Abstract

Abstract. Molecular hydrogen (H2), its stable isotope signature (δD), and the key combustion parameters carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4) were measured from various combustion processes. H2 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 H2 and CO emissions in combustion processes and suggest that previous H2 emissions from combustion may have been overestimated when scaled to CO emissions. For the gas and oil-fired heater exhausts, H2 and δD generally 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 an isotopically light H2 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 H2 and CO emissions during cold-start and some acceleration phases. While H2 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 H2/CO ratios (median 0.026, interpercentile range 0.12) are also significantly less compared to gasoline vehicle exhaust. Using H2/CO emission ratios, along with CO global emission inventories, we estimate global H2 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 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 H2 emissions, we derive a mole fraction ratio ΔH2/ΔCH4 (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 H2 emissions agree well with published global H2 emissions, suggesting that CH4 emissions may be a good proxy for biomass burning H2 emissions.

Highlights

  • The atmospheric budget of molecular hydrogen (H2) has recently gained increasing interest because of the ongoing discussion of a potential shift from our fossil fuel-based energy economy, to one that is based on H2 as an energy carrier

  • We will distinguish between the ratio based on measured mole fractions, here termed “absolute ratio”, and the ratio calculated after subtraction of the background dry air mole fraction, which are typically found in the combustion intake air, i.e. H2 ≈ 0.5 ppm and carbon monoxide (CO) ≈ 0.2 ppm

  • Our study has attempted to fill some of the gaps in the knowledge of H2 emissions quantification from fossil fuel and biomass combustion

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Summary

Introduction

The atmospheric budget of molecular hydrogen (H2) has recently gained increasing interest because of the ongoing discussion of a potential shift from our fossil fuel-based energy economy, to one that is based on H2 as an energy carrier. Such a shift could lead to increased anthropogenic emissions from leakage of H2, and potentially change the chemistry and physics of our atmosphere (Schultz et al, 2003; Warwick et al, 2004; Vogel et al, 2012). Vollmer et al.: Molecular hydrogen from residential combustion atmospheric H2 budget is a prerequisite for an assessment of potential future human impacts on the atmospheric H2 cycle

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