Overview of the FIELDVOC'94 experiment in a eucalyptus forest of Portugal

Abstract

The aim of the EU FIELDVOC'94 project (field study on the tropospheric degradation mechanisms of biogenic VOCs, isoprene and dimethylsulfide) was to understand the contribution of the isoprene chemistry to the budget of ozone and the formation of peroxy radicals (RO₂) under various conditions, particularly high and low nitrogen oxides concentrations (NO_x). An additional objective was to evaluate the involvement of nitrate radicals (NO₃) in the production of RO₂ during night-time. An experiment was undertaken in June–July 1994, at a forest site in central Portugal. During this campaign most of the compounds involved in the ozone/isoprene chemistry were measured, i.e., nitrogen oxides, carbon monoxide, degradation and secondary products (aldehydes, ketones, organic acids and peroxides), as well as free radicals (RO₂ and NO₃). Several intensive measurement periods were carried out under different conditions of high and low photochemical activity. Isoprene was the most abundant hydrocarbon measured at the site, with mixing ratios as high as 10–12 ppbv during periods of high photochemical activity and elevated temperatures. Terpenes were also produced in the ppbv level with maximum concentrations observed at night. Ozone exhibited a diurnal variability, with maximum levels in the afternoon of 80–100 ppbv attributed to the interaction of polluted air masses and the isoprene chemistry. NO₃ radicals never exceeded a maximum of 6 pptv. RO₂ radicals displayed a diurnal maximum of 200–250 pptv and in some case reached night-time levels of 10–20 pptv. Radicals and ozone budgets deduced from photostationary models or chemical box models showed that hydroxyl radical (OH) initiated isoprene chemistry contributed to about 45% of the observed NO₃ levels during daytime and that ozone initiated chemistry accounted for approximately half of the RO₂ observed. The chemistry of isoprene strongly enhanced the ozone net production and reduced by a factor 2–3 the OH concentration.