Estimation of the atmospheric hydrogen source from the oxidation of man-made and natural non-methane organic compounds using a Master Chemical Mechanism

Abstract

A Master Chemical Mechanism has been employed to describe the yield of hydrogen from the photolysis of the formaldehyde produced by the oxidation of up to one hundred volatile organic compounds (VOCs) in a polluted boundary layer in north west Europe. Yields were estimated from the ratio of the additional hydrogen production to the additional rate of each VOC oxidised. The hydrogen yields varied from close to zero for benzaldehyde, dimethyl ether, formic acid and propane to close to unity for limonene, in terms of additional hydrogen molecules produced per additional VOC molecule oxidised. The alkene class exhibited the largest mean hydrogen yield of 0.6 molecules per molecule oxidised. To put the estimated H2 yields into a global context, they were scaled by the global emissions of anthropogenic and natural biogenic emissions. Altogether, the twenty most prolific anthropogenic VOC species accounted for a global atmospheric H2 source of 1.3 Tg yr−1. Oxidation of ethylene was the most prolific H2 source, accounting for 0.27 Tg yr−1, followed by butadiene and formaldehyde. The total atmospheric H2 source from the oxidation of natural biogenic VOCs amounted to 18.6 Tg yr−1. The most prolific natural biogenic VOC species by a large margin was isoprene which accounted for about 70% of the natural biogenic total. The total atmospheric H2 source from anthropogenic and natural biogenic VOCs estimated here is 20 Tg yr−1 and is larger than literature estimates but within their uncertainty ranges.