Revising VOC emissions speciation improves the simulation of global background ethane and propane

Abstract

Non-methane volatile organic compounds (NMVOCs) generate ozone (O3) when they are oxidised in the presence of oxides of nitrogen, modulate the oxidative capacity of the atmosphere and can lead to the formation of aerosol. Here, we assess the capability of a chemical transport model (GEOS-Chem) to simulate NMVOC concentrations by comparing ethane, propane and higher-alkane observations in remote regions from the NOAA flask Network and the World Meteorological Organization's Global Atmosphere Watch (GAW) network. Using the Community Emissions Data System (CEDS) inventory, we find a significant underestimate in the simulated concentration of both ethane (35 %) and propane (64 %), consistent with previous studies. We run a new simulation in which the total mass of anthropogenic NMVOC emitted in a grid box is the same as that used in CEDS but with the NMVOC speciation derived from regional inventories. For US emissions, we use the National Emissions Inventory (NEI); for Europe, we use the UK National Atmospheric Emissions Inventory (NAEI); and for China, we use the Multi-resolution Emission Inventory model for Climate and air pollution research (MEIC). These changes lead to a large increase in the modelled concentrations of ethane, improving the mean model bias from −35 % to −4 %. Simulated propane also improves (from −64 % to −48 % mean model bias), but there remains a substantial model underestimate. There were relatively minor changes to other NMVOCs. The low bias in simulated global ethane concentration is essentially removed, resolving one long-term issue in global simulations. Propane concentrations are improved but remain significantly underestimated, suggesting the potential for a missing global propane source. The change in the NMVOC emission speciation results in only minor changes in tropospheric O3 and OH concentrations.