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Molecular hydrogen (H2), its stable isotope signature (δD), and thekey combustion parameters carbon monoxide (CO), carbon dioxide (CO2), andmethane (CH4) were measured from various combustion processes. H2 inthe exhaust of gas and oil-fired heaters and of waste incinerator plants wasgenerally depleted compared to ambient intake air, while CO was significantlyelevated. These findings contradict the often assumed co-occurring net H2and CO emissions in combustion processes and suggest that previous H2emissions from combustion may have been overestimated when scaled to COemissions. For the gas and oil-fired heater exhausts, H2 and δDgenerally decrease with increasing CO2, from ambient values of~0.5 ppm and +130‰ to 0.2 ppm and −206‰,respectively. These results are interpreted as a combination of anisotopically light H2 source from fossil fuel combustion and a D/H kineticisotope fractionation of hydrogen in the advected ambient air during itspartial removal during combustion. Diesel exhaust measurements fromdynamometer test stand driving cycles show elevated H2 and CO emissionsduring cold-start and some acceleration phases. While H2 and CO emissionsfrom diesel vehicles are known to be significantly less than those fromgasoline vehicles (on a fuel-energy base), we find that their molar H2/COratios (median 0.026, interpercentile range 0.12) are also significantly lesscompared to gasoline vehicle exhaust. Using H2/CO emission ratios, alongwith CO global emission inventories, we estimate global H2 emissions for2000, 2005, and 2010. For road transportation (gasoline and diesel), wecalculate 8.3 ± 2.2 Tg, 6.0 ± 1.5 Tg, and 3.8 ± 0.94 Tg,respectively, whereas the contribution from diesel vehicles is low(0.9–1.4%). Other fossil fuel emissions are believed to be negligiblebut H2 emissions from coal combustion are unknown. For residential(domestic) emissions, which are likely dominated by biofuel combustion,emissions for the same years are estimated at 2.7 ± 0.7 Tg,2.8 ± 0.7 Tg, and 3.0 ± 0.8 Tg, respectively. For biomass burning H2emissions, we derive a mole fraction ratio ΔH2/ΔCH4(background mole fractions subtracted) of 3.6 using wildfire emission datafrom the literature and support these findings with our wood combustionresults. When combining this ratio with CH4 emission inventories, theresulting global biomass burning H2 emissions agree well with publishedglobal H2 emissions, suggesting that CH4 emissions may be a good proxyfor biomass burning H2 emissions

atmospheric chemistry and physics

Molecular hydrogen (H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;) combustion emissions and their isotope (D/H) signatures from domestic heaters, diesel vehicle engines, waste incinerator plants, and biomass burning