Abstract
Abstract. Tropical rainforests act as a huge contributor to the global emissions of biogenic volatile organic compounds (BVOCs). Measurements of their oxidation products, such as formaldehyde (HCHO) and glyoxal (CHOCHO), provide useful indicators of fast photochemistry occurring in the lower troposphere. However, measurements of these species in tropical forest locations are extremely limited. To redress this, HCHO and CHOCHO were measured using the long-path (LP) and multi-axis (MAX) differential optical absorption spectroscopy (DOAS) techniques above the rainforest canopy in Borneo during two campaigns in spring and summer 2008, as part of the Oxidant and Particle Photochemical Processes above a south-east Asian tropical rainforest (OP3) project. The results were compared with concurrent measurements of hydroxyl radical (OH), isoprene (C5H8) (which was the dominant organic species emitted in this forest environment), and various meteorological parameters. Formaldehyde was observed at a maximum concentration of 4.5 ppb and glyoxal at a maximum of 1.6 ppb, significantly higher than previous measurements in rural locations. A 1-D chemistry model was then used to assess the diurnal evolution of formaldehyde and glyoxal throughout the boundary layer. The results, which compare well with the LP-DOAS and MAX-DOAS observations, suggest that the majority of the glyoxal and formaldehyde is confined to the first 500 m of the boundary layer, and that the measured ratio of these species is reproduced using currently accepted product yields for the oxidation of isoprene by OH. An important conclusion is that the measured levels of glyoxal are consistent with the surprisingly high concentrations of OH measured in this environment.
Highlights
Glyoxal (CHOCHO) is the smallest dicarbonyl and is produced in the atmosphere from the oxidation of a number of volatile organic compounds (VOCs)
Formaldehyde (HCHO) in the remote atmosphere is predominantly produced from the oxidation of methane by the hydroxyl (OH) radical (Arlander et al, 1995), but it is produced through the oxidation of isoprene, which can be a dominant source depending on the emission rates (Dufour et al, 2009)
In this paper we describe ground-based measurements of glyoxal and formaldehyde over a tropical rainforest canopy, using long-path differential optical absorption spectroscopy (DOAS) (LP-DOAS) and multi-axis DOAS (MAX-DOAS) instruments
Summary
Glyoxal (CHOCHO) is the smallest dicarbonyl and is produced in the atmosphere from the oxidation of a number of volatile organic compounds (VOCs). The tropical rainforests in South-East Asia act as a significant source of VOCs to the atmosphere; an estimated 10 Tg C yr−1 of isoprene is contributed by the rainforests of Borneo alone compared to a global total of ∼ 500 Tg C yr−1 (Guenther et al, 1995) Both formaldehyde and glyoxal have been used as indicators for fast organic photochemistry occurring in the boundary layer (Volkamer et al, 2005a). Satellite measurements indicate increased concentrations for both species over areas with high biogenic isoprene emissions (Wittrock et al, 2006) These measurements suggest that the ratio of glyoxal to formaldehyde can be an indicator of whether the sources of these species are predominantly biogenic or anthropogenic: higher ratios indicate a greater influence of biogenic activity. The observed diurnal variations and ratios of the two compounds are interpreted using a 1-dimensional model of the boundary layer above the forest canopy
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