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

Molecular hydrogen (H-2), its stable isotope signature (delta D), and the key combustion parameters carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4) were measured from various combustion processes. H-2 in the exhaust of gas and oil-fired heaters and of waste incinerator plants was generally depleted compared to ambient intake air, while CO was significantly elevated. These findings contradict the often assumed co-occurring net H-2 and CO emissions in combustion processes and suggest that previous H-2 emissions from combustion may have been overestimated when scaled to CO emissions. For the gas and oil-fired heater exhausts, H-2 and delta D generally decrease with increasing CO2, from ambient values of 0.5 ppm and +130 parts per thousand to 0.2 ppm and -206 parts per thousand, respectively. These results are interpreted as a combination of an isotopically light H-2 source from fossil fuel combustion and a D/H kinetic isotope fractionation of hydrogen in the advected ambient air during its partial removal during combustion. Diesel exhaust measurements from dynamometer test stand driving cycles show elevated H-2 and CO emissions during cold-start and some acceleration phases. While H-2 and CO emissions from diesel vehicles are known to be significantly less than those from gasoline vehicles (on a fuel-energy base), we find that their molar H-2/CO ratios (median 0.026, interpercentile range 0.12) are also significantly less compared to gasoline vehicle exhaust. Using H-2/CO emission ratios, along with CO global emission inventories, we estimate global H-2 emissions for 2000, 2005, and 2010. For road transportation (gasoline and diesel), we calculate 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 negligible but H-2 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 H-2 emissions, we derive a mole fraction ratio Delta H-2/Delta CH4 (background mole fractions subtracted) of 3.6 using wildfire emission data from the literature and support these findings with our wood combustion results. When combining this ratio with CH4 emission inventories, the resulting global biomass burning H-2 emissions agree well with published global H-2 emissions, suggesting that CH4 emissions may be a good proxy for biomass burning H-2 emissions.