Introduction

Biomass burning contributes significantly to greenhouse gases (GHGs) and atmospheric pollutants emissions (Zhang et al. 2016; Qiu et al. 2016; Chen et al. 2017). Emissions of GHGs from biomass burning mainly include carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4), accounting for 20–50% of global GHGs emissions (Yadav et al. 2017). At the same time, biomass burning also emits a large amount of atmospheric pollutants including particulate matter (PM), nonmethane organic compounds (NMVOC), sulfur dioxide (SO2), ammonia (NH3), black carbon (BC), carbon monoxide (CO), organic carbon (OC), and other trace gases (Akagi et al. 2011; Chen et al. 2017). These gases emissions produce significantly impacts on climate change, air quality, as well as public health and ecosystems at the global, regional, and local scales.

Crop straw open burning, as the major type of biomass burning, contributes to 34% of total biomass burning in Asia (Streets et al. 2003; Permadi and Oanh 2013). Quantification of GHGs and atmospheric pollutants emissions from crop straw open burning is of great important for mitigating climate change and improving air quality, which has been aroused widespread attention by researchers and policy-makers (Qiu et al. 2016; Wang et al. 2018; Zhang et al. 2019). Previous studies on these gases emissions primarily focus on the national or regional scales. However, there are large differences in climate condition, crop types, cropping structure, production method, and combustion efficiency among finer scales, such as provinces/states, cities, and counties. Ignoring these regional differences may lead to huge deviation in estimating GHGs and atmospheric pollutants emissions at the national level, resulting in high uncertainty. Therefore, mapping a high-resolution estimation of GHGs and atmospheric pollutants emission from open crop straw burning is essential to reduce the uncertainty and to improve the spatial accuracy.

China, as one of the major agricultural country worldwide, produces 653 million tons annually of crop straw with most being wheat, maize, and rice straw during 2000–2017 (Zhuang et al. 2020). Owing to the convenience handling, a large amount of crop straw burned directly in the field (Streets et al. 2003). Much work in the previous studies has evaluated the GHGs emissions or atmospheric pollutants from crop straw open burning at different scales using top-down or the bottom-up methods. However, these results exist higher uncertainties (Zhang et al. 2019). Top-down method is mainly affected by misleading heat-sensing information, satellite overpass periods, and weather conditions (Streets et al. 2003; He et al. 2011). By comparison, bottom-up method involves less uncertainties when using high-resolution activity data including emission factors, dry matter content, grain to straw ratio, and combustion efficiency (Zhang et al. 2013). Although the method has some advantage, collecting these activity data and obtaining the emission factors are a major challenge at the different scales. For example, Peng et al. (2016) obtained atmospheric pollutants emission for in-field burning emissions based on questionnaire surveys at the provincial scale in 2009; Zhang et al. (2019) estimated the pollutant emissions from crop straw open burning based on the 296 prefecture-level cities investigations. Although these results may reflect the status of atmospheric pollutants emission inventory from crop straw open burning to some extent, the existing studies exist several shortcomings as follows.

On the one hand, previous studies may not represent the finer emission estimation because of the limitation of surveys and samples at the finer scale, such as county. There exists difference in yield, cropping structure, dry matter content, and grain to straw ratio among finer scale in China, such as county. For example, Kang et al. (2020) found that the coefficient of variation in residue to grain ratio is 0.21 for rice, 0.17 for wheat, and 0.26 for maize, respectively. Similar, Liu et al. (2021) demonstrated maize yield ranges from 8.7 to 19.1 Mg ha-1 among counties in China. Ignoring these differences may overestimate or underestimate the gases emissions and may not clearly demonstrate the spatial distribution, consequently influencing the policy making for mitigation measures. On the other hand, most studies have focused on atmospheric pollutants emission or GHGs emission, while little research has simultaneously assessed the atmospheric pollutants emission and GHGs emission, especially for N2O and CH4. Previous studies have proven crop straw open burning is a significant source of GHGs (Shi and Yamaguchi 2014; Yadav et al. 2017; Wang et al. 2018). N2O and CH4 are characterized by high global warming potential, which significantly affect the climate change. Few existing studies have assessed the detail emissions and spatial pattern information. Third, emission factor (EF) is another factor influencing gases emission estimation. EF used in most previous studies is obtained according to early experiment results (Li et al. 2016). Zhang et al. (2019) found that the update EF varied considerably with early EF, consequently influencing the emission estimation. In addition to the limiting factors aforementioned, crop straw open burning calls for widely attention in China in recent years, and many incentive and mandatory polices such as “Opinions on Accelerating the Comprehensive Utilization of Agricultural Straw” in 2008 and “Notice on Comprehensive Utilization of Agricultural Straw” in 2019 have been released by the General Office of the State Council of the People’s Republic of China to promote straw returning to field or other uses including fodder, stroma, energy and fuel, and feed stock. Nevertheless, the portion of crop residue burned on field remains high, with 10–50% in most regions of China, and has even increased in some regions of China. To our knowledge, there is no study so far to assess these policies impacts on GHGs and atmospheric pollutants emissions. In this context, explicitly quantitating GHGs and atmospheric pollutants emission from crop straw open burning is crucial for improving the accuracy of emission inventory, and then better simulations of gases concentrations and their climate and air quality influences in models.

