**1. Introduction**

In our previous article, we discussed the adverse meteorological variables (such as precipitation, wind speed and direction, humidity, inversion, and mixing layer height) that affect air pollution and the surface synoptic situation patterns related to air pollution in eastern China, where the threshold values of meteorological elements are summarized [1]. From the previous article, we found that wind speed, RHs, inversion intensity (ITI), height difference in the temperature inversion (ITK), the lower height of temperature inversion (LHTI), and mixed layer height (MLH) in terms of a 25–75% high-probability range were, respectively, within 0.5–3.6 m s<sup>−</sup>1, 55–92%, 0.7–4.0 ◦C 100 m<sup>−</sup>1, 42–576 m, 3–570 m, and 200–1200 m. The probability of RPHPDs without rain was above 92% with the daily and hourly precipitation of all RPHPDs below 2.1 mm and 0.8 mm [1]. In this article,

**Citation:** Zhou, B.; Liu, D.; Yan, W. A Simple New Method for Calculating Precipitation Scavenging Effect on Particulate Matter: Based on Five-Year Data in Eastern China. *Atmosphere* **2021**, *12*, 759. https:// doi.org/10.3390/atmos12060759

Academic Editors: Mihalis Lazaridis and Dongshen Chen

Received: 26 April 2021 Accepted: 7 June 2021 Published: 11 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

we will discuss the precipitation scavenging effect conducive to air pollution removal. Scavenging of atmospheric aerosols by precipitation is a major removal mechanism for airborne particles [2]. Atmospheric aerosol wet scavenging directly affects the air quality by controlling the aerosol mass concentrations, and temporal and spatial distributions [3,4]. The scavenging of atmospheric aerosols also has a large impact on the chemical composition of rainwater [5,6]. Thus, the understanding and quantification of aerosol scavenging processes are very important for air quality and its improvements.

The wet process can be described by a wet scavenging coefficient [7–10]. The wet scavenging of atmospheric pollutants includes in-cloud scavenging processes [11–13] and below-cloud scavenging processes [14–27]. Below-cloud atmospheric particles are removed by raindrops via Brownian diffusion, interception, and impaction [28]. Bae et al. [16] noted that the collection efficiency, terminal velocity of raindrops, raindrop size distribution, and particle size distribution are important factors affecting below-cloud scavenging. In the later period of rainstorms, high concentrations of aerosols improved the precipitation efficiency significantly, resulting in more centralized clusters of intense precipitation [29].

Tai et al. [30] reported that precipitation is strongly negatively correlated with all PM2.5 components. The collection efficiency diameter is a function of both terminal velocity and collection mechanisms. When considering Brownian diffusion and interception, the most penetrating particle sizes increase as the drop diameter increases, which shows that the most penetrating particle sizes depend on the collection efficiency mechanism, vertical velocity, and collector diameter [28]. Chate et al. [20] found that the predicted rainwater concentration for a relative humidity (RH) of 50% is about two times larger than that for an RH of 95% in the case of hygroscopic particles. Using field observations and modeling, McLachlan and Sellström [31] found that the differences between ground-level and in-cloud temperatures should be considered when calculating the scavenging ratio.

The air pollution in the eastern part of China is quite serious and has become a serious environmental problem [32]; therefore, natural clearance (dry deposition and wet deposition) is very important, and especially precipitation scavenging is most important. Therefore, the understanding and quantification of aerosol scavenging processes are of extreme importance due to their impact on the physical and chemical characteristics of aerosols as well as precipitations [3].

In most parts of China, raindrop and aerosol particle spectra are not widely observed. However, atmospheric aerosol mass concentrations are widely observed. These common observation data of aerosol mass concentrations are helpful for us to analyze the clearance effect of precipitation on aerosols. In addition, scavenging schemes used by various aerosol transport models follow the theoretical estimation of scavenging and have become a high source of uncertainty for such models [33]. Therefore, it has become necessary to study precipitation scavenging in a more simple and quantitative way with a higher number of samples to analyze the dataset with high statistical significance. This would, in turn, reduce the uncertainties associated with the various chemical transport models used to study precipitation scavenging.

Thus, the present study is an attempt (1) to establish a "rain-only" method on particle aerosol removal from the atmosphere which is not only unique and novel but also very simple, (2) to investigate how aerosol scavenging depends on the precipitation intensity, precipitation duration, particle mass concentrations, and precipitation volumes, (3) to determine the threshold values of the precipitation intensity and duration below and above which aerosol scavenging behaves differently, and (4) to establish whether air pollution can be quantitatively predicted if one holds only the information of the precipitation intensity, precipitation duration, particle mass concentrations, and precipitation volumes for a given pollution level. 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.

We examine aerosol scavenging by precipitation in eastern China. The remainder of this paper is organized as follows: The study area, observations, and analysis methods used

are described in Section 2. We analyze precipitation scavenging on aerosols in Section 3. The conclusions are given in Section 4.

## **2. Study Area and Methodology**
