## 3 Methodology and Data Sources
### Analysis of the Impact of Individual Factors
#### 4.4.1 Impact of Meteorological Conditions on the Receptor City
4.2 Impact of Burning Area, Transport Distance, and Meteorological Conditions along the Transport Pathways  
It can be seen from Equation (6) that the PM\({}_{2.5\text{-inflow}}\) concentration is determined by the conditions of each HWFG on the transport pathway, including multiple factors, such as the SOB burning area in the HWFG, the HWFG area, the PBLH of the HWFGs, and others, and the quantitative relationship between them is iterative and nonlinear. To analyze the impact of the main factors on PM\({}_{2.5\text{-inflow}}\) concentration, taking each HWFG during each PE as a unit, we calculated the burning area of the SOB sources, their transport distance, the average PBLH along the transport pathway, and their resulting corresponding PM\({}_{2.5\text{-inflow}}\) concentrations. These main influence factors are shown in a 4D scatterplot in Figure 12. It should be noted that the area of HWFGs in the model parameters is replaced by the indicator of transport distance in the following analysis because transport distance is easier to measure and carry out in the formulation and implementation of SOB management policies. Note: ASOB, TD, and PBLH in the figure represent the area of the SOB (ha), the transport distance (km), and the PBLH along the transport pathway (km). \(C_{n}\) is the corresponding PM\({}_{2.5\text{-inflow}}\) concentration (\(\upmu\)g m\({}^{-3}\)). From different viewing angles of the 4D scatter plot, shown in Figure 12, we find that the burning area in each HWFG was within the two ranges of 0.005-0.6 ha and 1-2.5 ha during the 4 PEs, the transport distance was \(<\)250 km, and the average PBLH along the transport pathways was \(<\)2.5 km. When the burning area was at a high level of 1-2.5 ha, the PM\({}_{2.5\text{-inflow}}\) concentration reached 3000-6000 \(\upmu\)g m\({}^{-3}\). When the burning area was at a low level of 0.005-0.6 ha, the PM\({}_{2.5\text{-inflow}}\) concentration was in the range of 2-3000 \(\upmu\)g m\({}^{-3}\), and the variation in the PM\({}_{2.5\text{-inflow}}\) concentration was significant, with differences in the burning area, transport distance, and average PBLH along the transport pathway. According to the analysis results in the above section, the PM\({}_{2.5\text{-inflow}}\) concentration \(<\) 1000 \(\upmu\)g m\({}^{-3}\) is one of the \"safety limits\" to ensure urban hourly PM\({}_{2.5}\) concentrations do not exceed the standard. To determine the conditions for making the PM\({}_{2.5\text{-inflow}}\) concentration \(<\) 1000 \(\upmu\)g m\({}^{-3}\), the relationships between the burning area (at a low level of 0.005-0.6 ha) and the PM\({}_{2.5\text{-inflow}}\) concentration were fitted under four combinatorial conditions of transport distance and average PBLH along the transport pathway. Figure 12: Distribution of burning area, transport distance, average PBLH along the transport pathway, and the corresponding PM\({}_{2.5\text{-inflow}}\) concentration in the HWFGs during each PE. It can be seen from the fitting results shown in Figure 13 that under the conditions of shorter transport distance (<150 km) and lower PBLH along the transport pathway (<1.2 km), the burning area limit must be <0.18 ha to make the PM\({}_{2.5\text{-inflow}}\) concentration <1000 \(\upmu\)g m\({}^{-3}\). Second, under the conditions of shorter transport distance (<150 km) but higher PBLH along the transport pathway (1.2-2.5 km), the burning area limit should be <0.24 ha. Third, when the transport distance is longer than 150-250 km but the PBLH along the transport pathway is at a lower level of <1.2 km, the burning area limit is 0.27 ha. Fourth, when the transport distance is longer than 150-250 km and the PBLH along the transport pathway is at a higher level close to 1.2-2.5 km, the burning area limit is 0.35 ha.