# Effects of Agricultural Biomass Burning on Regional Haze in China: A Review
## 2 Biomass Burning and Regional Haze  
### Agricultural Open Field Biomass Burning in China  
Biomass burning is the burning of living and dead vegetation, including anthropogenic burning and natural fires [8]. Ninety percent of all biomass-burning is thought to be human initiated [8]. Anthropogenic burning is used in many applications, such as agricultural expansion, weed and residue burning, and harvesting practices. Natural fires include forest and grassland fires, caused by lighting [8]. Globally, it is estimated that 8700 Tg of dry matter is burnt each year [7]. China is one of the greatest sources of biomass burning emissions [14], predominantly due to anthropogenic burning such as post-harvest agricultural open field biomass burning. A previous study has estimated that about 122 Tg of crop residues are burned annually in China [14]. Since 1997, a series of regulations and laws to prohibit field burning were enacted by the Chinese government [2]. Farmers were encouraged to return crop residue to agricultural soils as fertilizer. However, despite these prohibitive laws, changes in practice were not widely adopted by farmers. For example, about 43% of crop residue was burned in the field in Suqian according to field investigations from 2001 to 2005 [21]. In China, crop residue burning accounts for a major part of total biomass burning [2]. There are several reasons for this. Firstly, China is an agricultural country, and produces large amounts of crop residue every year. Secondly, due to the high cost of straw collection, and limited room for straw storage, open field burning is the most effective option of all treatments. Finally, mechanized harvesting is another reason for crop residue burning in the field [21; 22]. After mechanized harvesting, the height of the stubble left in the fields is about 30 cm, making sowing difficult. Fragments are scattered throughout the field, making recovery difficult and reducing the potential for reuse as an industrial raw material. Crop residue burning has an obvious spatial and temporal pattern in China. Based on the daily fire point derived from MODIS from 2002 to 2015, fire peak month and intensity were documented by [PERSON] et al. [23]. Most of the burning was observed in Eastern China. Previous studies also demonstrated that the North China Plain, Northestern China and South China were the most important contributors to crop open burning in fields [17]. More than 80% of the agricultural burnings were in the agricultural regions, such as Anhui, Jiangsu, Shandong and Henan Provinces [10]. The temporal pattern of crop open burning is significantly associated with local sowing and harvest timing. In Mainland China, there were two distinct peaks corresponding to the summer and autumn harvest periods, especially in June and October [10]. The annual percentages of fires that occurred in June and October were 61-86% and 5-14%, respectively, according to active fire products derived from MODIS from 2009 to 2010 [10]. In the Northern China Plain, over 75% of the annual fires occurred in June, with another smaller peak in early October [17]. Besides the obvious spatial and temporal patterns, emissions of gaseous pollutants and aerosol particles from biomass burning also have distinct aerosol characteristics. Emissions from biomass burning are known as a source of greenhouse gases such as carbon dioxide, methane, and nitrous oxide. Furthermore, aerosols emitted from biomass burning have high mass concentrations of PM\({}_{2.5}\) (particles smaller than 2.5 \(\upmu\)m in diameters), OC (organic carbon), EC (elemental carbon) and K (potassium) [24]. Reports suggest that approximately 50-70% of the mass of all particle emissions from biomass burning is carbon, with OC and EC accounting for 55% and 8% of fine particle mass, respectively [25]. Trace inorganic species, mostly dominated by K, Cl (chlorine) and Ca (calcium), account for approximately 10% of the mass of fresh smoke [25]. In China, the PM mass concentrations tend to be higher in winter, due to the high coal consumption [26].