Observations and modelling of glyoxal in the tropical Atlantic marine boundary layer
Authors: Walker Hannah , Stone Daniel , Ingham Trevor , Hackenberg Sina , Cryer Danny , Punjabi Shalini , Read Katie , Lee James , Whalley Lisa , Spracklen Dominick V. , Carpenter Lucy J. , Arnold Steve R. , Heard Dwayne E. 
Published: April 2022

Abstract. In situ field measurements of glyoxal at the surface in the tropical marine
boundary layer have been made with a temporal resolution of a few minutes
during two 4-week campaigns in June–July and August–September 2014 at the
Cape Verde Atmospheric Observatory (CVAO; 16∘52′ N, 24∘52′ W). Using laser-induced phosphorescence spectroscopy with an
instrumental detection limit of ∼1 pptv (1 h averaging),
volume mixing ratios up to ∼10 pptv were observed, with 24 h averaged mixing ratios of 4.9 and 6.3 pptv observed during the
first and second campaigns, respectively. Some diel behaviour was observed,
but this was not marked. A box model using the detailed Master Chemical
Mechanism (version 3.2) and constrained with detailed observations of a
suite of species co-measured at the observatory was used to calculate
glyoxal mixing ratios. There is a general model underestimation of the
glyoxal observations during both campaigns, with mean midday (11:00–13:00) observed-to-modelled ratios for glyoxal of 3.2 and 4.2 for the two
campaigns, respectively, and higher ratios at night. A rate of production
analysis shows the dominant sources of glyoxal in this environment to be the
reactions of OH with glycolaldehyde and acetylene, with a significant
contribution from the reaction of OH with the peroxide HC(O)CH2OOH,
which itself derives from OH oxidation of acetaldehyde. Increased mixing
ratios of acetaldehyde, which is unconstrained and potentially
underestimated in the base model, can significantly improve the agreement
between the observed and modelled glyoxal during the day. Mean midday
observed-to-modelled glyoxal ratios decreased to 1.3 and 1.8 for campaigns 1
and 2, respectively, on constraint to a fixed acetaldehyde mixing ratio of
200 pptv, which is consistent with recent airborne measurements near CVAO.
However, a significant model under-prediction remains at night. The model
showed limited sensitivity to changes in deposition rates of model
intermediates and the uptake of glyoxal onto aerosol compared with
sensitivity to uncertainties in chemical precursors. The midday (11:00–13:00)
mean modelled glyoxal mixing ratio decreased by factors of 0.87 and 0.90 on
doubling the deposition rates of model intermediates and aerosol uptake of
glyoxal, respectively, and increased by factors of 1.10 and 1.06 on halving
the deposition rates of model intermediates and aerosol uptake of glyoxal,
respectively. Although measured levels of monoterpenes at the site (total of
∼1 pptv) do not significantly influence the model calculated
levels of glyoxal, transport of air from a source region with high
monoterpene emissions to the site has the potential to give elevated mixing
ratios of glyoxal from monoterpene oxidation products, but the values are
highly sensitive to the deposition rates of these oxidised intermediates. A
source of glyoxal derived from production in the ocean surface organic
microlayer cannot be ruled out on the basis of this work and may be
significant at night.