Abstract: Background, aim, and scope Biomass burning is a major source of black carbon(BC). China is a large agricultural country, with an average annual straw output of about 600 million tons. Open burning of crop residues accounts for about 26.8% of the total biomass burning. There are still limited studies focusing on the characteristics of BC from crop residues emissions. In this study, a custom-made combustion chamber was used to simulate the open burning of several representative types of crop residues. We obtained the characteristics of real-time emission factors of BC, and its emission inventory was established based on the crop yields in 2015 in China. Materials and methods The crop residues were mainly collected in the main grain producing areas of China. Test burns were conducted in a custom-made combustion chamber at the Institute of Earth Environment, Chinese Academy of Sciences (IEECAS) to simulate the open burning of crop residues. The combustion chamber is a ~8 m3 cavity container with a length, width, and height of 1.8, 1.8, and 2.2 m, respectively. The combustion chamber was equipped with a thermocouple, a thermoanemometer, and an air purification system. A dilution sampler (Model 18, Baldwin Environmental Inc., Reno, NV, USA) was installed downstream of the chamber to dilute the smoke before sampling. Results BC emission stability of crop residues during the process of flaming. The BC average emission factors for wheat straw, rice straw, corn stalk and soybean straw were 0.32 ± 0.05, 0.31 ± 0.13, 0.31 ± 0.09, and 0.44 ± 0.01 g·kg-1. The estimated BC emissions for open burning of wheat, rice, corn and soybean were 13,900 tons, 71,00 tons, 85,50 tons and 13,30 tons, respectively, in China in 2015. Discussion The carbon content of straw calculated by mass balance and the average emission factors are consistent with those of previous studies. The four kinds of crop residue mass did not change much during the initial combustion, and the mass decreased rapidly during the steady combustion process. The real-time emission factor of BC has little change in stable combustion. The quality reconstruction is inaccurate in this experiment at the initial and end of the combustion, and the BC emission factor is based on the mean value of the stable combustion stage. Conclusions Calculating the carbon content of crop residue can well reflect the changes of straw burning quality. The BC average emission factor of soybean straw was higher than that of the other three straws. The quality reconstruction method is not accurate due to the less BC emission in the initial and end of combustion. According to the estimation of BC emissions of opening burning crop straw in all Chinese mainland provinces in 2015, we can see that BC emissions are mainly concentrated in Jiangsu, Anhui, Henan and Hunan provinces. Recommendations and perspectives This experiment mainly simulates the change of BC emission in crop straw open burning during flaming stage, and the BC emissions from crop straw smoldering process is less.
Keywords: Black carbon; crop residues burning; high time resoluation；real-time emission factor