Effects of Precipitation on the Air Quality Index, PM 2.5 Levels and on the Dry Deposition of PCDD/Fs in the Ambient Air

The effects of 9 precipitation events in Suzhou City in Anhui Province, China, on the air quality index (AQI), PM 2.5 , and dry deposition flux of PCDD/Fs (polydibenzo-p -dioxins and polydibenzofurans) were investigated. A total of 7 precipitation events were positive contributes to the reduction of AQI; among them, the AQI were between 23 and 216, with an average of 75, the PM 2.5 concentrations were between 5.0 and 169 µ g m –3 , with an average of 25 µ g m –3 , while the total-PCDD/F-TEQ dry deposition flux ranged from 149 to 1034 pg WHO 2005 -TEQ m –2 day –1 and averaged 315 pg WHO 2005 -TEQ m –2 day –1 . By comparing the average AQI and PM 2.5 , respectively, during and after rainfall with that before rainfall, the results indicated that the average reduction fractions of AQI were 26% and 44%, respectively, while those of PM 2.5 were 58% and 43%. In addition, the effect of precipitation on the average reduction fraction of total PCDD/F-TEQ dry deposition flux was 31%. However, in the other 2 AQI elevation events, the AQI were between 23 and 100, and averaged 51; when comparing the average AQI and PM 2.5 concentrations, during and after the rain with that before the rain, the increases in AQI were 42% and 49%, respectively, while the increases in PM 2.5 concentration were 26% and 29%, respectively. The above results show that, on the whole, rain and snow improved the air quality. This is because rainwater removes particles or dissolved gaseous pollutants from the atmosphere and brings aerosols to the ground. However, in some cases, the increase of source emissions and atmospheric vertical convection, the effect of precipitation or air humidity increased the AQI and elevated the concentration of PM 2.5 , and dry deposition flux of PCDD/Fs. The results of this study provide useful information for both scientific communities and air quality management.


INTRODUCTION
Around the world, the combination of increasing population density and rapid industrialization has led to a serious deterioration in the degree of air pollution (Cao et al., 2009;Jia et al., 2020), especially urban air pollution, including particulate suspended matter (PM), sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3), carbon monoxide (CO), and other pollutants.The air pollution in these cities is so bad that it is reducing visibility, posing a great threat to public health, and undermining cities' sustainability (Hu et al., 2015;Shah and Patel, 2021;Zhang et al., 2022).
According to previous studies, precipitation can effectively improve air quality because rainwater can take away PM2.5 and other pollutants in the air and carry away some of the particulate matter to some extent, reducing the concentration of PCDD/Fs, and thus purifying the air and improving air quality (Tian et al., 2021a;Yu et al., 2021).
Air pollution releases pollutants due to human development and other natural or anthropogenic activities into the atmosphere and causes harm to humans, other organisms, and the natural environment.Therefore, to provide data for ambient air quality management, and protect public health (Wang et al., 2018;Suman, 2021), a class of air quality indices that are easy to read and easy to understand by citizens and policymakers have been presented (Bruno and Cocchi, 2002).The Air Quality Index (AQI) is an index that quantitatively describes air quality and is used to report the hourly or daily air quality of a region (Kyrkilis et al., 2007).
In recent years, the air quality of Qingdao, Jinan, Chengdu, and other cities in China was found to be much better in 2020 than in other years (Wan et al., 2020;Xu et al., 2020).This was because the epidemic control actions greatly reduced the air pollutant emissions from both stationary and mobile sources (Zhang et al., 2020;Yu et al., 2021).
Atmospheric fine particulate matter (PM2.5) are particles less than or equal to 2.5 µm in size and they can enter the lungs through the upper respiratory tract (DeCarlo et al., 2004;Nguyen et al., 2022;Pan et al., 2022).The sources of PM2.5 are divided into natural and anthropogenic sources.The natural sources include hurricanes, forest fires, volcanic eruptions, weathering of soil and rocks, tsunamis, and biological decay, while anthropogenic sources include vehicle exhaust and industrial emissions such as those from coal power plants and industrial production (Querol et al., 2001;Mutuku et al., 2021;Wu et al., 2021).
Dioxin (PCDD/Fs) is a group of toxic pollutants, which mainly comes from hazardous waste, medical waste, municipal solid waste, and medical waste incineration.In addition, it can also be produced in a wide range of activities, such as bleaching paper with chlorine (She et al., 2017;Lin et al., 2022).PCDD/Fs can be produced in pesticide manufacturing, automobile exhaust emissions, commercial detergents, petroleum refining, metallurgy, and other activities (Schuhmacher et al., 2000;Prange et al., 2002).The PCDDs, PCDFs, and PCBs constitute a group of persistent pollutants.These compounds have a certain structural correlation and a common mechanism of action (Van den Berg et al., 1998), because PCDD/Fs are lipophilic compounds that are difficult to degrade in nature.In addition, if humans are exposed to high concentrations of PCDD/Fs or consume contaminated food for a long time, it is likely to lead to impaired internal organ function, reduced perception, skin surface damage, and growth retardation (Chen et al., 2006;Hu et al., 2009;Ssebugere et al., 2019;Ngo et al., 2020).
POPs are mainly removed by sedimentation in the atmosphere, a process in which air pollutants settle in soil or water in a specific way, which is mainly divided into dry deposition and wet deposition (Brzuzy and Hites, 1996;Huang et al., 2011;Jeong et al., 2016).Wet deposition refers to the process of removing both particle and gaseous air pollutants in the atmosphere using snow and rainfall (Moon et al., 2005;Melymuk et al., 2011).Among them, in the process of rainfall, atmospheric temperature, humidity, rainfall, particle size and initial concentration of particles affect the process of wet deposition (Chang et al., 2004;Zhou et al., 2020).Dry deposition is the process with no rain in which particles and gases suspended in the atmosphere fall at their final speed.In addition, temperature, wind speed, humidity, and particle size affect the dry deposition flux of air pollutants (Mi et al., 2012;Suryani R et al., 2015).
In this study, a total of 9 precipitation events were selected and studied in Suzhou City, and they were divided into 7 AQI reduction events and 2 AQI increase events.The AQI and concentrations of PM2.5 before, during, and after precipitation are presented and discussed.Furthermore, the impacts of precipitation on total PCDD/Fs-WHO2005-TEQ dry deposition flux were investigated.

