4.2 Threshold analysis of the proportion of particulate matter emissions in wind erosion
The emission of particulate matter in the natural environment is mainly caused by wind. Under the influence of human activities in farmlands, when the crust is destroyed, particulate matter emission will increase significantly (Hoffmann and Funk, 2015). The change in the proportion of particulate matter emissions in the amount of wind erosion is closely related to the wind speed (Wiggs and Holmes, 2011). Zhou et al. (2020) showed that when the wind speed ranges between 6 m/s and -7 m/s, the dust emission flux can account for 23.60% to 26.58% of the total wind erosion. It reaches the maximum value, when the wind speed exceeds approximately 12 m/s. and the proportion of dust emission flux will be less than 1%. Similar conclusions have been obtained in this study. Table 3 shows that under low wind speed conditions, the proportion of particulate matter emissions is the largest. This is because for low wind speeds it is difficult to transport the larger soil particles in the BSC, but reach the threshold wind velocity for small particles. Therefore, the proportion of particulate matter emissions in this stage is the largest in the total wind erosion. When the wind speed gradually increases, the soil with a larger particle size will be carried by the wind and become part of the total wind erosion. At this point, the soil particles with a large particle size become the main part of the total wind erosion. If more particulate matter is to be emitted, a process of mechanical crushing of large-diameter soil particles is required. However, the soil particles in the BSC are structurally stable and not easily broken under the action of microorganisms. As a result, the proportion of particulate matter emissions decreases as the wind speed increases.
Based on the amount of particulate matter emissions in the total amount of wind erosion, the amount of wind erosion can be effectively used to estimate particulate matter emissions (Alfaro et al., 2004). The correlation between the particulate matter emissions and wind erosion can effectively predict wind erosion and particulate matter emissions events (Saxton et al., 2000). BSCs can effectively inhibit wind erosion. Thus, it is relatively difficult to obtain observational data on the amount of wind erosion of BSCs. However, the concentration of particulate matter emitted from the surface can be measured by instruments, which is relatively easy to obtain. Therefore, it is a rare but feasible method that uses the particulate matter emission of BSCs to calculate wind erosion amount. In this calculation process, the threshold value of the proportion of particulate matter emission in wind erosion is the parameter to be defined.