In this study, we present a comprehensive and high-resolution estimations of both GHGs and atmospheric pollutants emission from crop straw open burning in China. Because of the wheat, maize, and rice accounted for about 72% of total straw amount (Zhuang et al. 2020) and contributed much more than 85% of gases the emissions (Zhang et al. 2019), we choose these three types of crops research object. The GHGs include CO2, N2O, and CH4, while atmospheric pollutants contain PM10, PM2.5, NMVOC, NH3, NOx, SO2, CO, BC, and OC. The county-scale crop yield is obtained based on a national survey, covering 1815 counties of China. The EFs, grain to straw ratio, collection coefficient, open straw combustion ratio, and combustion efficiency adopted here are sourced from the newly research results shown in method section. Based on these parameters, we calculate the GHGs and atmospheric pollutants emissions. Building on this, we adopt Geographical Information System (GIS) to examine the spatial variation of GHGs and atmospheric pollutants emissions at county scale, identifying the hotspot of these gases emission.

Methods

National farmer survey

Based on a national farmer survey including 6.6 million farmers in 2013 counties organized by the Ministry of Agriculture and Rural Affairs of China during 2005–2014, we obtained the county-level yield for wheat, maize, and rice. The detailed information on survey and database can be seen in Zhang et al. (2020).

Emission estimation

The annual GHGs and pollutant emissions of wheat, maize, and rice are calculated at the county level based on the straw burning amounts and emission factors. The detailed calculation is as follows:

$$ {E}_{i,j,n}={\sum}_{n=1}^n\left({P}_{i,n}\times {R}_n\times {CF}_n\times {C}_i\times {F}_n\times {EF}_{n,j}/1000\right) $$
(1)

Where i, j, and n are county, GHGs and pollutants, and crop types; E is the emission amount (kg); P is the crop yield (kg); R is the straw-grain ratio (%); CF is the collection coefficient (%); C is the combustion ratio (%); F is the combustion efficiency (%); EF is the emission factor (kg/g).

The yield data is shown in Zhang et al. (2020). The straw-grain ratio for wheat, maize, and rice is 1.21, 1.40, and 0.95 (Kang et al. 2020). The combustion ratio is shown in Shi et al. (2017). The collection coefficient for wheat, maize, and rice is seen in Zhuang et al. (2020). The combustion efficiency is 93%, 92%, and 93% (Zhang 2009). The emission factors are shown in Table 1.

Table 1 Emission factors of GHGs and atmospheric pollutants from wheat, maize, and rice straw open burning
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Results and discussion

GHG and atmospheric pollutants emissions from crop straw open burning in China

Table 2 shows that mean emissions of CO2, CH4, and N2O derived from crop straw open burning are 69250.8 Gg, 242.9 Gg, and 4.2 Gg during 2005–2014, respectively, which account for 1.16%, 0.55%, and 0.33% of total CO2, CH4, and N2O emissions. The total GHGs emissions are estimated to reach 159735.6 Gg CO2-eq. Meanwhile, the emissions of PM10, PM2.5, NMVOC, NH3, NOx, SO2, CO, BC, and OC are 771.0 Gg, 539.7 Gg, 498.2 Gg, 34.7 Gg, 200.4 Gg, 24.8 Gg, 3426.5 Gg, 63.0 Gg, and 278.5 Gg, respectively, accounting for 4.64%, 4.42%, 2.11%, 0.35%, 0.69%, 0.09%, 2.01%, 3.57%, and 8.23% of total anthropogenic emissions, respectively (Li et al. 2017). In addition, the contribution of different crop type to GHG and atmospheric pollutants emissions are different. Results showed that maize straw burning was the largest contributor, followed by rice, wheat.

Table 2 Emission of GHGs and atmospheric pollutants from wheat, maize, and rice straw open burning
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Our results are not consistent with other previous studies, such as Li et al. (2016); Qiu et al. (2016), Zhang et al. (2019), and Wang et al. (2018). These disparities could mainly be triggered by prohibiting open straw burning policy that reduce the portion of field open crop residue burned (Peng et al. 2016; Zhou et al. 2017), but could be influenced by a combination of other factors: (1) Emission factors. Previous studies revealed that emission factors significantly affect the estimation results. Emission factors adopted in this study are mainly derived from Qiu et al. (2016), which differed partiality in comparison with Zhang et al. (2019) and Li et al. (2016) (Table S1). For example, the BC EF of maize straw burning adopted by Qiu et al. (2016) is significantly higher than that of Li et al. (2016) and Zhang et al. (2019), which the SO2 EF of rice straw burning is notably lower that of Li et al. (2016) and Zhang et al. (2019). (2) Crop yield. Previous studies adopted mainly the national or provincial yields data rather than the finer level (e.g., county), ignoring the difference may resulted in the uncertainty of estimation to some extent. In this study, we obtained the yield data at the county lever based on the national questionnaire, which at least improved the precision of estimation. (3) Grain to straw ratio. Grain to straw ratio used in this study is not completely consist with other studies (Li et al. 2016; Zhang et al. 2019). Such as, maize grain to straw ratio used in this study is lower than that of Zhang et al. (2019).

