Acetaldehyde and acetone in the Arctic snowpack during the ALERT2000 campaign. Snowpack composition, incorporation processes and atmospheric impact

Acetaldehyde and acetone were measured in the seasonal snowpack near Alert (Nunavut, Canadian Arctic) in February and April 2000. Acetaldehyde concentrations in fresh surface snow in February were about 2.5 ppbw, decreasing to 1 ppbw after several days, while gas-phase acetaldehyde was about 75 pptv. Values for aged layers were 1.3–2.6 ppbw. In April, values for fresh snow were 5–10 ppbw, decreasing to 1–4 ppbw after several days (gas-phase values were around 230 pptv). Values for aged layers were in the range 0.7–3 ppbw. Snow-phase acetaldehyde represented 67% of the (snow+atmospheric mixing layer) system in winter and 94% in spring. Preliminary acetone measurements yielded values 1.5–3 ppbw in surface snow several days after deposition. To understand the kinetics of exchange of acetaldehyde between the air and the snow, its mechanism of incorporation in snow was investigated. Surface incorporation by adsorption, and volume incorporation by dissolution were considered. Winter and spring measurements showed very different trends, and spring concentrations were higher than winter ones, which is contrary to thermodynamic expectations. The photolytical production of acetaldehyde in the snowpack is proposed as an explanation.