*3.4. Change in Particle Mass Concentration after Rain*

The concentration of particulate matter in the atmosphere depends on the balance between emissions and atmosphere self-cleaning. When the emission source is not changed and the weather system is stable, the particle mass concentration should be around the equilibrium state. In the absence of external transport, the PM concentration in the atmosphere depends on environmental emissions and dry or wet deposition. In a relatively short period of time, it can be considered that environmental emissions before and after precipitation do not change much; therefore, the impact of precipitation on the particle concentration can be analyzed. When the effects of dry deposition and environmental emissions cancel out, the concentration of particulate matter stabilizes, which is the equilibrium state. Then, how does the particle mass concentration approach the equilibrium state after a precipitation process?

The change in the particle mass concentration within 168 h after precipitation ends was analyzed using 6882 processes. The results are shown in Figure 7. The average particle mass concentration is low at the end of precipitation, being about 50 μg m−<sup>3</sup> for PM2.5 and 70 μg m−<sup>3</sup> for PM10. The particle mass concentration increases gradually after the end of precipitation. The average concentrations of PM2.5 and PM10 168 h after precipitation are more than 65 and 115 μg m<sup>−</sup>3, respectively. The growth of the particle mass concentration after precipitation was divided into two stages: 0–24 h after the end of precipitation is the rapid growth stage, and 24 h after the end of precipitation is the slow growth stage. The concentrations of PM2.5 and PM10 increase at 0.46 and 1.35 μg m−<sup>3</sup> per hour during the rapid growth phase, while they increase at 0.07 and 0.51 μg m<sup>−</sup>3, respectively, in the slow growth stage.

**Figure 7.** Changes in particle concentrations and wind speeds with time after precipitation ends.

In this section, the reason for the growth rate of the particle mass concentration within 24 h after precipitation being greater than that after 24 h is discussed.

The factors that influence air pollution include internal factors (emission sources) and external factors (such as precipitation, wind speed and direction, humidity, inversion, and mixing layer height [36]) and were discussed in the previous article [1], showing that

the threshold value is one of the criteria of pollution intensity. Analysis of the average wind speed within 168 h after the rain starts shows that the wind speed within 24 h after precipitation is greater than that after 24 h; the high wind is not conducive to the increase in the particle mass concentration. Meanwhile, emission sources are usually stable and can be considered as constant during the precipitation process. For a period of time after the end of precipitation, if the emission source is regarded as constant, the PM concentration change depends on the dry deposition and environmental emissions. When the concentration of particulate matter is high, the dry deposition effect is strong, exceeding the environmental emission, and the PM concentration decreases with time. When the PM concentration is low, the dry deposition effect is lower than that of environmental emissions, and the PM concentration increases with time. When the effects of dry deposition and environmental emissions cancel out, the concentration of particulate matter stabilizes, which is the equilibrium state. When the actual particle concentration is lower than the equilibrium concentration, dry deposition caused by the effect of the particle concentration decreases below the environmental emissions, which could lead to an increase in the particle concentration effect, where the PM concentration will increase to approach the equilibrium concentration. When there is a greater difference between the actual concentration and the equilibrium concentration, dry deposition caused by the effect of the PM concentration decreases below the environmental emissions, which could lead to an increase in the particle concentration effect, causing a greater particle concentration change over time. When the particle mass concentration approaches the equilibrium state, the closer it is to the equilibrium point, the more slowly it moves toward the equilibrium point. The particle mass concentration is far from the equilibrium point at the end of precipitation; therefore, the growth rate is relatively large. In the case of no rain within 168 h after the previous precipitation process, the weather is often sunny, and the particle mass concentration approaches the equilibrium state; therefore, the particle growth rate is slowed down significantly. Here, we choose the cases where there were more than 10 consecutive days without precipitation after the studied precipitation process. Additionally, we analyze particle concentration changes after the precipitation, in order to determine the above equilibrium concentration. According to the results of the 10-day or longer continuous observation, the arithmetic average concentrations of PM2.5 and PM10 are finally stabilized at about 80 and 120 μg m<sup>−</sup>3, which can be considered as the equilibrium points for PM2.5 and PM10.

#### **4. Conclusions**

Particle air pollution scavenging was jointly affected by the wind diffusion effect and precipitation scavenging effect. Precipitation is the most important factor in the balance of air pollution in ecosystems.

Deducing the threshold values of precipitation scavenging that were conducive to the pollution accumulation was very necessary to achieve better control of air pollution. This study provides a simple and quantitative way to establish a "rain-only" method on particle aerosol removal from the atmosphere. Such a simple methodology can be easily adapted to predict aerosol particle scavenging over any region across the world irrespective of the topographical, orographical, and climatic features. The threshold values of the precipitation intensity and duration below and above which aerosol scavenging behaves differently were developed.

A higher concentration, larger *RI*, and larger particle size lead to a higher *SE*. The greater the *RI*, the higher the *SE*, meaning the precipitation *SE* on PM10 is better than that on PM2.5. *RI* = 8.0 mm h−<sup>1</sup> has the best *SE* on PM2.5, and *RI* = 11.3 mm h−<sup>1</sup> has the best *SE* on PM10 when the total precipitation is fixed. The *SR* increases faster when accumulative precipitation is below 15 mm and more slowly when accumulative precipitation is above 15 mm. When accumulative precipitation is above 50 mm, the precipitation *SR*s of PM2.5 and PM10 are about 50% and 60%, respectively. The *SR* of coastal areas is less than that of inland Jiangsu. In the future, if the regional PM2.5 concentrations continue to decrease, the threshold values would remain applicable.

The growth of the particle mass concentration after precipitation was divided into two stages: the slow growth stage about 24 h after the end of precipitation, and the rapid growth stage 24 h after the end of precipitation. The concentrations of PM2.5 and PM10 increase at 0.46 and 1.35 μg m−<sup>3</sup> per hour, respectively, during the rapid growth phase, while they increase at 0.07 and 0.51 μg m<sup>−</sup>3, respectively, in the slow growth stage.

The methods in this study just studied the "rain-only" effect on particle aerosol removal from the atmosphere, and the influence of wind was not discussed. The present long-term and large datasets are able to quantitatively predict aerosol scavenging at any part if only the rain rate and duration are available.

**Author Contributions:** Conceptualization, D.L.; validation, D.L.; formal analysis, B.Z. and W.Y.; project administration, D.L.; resources, D.L.; writing—original draft preparation, B.Z.; writing review and editing, D.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was jointly supported by the National Key Project of MOST (2016YFC0203303, the open fund by the Key Laboratory for Aerosol–Cloud–Precipitation of CMA–NUIST in China (KDW1801), the Open fund by Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (KHK2005), and the Jiangsu Meteorological Bureau General project (KZ201902).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data are availability from http://106.37.208.233:20035/ (accessed on 10 June 2021).

**Acknowledgments:** Thanks to the Chinese Ministry of Environmental Protection for providing the environmental monitoring data.

**Conflicts of Interest:** The authors declare no conflict of interest.