METHODS
Suzhou (33°38′N, 116°58′E) (north of the Huaihe River) is located in Anhui Province, China, in the north of Anhui Province and in the northeast of Huaibei Plain, which belongs to a warm temperate zone and semi-humid monsoon climate.The average daily temperatures were between -7°C and 31°C in 2020 and 2021 were between -7°C and 32°C and the annual average temperatures were 15. 7°C and 15.8°C in 2020 and 2021, respectively.This study discusses the AQI and PM2.5 concentrations before, during, and after precipitation (http://www.aqistudy.cn/).

PCDD/F Concentration
The overall concentration of PCDD/Fs was simulated using a regression analysis of PM10 concentration.The PM10 value and the total mass concentration of PCDD/Fs have a high association (Lee et al., 2016).The following two linear equations can express their relationship: Y1, Y2: the total concentration of PCDD/F (pg m -3 ).
In this study, the dry deposition flux of PCDD/Fs was calculated using the technique provided by Liu's study (Liu et al., 2022), and the necessary parameters were acquired using methods used in previous research (Shih et al., 2006).

Air Quality Index (AQI)
The air quality index (AQI) is a quantitative representation of air quality data that may be used to reflect a city's short-term air quality state and changes (She et al., 2017;Shen et al., 2017).
The level of air quality affects people's life and health.The air quality index is obtained from the 24-hour average concentration of PM2.5, PM10, CO, NO2, and SO2, and the daily average 8-hour maximum O3 concentration.The U.S. Environmental Protection Agency (U.S. EPA) has established six AQI levels: Class I: 0-50, Good, Green.

Effects of Precipitation on the Reductions of AQI
In Suzhou, selecting 9 rainfall events of the same type, it can be found that the air quality index of 7 rainfall events decreased and the air quality index of 2 rainfall events increased, leading to deterioration of air quality.The dates, duration of precipitation, and amounts of precipitation are shown in Table 1.

