Reassessing the ratio of glyoxal to formaldehyde as an indicator of hydrocarbon precursor speciation

Abstract. The yield of formaldehyde (HCHO) and glyoxal (CHOCHO) from oxidation of volatile organic compounds (VOCs) depends on precursor VOC structure and the concentration of NO_x (NO_x = NO + NO₂). Previous work has proposed that the ratio of CHOCHO to HCHO (R_GF) can be used as an indicator of precursor VOC speciation, and absolute concentrations of the CHOCHO and HCHO as indicators of NO_x. Because this metric is measurable by satellite, it is potentially useful on a global scale; however, absolute values and trends in R_GF have differed between satellite and ground-based observations. To investigate potential causes of previous discrepancies and the usefulness of this ratio, we present measurements of CHOCHO and HCHO over the southeastern United States (SE US) from the 2013 SENEX (Southeast Nexus) flight campaign, and compare these measurements with OMI (Ozone Monitoring Instrument) satellite retrievals. High time-resolution flight measurements show that high R_GF is associated with monoterpene emissions, low R_GF is associated with isoprene oxidation, and emissions associated with oil and gas production can lead to small-scale variation in regional R_GF. During the summertime in the SE US, R_GF is not a reliable diagnostic of anthropogenic VOC emissions, as HCHO and CHOCHO production are dominated by isoprene oxidation. Our results show that the new CHOCHO retrieval algorithm reduces the previous disagreement between satellite and in situ R_GF observations. As the absolute values and trends in R_GF observed during SENEX are largely reproduced by OMI observations, we conclude that satellite-based observations of R_GF can be used alongside knowledge of land use as a global diagnostic of dominant hydrocarbon speciation.