Seasonal variation of peroxy radicals in the lower free troposphere based on observations from the FREE Tropospheric EXperiments in the Swiss Alps

Abstract

Continuous measurements of peroxy radicals were carried out using the chemical amplification technique during three intensive FREE Tropospheric EXperiments, (FREETEX) in 1996, 1998 and 2001 at the Jungfraujoch (3,580 m asl) in the Swiss Alps, in combination covering the period of ozone build‐up from late winter to late spring. The monthly medians of the peroxy radical measurements from 10:00 to 13:00 UT in February, March, April, and May, after filtering out the more cloudy conditions, show a clear seasonal behaviour with a gradual increase of the peroxy radical mixing ratios from around 12 pptv in February to around 20 pptv in May. This data represents among the first semi‐continuous seasonal observations of peroxy radical concentrations, which indicates the important and possibly dominant role of in‐situ photochemistry in the observed ozone build up from winter to spring in the lower free‐troposphere over the Alps.