AQI analysis
As shown in Fig. 1, among the top 9 precipitation events in Suzhou, 7 events showed the reduction of AQI (Table 1).The AQI before the rain ) ranged between 42 and 216 and averaged 95; while during the rain, AQI ranged from 25 to 133 and averaged 51 ).In addition, after the rain, AQI ranged from 39 to 118 and averaged 73 ).Furthermore, it can be seen that during the 7 events, the main pollutant was O3.The reason for this might be that the temperature was higher and the wind speed was slower in summer, so the after the rain 90 O3 concentration was higher.In addition, when comparing the AQI before and after the rain, the decrease in AQI during the rain ranged from 18% to 57%, with an average AQI of 44% lower than before the rain.When the AQI after the rain was compared to the AQI before the rain, the AQI reduction after the rain ranged from 2.4% to 54% and averaged 26% lower than that before the rain.As a result, rainwater would cling to the particles in the air and fall to the ground with them, reducing the concentration of particulate pollutants in the atmosphere significantly.Furthermore, the wind affected the horizontal dispersion of pollutants.When it rained, the wind speed increased  or decreased and swung irregularly up and down, and left and right, causing the rest of the particles to be mixed, which was conducive to the dispersion, dilution, and diffusion of contaminants.The above phenomenon proved that rain did indeed improve air quality to a certain extent, and the removal effect of particulate matter in the air was particularly significant.

PM2.5 concentration
As shown in Fig. 2, in the 7 events in which AQI decreased, before the rain, PM2.5 concentrations ranged from 11 to 169 µg m -3 and averaged 40 µg m -3 .During the rain, the PM2.5 concentrations ranged between 5.0 and 91 µg m -3 and its average concentration was 17 µg m -3 , while after the rain, the PM2.5 concentrations were between 9.0 and 50 µg m -3 and averaged 23 µg m -3 .Comparing the PM2.5 concentrations during the rain with those before the rain, the PM2.5 concentration reductions during the rain ranged from 13% to 67% and averaged 58% less than that before the rain, when comparing after the rain with before the rain, respectively, the PM2.5 concentration reductions after the rain were -17% to 58% and was 43% lower than that before the rain on average.The above phenomenon might have occurred because precipitation was mainly concentrated in summer, the atmospheric dispersion was better in summer, and the concentration of particulate matter itself was low.However, excessive precipitation in summer led to an increase in humidity.
There was a certain threshold of air humidity.Under this threshold, the concentration of particulate matter increased with the increase of humidity, and once this threshold was exceeded, the concentration of particulate matter decreased with the increase of humidity.Therefore, PM2.5 might rise after rain.and ranged from 149 to 470 pg WHO2005-TEQ m -2 day -1 , after the rain, averaging 257 pg WHO2005-TEQ m -2 day -1 .When the dry deposition flux of total PCDD/Fs-WHO2005-TEQ after the rain was compared to those before the rain, the reductions after the rain were between -2.0% and 55% and were 31% lower than those before the rain on average.
Overall, the rate of POPs removal by rainfall depends on human pollutants as well as ambient temperature.When the temperature is below 0°C, rainfall can effectively remove more organic vapors, thereby reducing the dry deposition flux of PCDD/Fs.

AQI analysis
From 2020 to 2021, this study selected the 9 precipitation events with the largest rainfall in Suzhou, where the AQI of 2 events (Fig. 4) showed an increasing trend (Table 1).
As shown in Fig. 4, for the 2 rainfall events in Suzhou with elevated AQI, the AQI values before the rain ranged from 29 to 80 with an average of 56, while the AQI during the rain ranged from 27 to 56 and averaged 43, and the AQI after the rain ranged from 62 to 102, with an average of 85.Furthermore, by comparing the AQI values before and during the rain, we can conclude that the AQI during the rain were 38% and 45%, respectively, and were 42% lower than those before the rain on average, while the AQI after the rain were 28% and 69%, respectively, and were 49% higher than those before the rain on average.

