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 + NO 2 ). 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 groundbased 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 satellitebased observations of R GF can be used alongside knowledge of land use as a global diagnostic of dominant hydrocarbon speciation.

Hotspot of glyoxal over the Pearl River delta seen from the OMI satellite instrument: implications for emissions of aromatic hydrocarbons

“…Validation with aircraft data indicates that the OMI HCHO and NO 2 retrievals are accurate within 20 and 30 %, respectively (Lamsal et al, 2014;Zhu et al, 2016). CHOCHO/HCHO column ratios from OMI are consistent with aircraft observations (Kaiser et al, 2015), whereas previous SCIAMACHY retrievals showed large discrepancies (DiGangi et al, 2012).…”

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

“…However, the opposite behavior is observed in ground-based studies (DiGangi et al, 2012). Our recent CHOCHO retrieval from the OMI satellite instrument (Chan Miller et al, 2014) is in better agreement with surface observations of CHOCHO and R GF (Kaiser et al, 2015) compared to those from GOME-2 (Vrekoussis et al, 2010) and SCIAMACHY (Wittrock et al, 2006) as a result of improved background corrections and removal of NO 2 interferences. There remains the question of how observed CHOCHO-HCHO relationships are to be interpreted.…”

“…The CHOCHO observations in the free troposphere (> 3 km) have to be treated with caution since they are below the reported instrument precision (32 pptv, Kaiser et al, 2015). It is therefore difficult to determine whether the bias is due to a missing CHOCHO source in the model or instrument artifact.…”

“…Another possible source of CHOCHO in the transition layer is via heterogeneous aerosol oxidation (Volkamer et al, 2015). However, specific aerosol precursors that produce CHOCHO at yields required to match the SENEX observations are currently unknown (Kaiser et al, 2015).…”

Tropospheric emissions: Monitoring of pollution (TEMPO)

“…TEMPO will resolve a wide range of NO x regimes due to its high spatial resolution, making NO 2 , H 2 CO and C 2 H 2 O 2 crucial datasets for validating isoprene chemistry. The ratio of C 2 H 2 O 2 to H 2 CO can provide insight on the VOC speciation and dominant chemical pathways in different locations [54]. Satellite C 2 H 2 O 2 retrievals are a good aromatic emissions constraint where these are high [15,68].…”