Improved Crop-specific NOx Emissions from Croplands in Southeast Asia over 1980–2019

Nitrogen oxides (NOx) play an important role in the formation of tropospheric ozone and fine particulate matter. Although regulatory policies in Southeast Asia have successfully limited combustion sources of NOx emissions, fertilized cropland NOx emissions have become increasingly prominent. However, the amount and geographic distribution of NOx emissions from fertilized croplands over Southeast Asia remain largely uncertain. Here, we use a bottom-up spatial model that combines temperature dependence and the Yienger and Levy (YL95) scheme to quantify the spatiotemporal changes in crop-specific fertilized cropland NOx emissions at five-arcminute resolution (about 10 km) from 1980 to 2019. The results show a 4-fold increase in NOx emissions from fertilized croplands in Southeast Asia from 12.9 Gg N yr-1 (R50, the difference between the 25% and 75% quantiles: 8.8-17.7 Gg N yr-1) in 1980 to 53.3 Gg N yr-1 (R50: 35.6-72.0 Gg N yr-1) in 2019. During 1980-2019, the annual NOx emissions of rice, maize and other crops showed increasing spatial trends (0.20, 0.21, and 0.63 Gg N yr-2, respectively), whereas those of wheat remained flat (-0.0006 Gg N yr-2). Our new estimate averages 19% lower than previous bottom-up and top-down estimates. The NOx emissions hotspots are mainly located in Indonesia, Vietnam, Thailand and the Philippines for rice, maize and other crops and in Myanmar for wheat. Our crop-specific and spatially explicit NOx emissions inventories can contribute to identifying emissions and reduction hotspots and assessing future policy implications regarding mitigation options for improving air quality and food security in Southeast Asia. Future studies that comprehensively consider meteorological conditions, soil properties, management practices and small-scale land-use changes can potentially lower the uncertainty of this estimation.