PM2.5 concentration
As shown in Fig. 5, in Suzhou, during the two AQI elevation events, PM2.5 concentrations before the rain ranged from 32 to 38 µg m -3 with an average of 34 µg m -3 .During the rain, PM2.5 concentrations ranged from 7.0 to 46 µg m -3 , with an average of 25 µg m -3 ; after the rain, PM2.5 concentrations were between 25 and 65 µg m -3 , with an average of 44 µg m -3 .The PM2.5 concentrations during the rain were between -13% and 62% and were 26% higher on average than those before the rain, while after the rain compared with before the rain, the PM2.5 concentrations after the rain were between -21% and 62%, with a 29% higher average than those before the rain.This was because the heavy rain in summer effectively washed out the particulate pollutants in the air.When the rainfall lasted for a long time and the amount was large,  the particulate matter in the air was washed away, and the weather was fine after the rain.After that, the PM2.5 concentrations declined.However, if the rainfall was not large enough, or the air humidity was high, the PM2.5 concentrations were higher.
3.2.3Dry deposition flux for the total-PCDD/Fs-WHO2005-TEQ According to Fig. 6, the two events in which the AQI value increased and the dry deposition fluxes in the total PCDD/Fs-WHO2005-TEQ before the rain were between 188 and 219 pg WHO2005-TEQ m -2 day -1 with an average of 204 WHO2005-TEQ pg m -2 day -1 ; after the rain were between 196 and 371 pg WHO2005-TEQ m -2 day -1 and averaged 284 pg WHO2005-TEQ m -2 day -1 .the increases after the rain ranged between 4.0% and 69% and were 39% higher than those before the rain on average when comparing the dry deposition flux in the total PCDD/Fs-WHO2005-TEQ after the rain with that before the rain.The above results revealed that when the AQI elevated, normally, the dry deposition of total PCDD/Fs-WHO2005-TEQ increased as well.

CONCLUSION
This study investigated the effects of 9 precipitation events on the AQI, PM2.5, and wet deposition of total PCDD/Fs-WHO2005-TEQ.The major results are summarized as follows.1.In Suzhou, for the 7 precipitation events, the AQI ranged from 23 to 216 and averaged 75, Comparing AQI during and after the precipitation, respectively, with that of before the precipitation, the reduction fractions in the AQI were 26% and 44%, respectively, and in two cases where the AQI index elevated, the AQI were between 23 and 100, with an average of 51.
As a whole, comparing AQI during and after the precipitation, respectively, with that before the precipitation, the increased fractions of AQI were 42% and 49%. 2. For the 7 precipitation events, the PM2.5 concentration was between 5 and 169 µg m -3 , with an average of 25 µg m -3 and the reductions in the PM2.5 concentrations during and after the precipitation were 58% and 43%, respectively.In the other 2 cases, the PM2.5 concentrations were from 4.0 to 29 µg m -3 , with an average of 14 µg m -3 .The PM2.5 concentrations during and after the rain were 26% and 29% higher than those before the precipitation, respectively.3.For the 7 precipitation events demonstrating a reduction in the AQI, the dry deposition flux of total PCDD/Fs-WHO2005-TEQ in Suzhou from 2020 to 2021 were between 149 and 1034 pg WHO2005-TEQ m -2 day -1 , with an average of 315 pg WHO2005-TEQ m -2 day -1 .When the values after the precipitation were compared to those before the precipitation, the dry deposition flux was reduced by 31%.The dry deposition flux in the total PCDD/Fs-WHO2005-TEQ were between 188.5 and 371.2 pg WHO2005-TEQ m -2 day -1 and averaged 244 pg WHO2005-TEQ m -2 day -1 in the other 2 precipitation events.Comparing the values after the precipitation with those before the precipitation, the increase in the PCDD/Fs dry deposition flux was 39%. 4. The results show that, in general, rainfall can improve air quality because particulate matter or dissolved gaseous pollutants are removed and aerosols are brought to the ground.This study selected the top 9 events of rainfall intensity in Suzhou, Anhui Province, and found that when the rainfall intensity is high, it can indeed reduce AQI and reduce the air pollutant concentration in the ambient air.However, in some cases, the increase of source emissions or the decrease of atmospheric vertical convection leads to the increase of AQI, PM2.5, and PCDD/Fs dry deposition fluxes in ambient air.

Fig. 1 .
Fig. 1.The proportions of the seven AQI categories for Suzhou before the rain, during the rain, and after the rain for 2020-2021 for AQI reduction events.

Fig. 2 .
Fig. 2. variations in the PM2.5 concentrations in Suzhou (a-c, e-h) before the precipitation, during the precipitation, and after the precipitation from 2020-2021 when the AQI was reduced.

Fig. 4 .Fig. 5 .
Fig. 4. The percentages of the two AQI categories for Suzhou before, during, and after precipitation in 2020-2021 for the elevated AQI events.

Table 1 .
The AQI values and precipitation intensity in Suzhou from 2020 to 2021.