Nitrous acid emission from open burning of major crop residues in mainland China

Nitrous acid (HONO) plays a significant role in tropospheric chemistry. The emission sources of HONO, however, are poorly characterized, which constraints the predictive capabilities of HONO and the associated environmental influences in models. This study measured the emission factors of HONO (EFHONO) from the agricultural residue (including wheat, rice, corn and soybean straws) open burning, the largest biomass combustion source in China. Based on this, a high-resolution (0.25 degrees x 0.25 degrees) emission inventory of HONO from 2011 to 2015 was established using the city-level activity data compiled through statistical yearbook review in combination with machine learning algorithms, including linear regression (LR), back-propagation neural network (BPNN), general regression neural network (GRNN) and random forest (RF). Results showed that the averaged EFHONO from open burning of wheat, rice, corn and soybean straws were 0.10, 0.25, 0.98 and 0.97 g kg(-1), respectively. Total annual emissions of HONO were estimated to be 101347.3, 100232.1, 104278.3, 98383.0 and 107783.9 tons, respectively, for 2011-2015. Regions with high HONO emission intensities were mainly concentrated in North China Plain and Northeast China, whereas low intensities were mainly allocated to western regions. The temporal distribution of average provincial emissions showed the peaks in March, April, June and October, respectively. Results shown herein should be useful for improving the accuracy of HONO budgets and air quality simulations in China.