Seasonal variation of VOC concentrations above a boreal coniferous forest

Biogenic volatile organic compound (VOC) concentrations in ambient air were measured in Central Finland from April 2000 to April 2002. The concentrations of isoprene increased in May and declined in September, but the concentrations of its oxidation products, methacrolein and methyl vinyl ketone, remained at about 100 ppt level also after that suggesting they might have anthropogenic sources as well. The winter and summer mean midday concentrations of monoterpenes were 97 and 250 pptv, respectively. The winter concentrations of monoterpenes were unexpectedly high, but they can be explained by small, but existing, emissions from at least Picea abies and also by the much longer atmospheric lifetime of monoterpenes in winter than in summer. The hydroxyl and nitrate radical concentrations in Central Finland were calculated for the year 2000 using an observationally constrained photochemical box model. Using the calculated hydroxyl radical concentrations the atmospheric lifetime of α-pinene with respect to the OH reaction was estimated to be 50 times longer in December than in July. The nitrate and ozone reactions extend the atmospheric lifetime of α-pinene to 3–4 fold in winter in comparison with the summer lifetime. The mean afternoon (12–4 p.m.) emission potentials (at 30°C according to Guenther et al., J. Geophys. Res. 98 (1993) 12609) were 0.4, 0.2, 1.4, 0.7, 0.4, and 0.2 μg g−1dry weight h−1 in January, April, May, June, July, and October, respectively. In summer the VOC emissions also included isoprene and sesquiterpenes. The highest isoprene emission potential was observed in June (1.3 μg g−1dry weight h−1, (at T=30°C and PPFD=1000 μmol m−2 s−1 according to Guenther et al.) and the highest sesquiterpene emission potential in July (0.6 μg g−1dry weight h−1, scaled to 30°C similarly to the monoterpenes).