The county-level emissions distribution for GHGs and atmospheric pollutants from crop straw open burning in China

Figures 1 and 2 illustrate the county-level emissions distribution for GHGs and atmospheric pollutants. The county-level emissions distribution of total GHGs and atmospheric pollutants from crop straw open burning are basically similar. Hotspots for total GHGs and atmospheric pollutants emissions from crop straw open burning are mainly concentrated in 54 counties of the northeast regions, such as Nonga, Yushu, Gonganling, Zhaodong, and Lishum, which are the largest emission regions (Figs. 1 and 2). These 54 counties together emitted 13962.9 Gg CO2, 49.5 Gg CH4, 0.80 Gg N2O, 164.7 Gg PM10, 115.3 Gg PM2.5, 109.6 Gg NMVOC, 7.6 Gg NH3, 46.3 Gg NOx, 4.9 Gg SO2, 619.7 Gg CO, 16.5 Gg BC, and 73.0 Gg OC, respectively, accounting for 20.2%, 20.4%, 19.2%, 21.4%, 21.4%, 22.0%, 21.8%, 23.1%, 18.1%, 16.5%, and 26.2% of China’s total corresponding GHGs and atmospheric pollutants emissions. Specifically, we also found north China, such as Jing-Jin-Ji regions, where the main areas of cereal production are the additional GHGs and atmospheric pollutants hotspots (Zhang et al. 2019).

Fig. 1
figure 1

County-level emissions distribution of GHGs from wheat, maize, and rice straw burning

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Fig. 2
figure 2

County-level emissions distribution of atmospheric pollutants from wheat, maize, and rice straw burning

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We also analyze the crop-specific emissions distribution for GHGs and atmospheric pollutants at the county level (Figs. 1 and 2). Hotspots for GHGs and atmospheric pollutants emissions from wheat straw burning are located at the north China particularly in the counties of Dengzhou, Hua, Yongcheng, Mengcheng, Cao, and Tanghe, while maize is spatially concentrated in the counties of Nongan, Yushu, Gongzhuling, Zhaodong, Lishu, and Changtu in the north China. By contrast, GHGs and atmospheric pollutants emission hotspots for rice straw burning are mainly emerge in counties in the Middle-Lower Yangtze River Region, such as Poyang, Jianli, Wuchang, Wuchang, Fengcheng, and Fuyuan. Additional emission hotspots are scattered across northeast China, where rice production areas are located, such as in the counties of Fujin, Hulin, Yushu, and Qingan.

Conclusion and implications

This study developed a new high-resolution estimation of GHGs and pollutants emission from cereal crop straw open burning in China based on the national questionnaire in combination with the up-to-data emission factors. Our results show that GHGs and pollutants emission from crop straw open burning are remarkably lower than those previous studies, which are largely attributed to the policy on prohibiting open straw burning and accelerating the comprehensive utilization of straw, county-level yield database, and emission factor selection. High emissions distribution for GHGs and atmospheric pollutants are mainly emerged in the northeast China, and detailed distribution varied with crop types. Emissions in wheat, maize, and rice are respectively distributed in the north China, northeast China, and middle-lower Yangtze River Region. These results in this study in combination with cropland use, MODIS fire counts, and rural population data provide the basic database to redistribute the spatial distribution of GHGs and atmospheric pollutants emissions at the county level, and further can improve simulation results of GHGs and atmospheric pollutants concentrations. Meanwhile, this study also reveals detailed the high emission counties, providing the possible to targeted decrease the straw open burning via policy and other control measures. In addition, proper utilization of these crop straw, for example, returning straw to cropland, has been proven to improve environmental and economic benefits in agroecosystem (Xia et al. 2014; Xia et al. 2018).

Although this study makes a lot of progress on accurately estimation of GHGs and atmospheric pollutants emissions from crop straw open burning, there still exists uncertainty, such as grain to straw ratio, collection coefficient, combustion ratio, and emission factors. For example, the variation of residue to grain ratio adopted for wheat, maize, and rice is 0.17, 0.26, and 0.21, respectively (Kang et al. 2020). Meanwhile, emission factors also show a large variation (Qiu et al. 2016; Zhang et al. 2019). In addition, we do not consider the spatial variation of both grain to straw ratio, collection coefficient, combustion ratio, and emission factors. Therefore, future study should obtain the high-precision grain to straw ratio, collection coefficient, combustion ratio, and emission factors through field observations and experiments, especially at the county level, to reduce the uncertainty.