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 hour averaging), volume mixing ratios up to ~10 pptv were observed, with 24 hour averaged mixing ratios of 4.9 pptv 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 (1100–1300 hours) 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 glycoaldehyde 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 underprediction remains at night. The model was sensitive to changes in deposition rates of model intermediates and the uptake of glyoxal onto aerosol. The midday (1100–1300) 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.

Highlights

  • Reactive organic compounds in the marine atmosphere have the potential to influence climate through changing the atmospheric oxidation capacity (Read et al, 2012; Whalley et al, 2010) and by modifying the composition and size distribution of remote aerosol (O’Dowd et al, 2004; Spracklen et al, 2008)

  • Satellite observations suggest a relationship between ocean biology, cloud albedo and cloud droplet number concentration (CDNC) in remote marine regions (Meskhidze & Nenes, 2006; Krüger & Graßl, 2011), implying a biological source for marine cloud condensation nuclei (CCN)

  • Two intensive measurement campaigns were undertaken as part of the Oceanic Reactive Carbon: Chemistry-Climate Impacts (ORC3) project at the Global Atmospheric Watch (GAW) Cape Verde Atmospheric Observatory (CVAO) station

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Summary

Introduction

Reactive organic compounds in the marine atmosphere have the potential to influence climate through changing the atmospheric oxidation capacity (Read et al, 2012; Whalley et al, 2010) and by modifying the composition and size distribution of remote aerosol (O’Dowd et al, 2004; Spracklen et al, 2008). In situ observations have been made using the Fast Light-Emitting Diode 20 Cavity-Enhanced Differential Optical Absorption Spectroscopy (Fast LED-CE-DOAS) technique, showing average glyoxal mixing ratios of 43 pptv and 32 pptv in the Tropical Pacific MBL in the northern and southern hemisphere, respectively 22 (Coburn et al, 2014). We use a sensitive in situ laser-induced phosphorescence (LIP) technique, allowing measurements with a detection limit of ~1 pptv for 1 hour averaging but recorded with a duty cycle of 7 minutes (with a corresponding average limit of detection (LOD) of ~3 pptv) We use these new observations in conjunction with photochemical box modelling and direct observations of glyoxal precursor species, to investigate the dominant processes controlling its formation and loss in the remote marine atmosphere, and the extent to which observed concentrations can be reconciled with our knowledge of sources and sinks.

Oceanic Reactive Carbon
Glyoxal measurements
Instrument Description
Instrument Calibration
Limit of Detection
Determination of the instrument zero 6
Measurement uncertainty 14
Supporting measurements
Deployment of the instrument at Cape Verde
Impact of acetaldehyde 2
Impact of terpenes 18 Measurements at the Cape Verde Atmospheric
Local terpene sources 26
Remote monoterpene sources 26
Impact of physical processes
Conclusions and implications
Figure 5

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