Glyoxal yield from isoprene oxidation and relation to formaldehyde: chemical mechanism, constraints from SENEX aircraft observations, and interpretation of OMI satellite data

Glyoxal (CHOCHO) is produced in the atmosphere by oxidation of volatile organic compounds (VOCs). It is measurable from space by solar backscatter along with formaldehyde (HCHO), another oxidation product of VOCs. 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 time-dependent yields 5 from isoprene oxidation, their dependences on nitrogen oxides (NOx≡NO +NO2), the behaviour of the CHOCHO-HCHO relationship, the quality of OMI satellite observations, and the implications for using satellite CHOCHO observations as constraints on isoprene emission. We simulate the SENEX and OMI observations with the GEOS-Chem chemical transport model 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 ob10 servations 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 1 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1042, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 29 November 2016 c © Author(s) 2016. CC-BY 3.0 License.