Spatial and temporal variations of open straw burning based on fire spots in northeast China from 2013 to 2017

Open straw burning releases significant amounts of particulate matter (PM) and trace gases into the air and thus is an important source of atmospheric pollutants in China. In this study, based on fire spots in northeast China from 2013 to 2017, we analyzed pollution concentration degree (PCD) and pollution concentration period (PCP), and assessed spatial and temporal variations of open straw burning. Straw types and burning areas were then determined according to the distributions of fire spots and land use dataset. The amounts of several major atmospheric pollutants (PM10, PM25, CO, NOx and NMVOCs (non-methane volatile organic compounds)) emitted into the atmosphere from open straw burning were estimated according to crop production, Residue/Grain ratio (R/G), combustion efficiency and emission factor. Open straw burning occurred frequently in dry and paddy fields, mainly in areas of West and Northeast Heilongjiang Province, Midwest Jilin Province and Central Liaoning Province. The temporal variations of open straw burning tended to be dispersed with PCD varying from >0.8 during 2013-2015 to <0.4 during 2017 and PCP changing from 28.2 to 36.0 during autumn-winter seasons of 2013-2015 to 5.4-7.9 during winter-spring seasons of 2016-2017. During the five-year period (2013-2017), the total straw yield reached 628.7 million tons, of which 10.9 million tons were open burned in northeast China. The total amounts emitted from open straw burning were 108.9, 106.9, 552.9, 36.9 and 99.4 thousand tons for PM10, PM25, CO, NOx and NMVOCs, respectively. Among these emissions, corn straw burning contributed 81.7%, 81.6%, 71.4%, 86.8% and 74.9% while rice straw burning contributed 18.2%, 18.4%, 28.5%, 13.2% and 25.0%, respectively. Emissions from open straw burning have significant impacts on pollutants ambient concentrations, especially for PM2.5 in the autumn-winter seasons as indicated by their good correlations (p < 0.05).