Global levels of the hydrocarbons ethane and propane in the atmosphere have been underestimated by more than 50%, according to a new study by a team of scientists from Europe and the US.
When ethane and propane—the most abundant non-methane hydrocarbons (NMHC) in the atmosphere—mix with nitrogen oxides from vehicles and power plants they form tropospheric ozone—a key component of smog and directly linked to increases in mortality. This new study shows that global fossil fuel emissions of these hydrocarbons have been underestimated and are a factor of 2-3 times higher than previously thought.
Accounting for these enhanced ethane and propane emissions results in simulated surface ozone concentrations that are 5–13% higher than previously assumed in some polluted regions in Asia.
The team used the OsloCTM3 model to simulate the pre-industrial (year 1750) and current (year 2011) distributions of atmospheric ethane and propane. The OsloCTM3 simulations are driven with three-hourly year 2011 meteorological forecast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) model. These data are 36-hour forecasts produced with 12 hours of spin-up starting from an ERA-Interim analysis at noon on the previous day.
The study used data collected from 20 observatories world-wide. The researchers showed that observations of pre-industrial and present-day ethane and propane can be reproduced in simulations with a detailed atmospheric chemistry transport model—provided that natural geologic emissions are taken into account and anthropogenic fossil fuel emissions are assumed to be two to three times higher than is indicated in current inventories.
Levels of ethane and propane declined in many places the 1980s and 1990s, but global growth in demand for natural gas means these trends may be reversing. The effects of higher ozone would be felt in the rural environment where it damages crops and plants, and in cities on human health.—Professor Ally Lewis, a co-author of the study
The authors noted that a recent study also suggests underestimation of fossil methane emissions; methane constitutes the largest share of hydrocarbons emitted from fossil sources.
Compared with previous inventories the much higher fossil fuel ethane and propane emissions in the new data sets used in ALT1 and ALT2 in this study are mainly due to higher NMHC to methane emission ratios. The improved agreement with ice-core ethane measurements for the simulation with geologic emissions supports the idea that there is a considerable geologic methane emission source. As for ethane and propane, geologic emissions of methane have been neglected in many model studies. In accordance with an earlier study, we suggest a need for more studies evaluating the reported level of fossil methane emissions in current emission inventories. Understanding the contribution from different natural and anthropogenic emission sources is a critical precursor to design efficient measures to reverse ongoing atmospheric ethane, propane and methane increases.—Dalsøren et al.
Stig B. Dalsøren, Gunnar Myhre, Øivind Hodnebrog, Cathrine Lund Myhre, Andreas Stohl, Ignacio Pisso, Stefan Schwietzke, Lena Höglund-Isaksson, Detlev Helmig, Stefan Reimann, Stéphane Sauvage, Norbert Schmidbauer, Katie A. Read, Lucy J. Carpenter, Alastair C. Lewis, Shalini Punjabi & Markus Wallasch (2018) “Discrepancy between simulated and observed ethane and propane levels explained by underestimated fossil emissions” Nature Geoscience doi: 10.1038/s41561-018-0073-0