Hydrogen peroxide, organic peroxides and organic acids in a forested area during FIELDVOC'94

Abstract

Gas-phase H₂O₂, organic peroxides and organic acids were measured from mid-June to mid-July 1994 during the second FIELDVOC'94 campaign in a forested area near Tábua (Portugal). The site was located in a plantation of eucalyptus trees. Methylhydroperoxide (MHP, CH₃OOH) and H₂O₂ were quantified in the air samples. Hydroxymethylhydroperoxide (HMHP, HOCH₂OOH) was observed occasionally. Formic and acetic acids were the organic acids determined in the ambient air. The hydroperoxides and the organic acids showed pronounced diurnal variations with peak mixing ratios in the early afternoon (12:00–16:00 UT). Sometimes a second maximum was observed in the late afternoon (16:00–20:00 UT). In comparison with the peroxides the maximum organic acid mixing ratios were shifted by 1–2 h. The maximum mixing ratios were 1.38 ppbv (H₂O₂), 0.64 ppbv (MHP), 0.22 ppbv (HMHP), 2.8 ppbv (formic acid), and 1.5 ppbv (acetic acid). The ratio of formic acid over acetic acid mixing ratios was 1.5. The H₂O₂ mixing ratio was strongly dependent on O₃,NO_x, isoprene, RO_x and relative humidity. Mixing ratios of all peroxides were below detection limit (d.l.) (<15 pptv) during night-time. Deposition processes were found to determine the mixing ratios of the hydroperoxides and organic acids during the nights.