Molecular hydrogen (H2) combustion emissions and their isotope (D/H) signatures from domestic heaters, diesel vehicle engines, waste incinerator plants, and biomass burning

摘要

Molecular hydrogen (H), its stable isotope signature (δD), and thekey combustion parameters carbon monoxide (CO), carbon dioxide (CO), andmethane (CH) were measured from various combustion processes. H 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 Hand CO emissions in combustion processes and suggest that previous Hemissions from combustion may have been overestimated when scaled to COemissions. For the gas and oil-fired heater exhausts, H and δDgenerally decrease with increasing CO, 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 H 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 H and CO emissionsduring cold-start and some acceleration phases. While H 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 H/COratios (median 0.026, interpercentile range 0.12) are also significantly lesscompared to gasoline vehicle exhaust. Using H/CO emission ratios, alongwith CO global emission inventories, we estimate global H 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 H 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 Hemissions, we derive a mole fraction ratio ΔH/ΔCH(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 CH emission inventories, theresulting global biomass burning H emissions agree well with publishedglobal H emissions, suggesting that CH emissions may be a good proxyfor biomass burning H emissions.