DOAS measurements of formaldehyde and glyoxal above a south-east Asian tropical rainforest

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 longpath (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 (C 5 H 8 ) (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.

Modeling of HCHO and CHOCHO at a semi-rural site in southern China during the PRIDE-PRD2006 campaign

“…The top-10 precursor NMHCs of HCHO and CHOCHO in terms of their production rate are listed in Table 2. Compared to similar studies (e.g., Volkamer et al, 2010;Huisman et al, 2011;Washenfelder et al, 2011;MacDonald et al, 2012;Parrish et al, 2012), anthropogenic and biogenic sources contribute almost equally to the chemical formation of HCHO and CHOCHO at the BG site. However, production of HCHO and CHOCHO from anthropogenic precursors is larger than from isoprene before noon; in the afternoon, the contribution of isoprene to HCHO and CHOCHO production becomes higher than from anthropogenic NMHCs.…”

“…Based on satellite observations, Vrekoussis et al (2010) conclude that regions with R GF lower than 0.045 are under influence of anthropogenic emissions, whereas R GF higher than 0.045 often indicates that VOC emissions mostly originate from biogenic sources. Average R GF up to 0.2-0.4 were observed by MacDonald et al (2012) in an Asian tropic forest. However, after analyzing the measured R GF and relevant trace gases at a rural site, DiGangi et al (2012) found that higher R GF corresponded to increased anthropogenic impact on local photochemistry.…”

“…But, during a chamber study, Kroll et al (2005) did not find any growth of both neutral ((NH 4 ) 2 SO 4 ) and acidic (NH 4 HSO 4 ) aerosols in the presence of gaseous HCHO, suggesting that the uptake of HCHO by aerosols is unlikely taking place. Although numerical simulations of HCHO, either by multi-dimensional models or box models, can in some cases reproduce HCHO observations (Wagner et al, 2001;Fried et al, 2003b;MacDonald et al, 2012), significant discrepancies between modeled and measured HCHO concentration have been frequently found (Choi et al, 2010;Fried et al, 2011, and references therein). The model underestimation can arise from the following: deviation from the steadystate assumption (Fried et al, 2003a), direct emissions and their transport (Fried et al, 2011), missing consideration of HCHO production from unmeasured precursors (Kormann et al, 2003;Choi et al, 2010), etc.…”

Glyoxal observations in the global marine boundary layer

“…In the case of the glyoxal retrieval using DOAS, several different analysis procedures have been commonly used [Sinreich et al, 2010;Li et al, 2012;Liu et al, 2012;MacDonald et al, 2012]. Most of the retrievals involve two major CHOCHO absorption bands including one at about 440 nm and the relatively strong band at around 455 nm.…”

“…Initial interest in glyoxal within the atmospheric boundary layer was generated by reports of elevated concentrations at urban locations [Grosjean et al, 1990;Volkamer et al, 2005a;Sinreich et al, 2007] and in forest environments [Huisman et al, 2011;MacDonald et al, 2012], with peak levels ranging between a few hundred parts per trillion (pptv, equivalent to pmol mol À1 ) to low parts per billion (ppbv, equivalent to nmol mol À1 ) observed using ground-based instruments. Satellite observations have also been reported using the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography [Wittrock, 2006;Wittrock et al, 2006], the Ozone Monitoring Instrument (K. Chance, personal communication, 2013), and the Global Ozone Monitoring Experiment-2 [Lerot et al, 2010;Vrekoussis et al, 2010].…”