Observational constraints on glyoxal production from isoprene oxidation and its contribution to organic aerosol over the Southeast United States

Abstract

We use a 0‐D photochemical box model and a 3‐D global chemistry‐climate model, combined with observations from the NOAA Southeast Nexus (SENEX) aircraft campaign, to understand the sources and sinks of glyoxal over the Southeast United States. Box model simulations suggest a large difference in glyoxal production among three isoprene oxidation mechanisms (AM3ST, AM3B, and Master Chemical Mechanism (MCM) v3.3.1). These mechanisms are then implemented into a 3‐D global chemistry‐climate model. Comparison with field observations shows that the average vertical profile of glyoxal is best reproduced by AM3ST with an effective reactive uptake coefficient γglyx of 2 × 10−3 and AM3B without heterogeneous loss of glyoxal. The two mechanisms lead to 0–0.8 µg m−3 secondary organic aerosol (SOA) from glyoxal in the boundary layer of the Southeast U.S. in summer. We consider this to be the lower limit for the contribution of glyoxal to SOA, as other sources of glyoxal other than isoprene are not included in our model. In addition, we find that AM3B shows better agreement on both formaldehyde and the correlation between glyoxal and formaldehyde (RGF = [GLYX]/[HCHO]), resulting from the suppression of δ‐isoprene peroxy radicals. We also find that MCM v3.3.1 may underestimate glyoxal production from isoprene oxidation, in part due to an underestimated yield from the reaction of isoprene epoxydiol (IEPOX) peroxy radicals with HO2. Our work highlights that the gas‐phase production of glyoxal represents a large uncertainty in quantifying its contribution to SOA.

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

“…As shown below, we find that this pathway can explain SENEX observations of prompt CHOCHO production under low-NO x conditions. The mechanism presented here differs substantially from the AM3 mechanism previously used by Li et al (2016) to analyze the SENEX observations. Li et al (2016) tested branching ratios of 22 and 0 % for δ-ISOPO 2 + NO, with the latter intended to reflect ISOPO 2 isomer interconversion.…”

“…The mechanism presented here differs substantially from the AM3 mechanism previously used by Li et al (2016) to analyze the SENEX observations. Li et al (2016) tested branching ratios of 22 and 0 % for δ-ISOPO 2 + NO, with the latter intended to reflect ISOPO 2 isomer interconversion. The 10 % branching ratio in this study is constrained by SEAC 4 RS organic nitrate observations .…”

“…Inclusion of DHDC increases the yield of CHOCHO via ISOPO 2 isomerization by 18 %, which is comparable to the AM3 yield. Li et al (2016) found that CHOCHO concentrations are sensitive to aerosol reactive uptake. Our standard model simulation does not include this uptake, but we conducted a sensitivity simulation with a reactive uptake coefficient γ = 2 × 10 −3 from Li et al (2016).…”

“…The 10 % branching ratio in this study is constrained by SEAC 4 RS organic nitrate observations . Li et al (2016) report a CHOCHO yield from GLYC oxidation (Sect. S1 in the Supplement), which is mainly due to a lower CHOCHO yield from GLYC + OH (13 % vs. 20 %).…”

“…Thirteen daytime flights were conducted over the campaign with extensive boundary layer coverage. Li et al (2016) recently used the SENEX observations to evaluate CHO-CHO formation from isoprene in the AM3 chemical transport model (CTM). They found that the AM3 mechanism had closer agreement with observations than the explicit Master Chemical Mechanism v3.3.1 (MCMv3.3.1; Jenkin et al, 2015), and suggested that CHOCHO yields from isoprene epoxydiols are underestimated in MCMv3.3.1.…”