# 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
Approximately 90% of particles emitted from biomass burning are PM\({}_{2.5}\)[36]. A previous study revealed that atmospheric particles, especially fine particles from wheat straw burning in Northern China Plain, exert considerable influence on Beijing air quality [37]. Secondly, chemical compositions of particles emitted from agricultural straw burning contained notable amounts of some elements such as K and C. Previous studies have demonstrated that the combustion-derived particles emitted from wheat are enriched in K and Cl, whereas the particles emitted from rice are largely carbonaceous [38]. Some fraction indicators were selected for assessing the relationship between biomass burning and regional haze. In general, high PM\({}_{2.5}\)/PM\({}_{10}\), OC/EC, and K/EC ratios were indices for biomass burning [9; 24]. High ratios of PM\({}_{10}\)/SO\({}_{2}\) and CO/SO\({}_{2}\) showed that air quality was heavily impacted by wheat straw burning, and these ratios can be employed as indicators of the contribution of wheat straw burning to the degradation of Beijing air quality [37]. Here, we focus on Changchun City as a case study. There are two reasons for choosing Changchun City: (1) it lies in a high-frequency fire region, and suffers from regional haze; and (2) fire locations and PM\({}_{2.5}\) concentrations can be monitored simultaneously. PM\({}_{2.5}\) concentrations, fire points, temperature and precipitation during harvesting season and the regional haze period in Changchun City in 2014 and 2015 are shown in Figure 1. The PM\({}_{2.5}\) concentrations were downloaded from Air Quality OnlineMonitoring of China [39]. There are 10 measurement stations in Changchun for long time online monitoring. The fire points were derived from the NASA Earth Observatory's Active Fire Data [40]. In general, harvesting season was from 1 October to 10 November, with biomass burning occurring one week later than harvesting. The PM\({}_{2.5}\) concentrations and fire points presented same temporal pattern both in 2014 and 2015 (Figure 1). Figure 1 shows a high PM\({}_{2.5}\) concentration during biomass burning. The PM\({}_{2.5}\) concentration peak in 2015 was later than that in 2014, due to the later harvesting and biomass burning in 2015. PM\({}_{2.5}\) concentrations and fire points were lower in 2015 compared with 2014, because of serious prohibition of biomass burning and collection of agricultural residue by the local government in 2015. Nevertheless, a strong peak occurred on 9 November, because most agricultural residue was abandoned in the fields waiting for collection, while weather reports predicted heavy snow on the night of 9 November. If the agricultural residues were not burned before the heavy snow, they would have remained in the field until next spring, thereby delay sowing. Consequently, most farmers burned the residue before 9 November in order to dispose of the residue in time, resulting the emission of large quantities of particulate matter and pollutant gases into the atmosphere in one day. This triggered a regional haze event. Besides biomass burning emission, the weather conditions (temperature and precipitation) between 2014 and 2015 were the same. It suggested that dramatically biomass burning contributed the regional haze in Changchun city. If a prescribed burning had been conducted, and the crop residue was burned over a longer time period instead of on one or two days, there would have been no PM\({}_{2.5}\) concentration peak. Despite the qualitative understanding of biomass burning and regional haze, the determination of key parameters needed for estimating the effects of biomass burning on regional haze is not straightforward.