. The more northerly Mont Tremblant site had higher ion concentrations and higher acidity than did the Roundtop Mountain site. The dominant anion was SO 4 2− followed by NO 3 −. The dominant cation was NH 4 +. The NO 3 − SO 4 2− equivalents ratio showed a strong wintertime peak from year to year with values sometimes exceeding 1.0. Over the 6 yr data set, the average concentrations of the major ions in the mixed and precipitation samples showed small decreases with time in all seasons. In contrast, the average and median wintertime H +, SO 4 2− and NO 3 − concentrations, in the high elevation fog samples, increased. The results show that the mountain areas of southern Quebec have a potential for total wet deposition that is distinct from low elevation sites in the region.