Reply on RC3

Mengze Li

doi:10.5194/essd-2021-246-ac3

Abstract

Methane, ethane and propane are among the most abundant hydrocarbons in the atmosphere. These compounds have many emission sources in common and are all primarily removed through OH oxidation. Their mixing ratios and long-term trends in the upper troposphere and stratosphere are rarely reported due to the paucity of measurements. In this study, we present long-term (2006–2016) global ethane, propane, and methane data from airborne observation in the Upper Troposphere - Lower Stratosphere (UTLS) region, combined with atmospheric model simulations for ethane at the same times and locations, to focus on global ethane trends. The model uses the Copernicus emission inventory CAMS-GLOB and distinguishes 12 ethane emission sectors (natural and anthropogenic): BIO (biogenic emission), BIB (biomass burning), AWB (agricultural waste burning), ENE (power generation), FEF (fugitives), IND (industrial processes), RES (residential energy use), SHP (ships), SLV (solvents), SWD (solid waste and waste water), TNR (off-road transportation), and TRO (road transportation). The results from the model simulations were compared with observational data and further optimized. The Northern Hemispheric (NH) upper tropospheric and stratospheric ethane trends were 0.33 ± 0.27 %/yr and −3.6 ± 0.3 %/yr, respectively, in 2006–2016. The global ethane emission for this decade was estimated to be 19.28 Tg/yr. Trends of methane and propane, and of the 12 model sectors provided more insights on the variation of ethane trends. FEF, RES, TRO, SWD and BIB are the top five contributing sectors to the observed ethane trends. An ethane plume for NH upper troposphere and stratosphere in 2010–2011 was identified to be due to fossil fuel related emissions, likely from oil and gas exploitation. The discrepancy between model results and observations suggests that the current ethane emission inventories must be improved and higher temporal-spatial resolution data of ethane are needed. This dataset is of value to future global ethane budget estimates and the optimization of current ethane inventories. The data are public accessible at https://doi.org/10.5281/zenodo.5112059 (Li et al., 2021b).

Highlights

Ethane (C2H6) is among the most abundant non-methane hydrocarbons (NMHC) present in the atmosphere
Ethane oxidation forms acetaldehyde, which in turn contributes to the formation of peroxyacetyl nitrate (PAN) or peracetic acid depending on the levels of nitrogen oxides (NOx) (Millet et al, 2010)
Franco et al (2015) showed the ethane trend at Jungfraujoch to be -0.92%/yr during 1994-2008, followed by a strong positive trend of 4.9%/yr during 2009-2014, which may be related to the growth of shale gas exploitation in North America

Summary

Introduction

Ethane (C2H6) is among the most abundant non-methane hydrocarbons (NMHC) present in the atmosphere. 84% of its total emissions are from the Northern Hemisphere (NH) (Xiao et al, 2008). Oxidation by hydroxyl (OH) radicals is the major atmospheric loss process for tropospheric ethane while in the stratosphere the reaction with chlorine (Cl) radicals provides an additional loss processes (Li et al, 2018). Due to the seasonal variation of ethane emissions and the photochemically generated OH radicals, ethane 35 has a clear annual cycle in concentration, showing higher levels in winter. Ethane oxidation forms acetaldehyde, which in turn contributes to the formation of PAN (peroxyacetyl nitrate) or peracetic acid depending on the levels of NOx (Millet et al, 2010). PAN acts as a reservoir species of nitrogen oxides (NOx) 40 and can strongly affect tropospheric ozone distributions by transporting NOx from the point of emission to remote locations. PAN is known to be a secondary pollutant like ozone with negative impacts on regional air quality and human health (Dalsøren et al, 2018; Fischer et al, 2014; González Abad et al, 2011; Kort et al, 2016; Monks et al, 2018; Pozzer et al, 2020; Rudolph, 1995; Tzompa-Sosa et al, 2017)

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