# Effects of Agricultural Biomass Burning on Regional Haze in China: A Review
## 2 Biomass Burning and Regional Haze
### Relationship between Biomass Burning and Regional Haze  
Haze is defined as an air clarity phenomenon when visibility is less than 10 km, caused by dry and invisible airborne fine particles according to the criteria prescribed by the China Meteorological Administration [28]. Biomass burning was the major contributor of local particulate matter, surface ozone, and regional haze [29]. Under economic development, fossil fuel consumption has also been increasing, and domestic biofuel has been decreasing in rural regions. As a result, the demand of fuel is lower, and more agricultural crop biomass is burned directly in the field [10]. Agricultural open field crop burning can result in intense local and regional haze and environmental pollution episodes, and can even affect climate change. A previous study documented that crop residue burning was responsible for intensive air pollution during harvest season [30]. The relationship between biomass burning and regional haze can be deduced from their temporal and spatial pattern and chemical composition over China. Both biomass burning and regional haze show obvious temporal patterns. In general, aerosol concentrations tend to be higher in winter and lower in summer [31], with a secondary peak in spring [31]. Because of the increasing aerosol concentrations, the air quality has been deteriorating and hazy condition has been increasing in the past decades [26]. Regional haze is more frequent in winter and spring, and less frequent in summer [32], due to both increasing aerosol emissions from heating and stable atmospheric stratification in winter. However, previous studies indicated that crop burning emits large quantities of particulate matter and gaseous pollutants, and can seriously deteriorate air quality during the harvest season, especially in autumn [33]. Several active fires were detected in most parts of China during the harvest season [30], likely due to the common management practice of tilling directly after burning. Notably, in the past few years, October and June are the two months when both biomass burning and regional haze occurred in Northeastern China and Southeastern China, respectively [10]. In October, large quantities of gaseous pollutants and aerosol particles were emitted into the atmosphere during biomass burning episodes, and the stable atmospheric stratification prevented short term dispersion [10]; as a result, a regional haze event occurred after the biomass burning period. Based on the surface visibility observations at meteorological stations in China, four major haze areas are identified: (1) Hua Bei Plain in Northern China, and the Guanzhong Plain; (2) Eastern China with the main body in the Yangtze River Delta area; (3) Southern China with most areas of Guangdong and the Pearl River Delta area; and (4) the Sichuan Basin in Southwestern China [26]. (Furthermore, Northeastern China has two sub-regions: the major grain producing area mainly in Heilongjiang Province and an old industrial region mainly in Liaoning Province; these could comprise a fifth regional haze area. Northeastern China and the North China Plain are often reported to suffer from heavy regional air pollution during harvest season. Increased crop residue burning is likely to be one of the reasons for an increase in regional haze in Northeastern China. A previous paper documented that Heilongjiang Province (in Northeastern China) was one of three high-density biomass burning regions [34; 35]. Lower population and a large area of crop land in Heilongjiang Province caused most agricultural biomass to be disposed of by open burning in the field. In summary, the major regional haze distribution is focused in Eastern China, consistent with fire distribution in China. While temporal and spatial patterns of agricultural burning and regional haze show the same trends, the chemical compositions of particles are another key parameter for estimating the effects of biomass burning on regional haze. Firstly, both biomass burning and regional haze are dominated by fine particles.