Quantum yields for the photolysis of glyoxal below 350 nm and parameterisations for its photolysis rate in the troposphere

Robert J Salter,Mark A Blitz,Michael J Pilling,Paul W Seakins,Tamás Kovács,Dwayne E Heard,Andrew R Rickard

doi:10.1039/c3cp43597k

Robert J Salter, Mark A Blitz + Show 5 more

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https://doi.org/10.1039/c3cp43597k

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Abstract

The formation of HCO and of H in the photolysis of glyoxal have been investigated over the wavelength ranges 310-335 nm for HCO and 193-340 nm for H. Dye laser photolysis was coupled with cavity ring-down spectroscopy for HCO, and with laser induced fluorescence spectroscopy for H. Absolute quantum yields were determined using actinometers based on (a) Cl2 photolysis and the Cl + HCHO reaction for HCO and (b) N2O photolysis (and O(1)D + H2) and CH2CO photolysis (and CH2 + O2) for H. The quantum yields were found to be pressure independent in this wavelength region. Quantum yields for all product channels under atmospheric conditions were calculated and compared with literature values. Differences between this work and previously published work and their atmospheric implications are discussed.

Highlights

In our companion paper[1] we discussed the photolysis mechanism of glyoxal (HCO)[2] in the wavelength region 355–414 nm showing that above 395 nm, photolysis can occur through two electronic states
We show that the major photolysis channels in this wavelength region are (P1a), (P1b) and (P1c) and that the yield of (P1d) is less than 0.1
Where kq/kd is the ratio of the quenching to the dissociation rate coefficients, where dissociation refers to all operative photolysis channels

Summary

Introduction

In our companion paper[1] we discussed the photolysis mechanism of glyoxal (HCO)[2] in the wavelength region 355–414 nm showing that above 395 nm, photolysis can occur through two electronic states. In this paper we report data from shorter wavelength (193–355 nm) photolysis, detecting both H and HCO products, and focus on the atmospheric implications of the measurements. Elevated concentrations (up to 1–1.2 ppbv at midday in Mexico City)[15] have been measured in urban areas.[16] Glyoxal reacts relatively slowly with OH (k296K = 8.8 Â 10À12 cm[3] molÀ1 sÀ1),[17] giving it an atmospheric lifetime with respect to removal by OH, tOH, of B1 day for [OH] B 1 Â 106 mol cmÀ3.18 The major removal process for glyoxal in the troposphere is photolysis, with tPhot B 2–3 hours.[18,19] Photolysis of glyoxal can take place at wavelengths that are available within the troposphere. Photolysis can lead to the production of the formyl radical, HCO, which plays an important atmospheric role as it reacts quickly with O2 to form HO2 radicals:[20,21]

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Discussion

Conclusion