Assessment of TROPOMI CH4 product for estimating methane emissions from an agricultural region in Eastern Ontario

Abstract:

The two main sources of CH4 from the agricultural sector are enteric fermentation and manure management systems. Canada uses the IPCC Tier-II methodology to estimate CH4 emissions for its national inventory report of GHG emissions to UNFCCC, which is based on a bottom-up approach using activity data and emission factors obtained through site level experimental data. Satellite observations provide a top-down approach. However, because of the presence of wetlands in some agricultural regions, it is challenging to obtain accurate CH4 emission estimates at a regional scale. This study explores the use of TROPOMI methane product (xCH4) for verifying agricultural methane emission estimates in Eastern Ontario. We analyzed the spatial and temporal variability of CH4 emission from wetlands using the WetCHARTs dataset, and from agricultural sector using the EDGAR dataset. The simulated wetland CH4 emissions show strong seasonal variations related with air temperature, and the spatial variability is very pronounced during the growing season. The agricultural CH4 emissions are relatively constant during the year and are smaller than those from wetlands during the summer.The monthly average TROPOMI xCH4 do not show clear seasonal variations compared to the expected methane emissions from wetlands. We investigated the relationship between agricultural emissions and daily xCH4 data during a few cold winter days. A positive correlation is observed between the two variables, but the relationship changes from day to day. Using TROPOMI data acquired on March 18, 2018, CH4 emissions at the 0.1°grid can be estimated with a R² of 0.62 and RMSE of 1.02 mg /m²/day (37%; n=126). We conclude that there is great potential to map the spatial variability of agricultural CH4 emissions where it is the dominant source; however, further efforts are needed to understand the variability of other emission sources, and the factors driving the daily variations of TROPOMI xCH4 data. This is important because, large amount of xCH4 samples are needed to generate detailed CH4 emission maps; however, the number of successful TROPOMI retrievals in a given day is usually limited.