Abstract
Abstract. Glyoxal (CHOCHO) is produced in the atmosphere by the oxidation of volatile organic compounds (VOCs). Like formaldehyde (HCHO), another VOC oxidation product, it is measurable from space by solar backscatter. Isoprene emitted by vegetation is the dominant source of CHOCHO and HCHO in most of the world. We use aircraft observations of CHOCHO and HCHO from the SENEX campaign over the southeast US in summer 2013 to better understand the CHOCHO time-dependent yield from isoprene oxidation, its dependence on nitrogen oxides (NOx ≡ NO + NO2), the behavior of the CHOCHO–HCHO relationship, the quality of OMI CHOCHO satellite observations, and the implications for using CHOCHO observations from space as constraints on isoprene emissions. We simulate the SENEX and OMI observations with the Goddard Earth Observing System chemical transport model (GEOS-Chem) featuring a new chemical mechanism for CHOCHO formation from isoprene. The mechanism includes prompt CHOCHO formation under low-NOx conditions following the isomerization of the isoprene peroxy radical (ISOPO2). The SENEX observations provide support for this prompt CHOCHO formation pathway, and are generally consistent with the GEOS-Chem mechanism. Boundary layer CHOCHO and HCHO are strongly correlated in the observations and the model, with some departure under low-NOx conditions due to prompt CHOCHO formation. SENEX vertical profiles indicate a free-tropospheric CHOCHO background that is absent from the model. The OMI CHOCHO data provide some support for this free-tropospheric background and show southeast US enhancements consistent with the isoprene source but a factor of 2 too low. Part of this OMI bias is due to excessive surface reflectivities assumed in the retrieval. The OMI CHOCHO and HCHO seasonal data over the southeast US are tightly correlated and provide redundant proxies of isoprene emissions. Higher temporal resolution in future geostationary satellite observations may enable detection of the prompt CHOCHO production under low-NOx conditions apparent in the SENEX data.
Highlights
Glyoxal (CHOCHO) and formaldehyde (HCHO) are shortlived products of the atmospheric oxidation of volatile organic compounds (VOCs)
Satellite observations of HCHO have been widely used as a proxy to estimate isoprene emissions (Abbot et al, 2003; Palmer et al, 2006; Millet et al, 2008; Curci et al, 2010; Barkley et al, 2013), but there are uncertainties related to the HCHO yield from isoprene oxidation (Marais et al, 2012) and the role of other NMVOCs as HCHO precursors (Fu et al, 2007)
ISOPO2 isomerization in the previous GEOSChem mechanism of Travis et al (2016) produced solely hydroperoxyaldehydes (HPALDs), but here we include the formation of dihydroperoxy α-formyl peroxy radicals following MCMv3.3.1. di-HPCARPs in MCMv3.3.1 have a low CHOCHO yield, but here we introduce a (1,5)H-shift isomerization of diHPCARPs that could be competitive with the (1,4)H-shift isomerization due to the presence of the terminal-peroxide functional group (Crounse et al, 2013)
Summary
Glyoxal (CHOCHO) and formaldehyde (HCHO) are shortlived products of the atmospheric oxidation of volatile organic compounds (VOCs). We use CHOCHO and HCHO aircraft observations over the southeast US from the summer 2013 Southeast Nexus (SENEX) campaign (Warneke et al, 2016), interpreted with the Goddard Earth Observing System chemical transport model (GEOSChem), to test understanding of the CHOCHO yield from isoprene oxidation, its dependence on nitrogen oxide radicals (NOx ≡ NO + NO2), and the combined value of the CHOCHO–HCHO pair measured from space to constrain isoprene emissions and chemistry. We present an improved chemical mechanism for CHOCHO formation from isoprene for the GEOS-Chem CTM, and evaluate it against the SENEX observations, including the time and NOx dependence of the CHOCHO yield from isoprene. We discuss the implications of the new mechanism for the interpretation of satellite observations, and present a first validation of the CHOCHO retrieval from the OMI satellite instrument (Chan Miller et al, 2014)
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