Characterization of total ecosystem-scale biogenic VOC exchange at a Mediterranean oak–hornbeam forest

Recently, the number and amount of biogenically\r\nemitted volatile organic compounds (VOCs) has been discussed\r\nin great detail. Depending on the ecosystem, the published\r\nnumber varies between a dozen and several hundred\r\ncompounds. We present ecosystem exchange fluxes from\r\na mixed oak–hornbeam forest in the Po Valley, Italy. The\r\nfluxes were measured by a proton transfer reaction-timeof-\r\nflight (PTR-ToF) mass spectrometer and calculated using\r\nthe eddy covariance (EC) method. Detectable fluxes were\r\nobserved for up to 29 compounds, dominated by isoprene,\r\nwhich comprised over 60% of the total upward flux (on a\r\nmolar basis). The daily average of the total VOC upward flux\r\nwas 10.4 nmolm2 s1. Methanol had the highest concentration\r\nand accounted for the largest downward flux. Methanol\r\nseemed to be deposited to dew, as the downward flux happened\r\nin the early morning, right after the calculated surface\r\ntemperature came closest to the calculated dew point temperature.\r\nWe estimated that up to 30% of the upward flux of methyl\r\nvinyl ketone (MVK) and methacrolein (MACR) originated\r\nfrom atmospheric oxidation of isoprene. A comparison between\r\ntwo methods for the flux detection (manual and automated)\r\nwas made. Their respective advantages and disadvantages\r\nwere discussed and the differences in their results\r\nshown. Both provide comparable results.