The anthropogenic contribution to isoprene concentrations in a rural atmosphere

Atmospheric hydrocarbons are continuously monitored at the rural site of Ta enikon, Switzerland. As expected for a rural area, highest isoprene concent rations are found in summer. However, elevated concentrations are also meas ured on some occasions in winter, in particular during events with long-las ting surface inversions, temperatures constantly below 0 degrees C and snow covering the vegetation. During such events, concentrations of isoprene ar e strongly correlated with those of 1,3-butadiene, a substance that is main ly due to human activities. For these periods, a molar ratio between the co ncentrations of isoprene and those of 1,3-butadiene of 0.42 is observed. Th is value, together with the concentrations of 1,3-butadiene, is used to est imate the anthropogenic fraction of the atmospheric isoprene for the whole of 1997. It is found that the fraction is close to 100% in January-February and again in November-December. On the other hand, as early as March, a co nsiderable amount of the observed isoprene appears to be of biogenic origin , although isoprene emissions by trees are negligible. The relative anthrop ogenic contribution is minimal in midsummer, when biogenic emissions are hi ghest. For this time of the year, the anthropogenic contribution is largest during the early morning hours, in agreement with the traffic peak on near by country roads. (C) 1999 Elsevier Science Ltd. All rights reserved.