## 3 Methodology and Data Sources
### Analysis of the Impact of Individual Factors
#### 4.4.1 Impact of Meteorological Conditions on the Receptor City  
From Equations (11)-(13), it is found that the hourly contribution of PM\({}_{\text{2.5-inflow}}\) is closely related to the PM\({}_{\text{2.5-inflow}}\) concentration and coefficients \(\alpha\) and \(\beta\). Since both \(\alpha\) and \(\beta\) are \(<\)1, the hourly contribution of a certain PM\({}_{\text{2.5-inflow}}\) during the first hour of its residence time is greater than that of each hour thereafter. Taking the urban hourly average PM\({}_{\text{2.5}}\) concentration of 44.39 \(\mu\)g m\({}^{-3}\) during the SOB season of 2020-2021 as the background concentration, we can deduce that when the hourly contribution of a certain PM\({}_{\text{2.5-inflow}}\) during the first hour is \(<\)30 \(\mu\)g m\({}^{-3}\), the urban hourly PM\({}_{\text{2.5}}\) during its residence time will not exceed the standard (under the premise of no other PM\({}_{\text{2.5-inflow}}\)). Therefore, in the following, we discuss the influencing factors of the hourly contribution of PM\({}_{\text{2.5-inflow}}\) during the first hour of residence time. According to Equation (11), since the PM\({}_{\text{2.5}}\) dry deposition rate and calculation time are constant values (\(v_{d}\) = 0.0005 m/s, \(\tau_{1}\) =3600 s), the hourly contribution of a certain PM\({}_{\text{2.5-inflow}}\) during the first hour of its residence time is determined by the PM\({}_{\text{2.5-inflow}}\) concentration and urban PBLH. Therefore, we investigated the relationship between the hourly contribution of PM\({}_{\text{2.5-inflow}}\) during the first hour and the urban PBLH levels (\(H_{rc}(\tau_{1})\)) at different specific PM\({}_{\text{2.5-inflow}}\) concentrations (\(C_{n}\)). As the results show in Figure 11, when urban PBLH is \(>\)600 m, even if the PM\({}_{\text{2.5-inflow}}\) concentration is at an extremely high level of 10,000 \(\mu\)g m\({}^{-3}\), its hourly contribution is still less than 30 \(\mu\)g m\({}^{-3}\). This owes to the strong dilution and diffusion of PM\({}_{\text{2.5}}\) in the vertical direction. Moreover, when the PM\({}_{\text{2.5-inflow}}\) concentration is \(<\)1000 \(\mu\)g m\({}^{-3}\), the hourly contribution of PM\({}_{\text{2.5-inflow}}\) generally does not exceed 30 \(\mu\)g m\({}^{-3}\), even if the urban PBLH is at a lower height of \(<\)600 m. Nevertheless, according to Equations (14) and (15), if continuous PM\({}_{\text{2.5-inflow}}\) within a few hours enters the city sequentially, its cumulative contributions are likely to induce the urban hourly PM\({}_{\text{2.5}}\) to exceed the standard. According to the calculation of residence time, as in Equation (10), for the average diameter of the urban geographic space of Changchun, the residence time of a specific PM\({}_{\text{2.5-inflow}}\) will not exceed 4 h when the average wind speed is \(>\)2 m/s. Based on the above analysis, we can see that urban PBLH mainly affects the hourly contribution of a specific PM\({}_{\text{2.5-inflow}}\), and the average urban wind speed further determines its total contribution by affecting the residence time of PM\({}_{\text{2.5-inflow}}\). In addition, it can be deduced that under the conditions of the average urban wind speed being \(>\)2 m/s and the interval time of each straw burning event being \(>\)4 h, when the urban PBLH is \(>\)600 m  
Figure 11: Relationship between the hourly contribution of PM\({}_{\text{2.5-inflow}}\) and urban PBLH at different specific PM\({}_{\text{2.5-inflow}}\) concentrations. or the PM\({}_{2.5\text{-inflow}}\) concentration is \(<\)1000 \(\upmu\)g m\({}^{-3}\), the SOB sources will not lead the urban hourly PM\({}_{2.5}\) concentration to exceed the standard. Previous studies have also shown that PM\({}_{2.5}\) concentrations during a haze period in the city of Northeast China were mostly influenced by biomass burning emissions and the atmospheric diffusion conditions of the variation in PBLH and wind speed [53].