The Effects Of Straw Burning On Air Quality In China

Agricultural residue burning, which emits a large amount of pollutants including PM2.5, has been recently limited or banned during harvest seasons in China due to air pollution concern. But this policy is unlikely to be a long-term solution without providing an effective alternative approach for farmers to remove straw. Straw burning in many areas is still very common, indicating a need to explore other solutions to air pollution problem from straw burning. The purpose of this study was to quantitatively evaluate the air quality effects of straw burning in China and seek for mitigation approach based on weather-air quality relationships. Straw burning emissions were estimated based on air pollution, global fire, and crop yield data. The key regions of the weather-air quality relationships were identified using singular value decomposition (SVD). Case simulations and experiments of the haze events in the major straw burning regions were conducted using HYSPLIT.The results show that (1) Open straw burning is one of the major air pollution sources in China. Straw burning PM2.5 emission in East China accounts for about 1/3 of the total anthropogenic PM2.5 emissions during the harvest seasons. Different from other emission sources in the air pollution inventories developed based on annual emissions, monthly emission should be used for straw burning, or the contribution from straw burning would be substantially underestimated during the harvest seasons. (2) The two main coupled spatial patterns between air pollution index (API) and meteorology, represent the same API anomalies across the country with the key regions in eastern China, and opposite API anomalies between northern and southern China, with separate key regions. The spatial correlations are more statistically significant in both magnitude and geographic coverage than the correlations at individual locations. They therefore could be used to improve long-term air quality predictions, including the air pollution impacts of straw burning. (3) Straw burning and the unfavorable weather conditions in East China caused the regional haze in June 2014. If crop straw burning date was changed to just before a big precipitation event during the harvest period, air pollutant severity would be dramatically reduced. It can be concluded based on the results that a substantial reduction of or complete ban on straw burning would significantly improve air quality in many parts of China. However, considering that there are no effective approaches yet to remove crop straw, in the case of policy permitting, farmers could be advised of selecting burning date according to the forecast precipitation so that straw is burned in a scientific way to minimize the air pollution effects.