**1. Introduction**

Fine particulate matter smaller than 2.5 μm (PM2.5) is already a serious and growing environmental problem in China. The smog situation in China is getting even worse. Extremely high levels (several tens of times higher than the concentration of WHO's recommendation (<25 μg m<sup>−</sup><sup>3</sup> of 24-hour exposure) can last more than a week.

Now, the damage caused by PM2.5 is not just an issue for China. It has been proven that the worsening of air pollution in China affects Korea and Japan as well, with the fine particles moving with the winds. Especially, air quality in South Korea has become an increasing source of concern for the nation. According to the survey conducted by the Seoul Development Institute [1], 52% of the residents in Seoul

consider air pollution to be the most imminent environmental issue, and 68% of them consider that the level of air pollution is serious.

In the case of Japan, in recent years, high levels of PM2.5 were recorded in parts of western Japan, especially Fukuoka Prefecture [2]. Under the growing concern to this pollution crisis, many regional organizations in Japan are planning various measures like providing the real-time hourly PM2.5 data on website.

This fiendish PM2.5 are removed from the atmosphere by wet and/or dry depositions, the former proceeds more efficiently in the form of precipitation such as rain, fog, and snow [3–5]. Most people have probably experienced a clear view of the sky and landscapes after rainfall. This is a good example to understand that the rain is a great cleaner of PM2.5. Therefore, in the assessment of public health risks associated with PM2.5, a study of the chemical nature of the rain have fallen during high atmospheric loading for PM2.5 is no less important than that of the ambient PM2.5 itself.

One of major mechanisms of the incorporation of ambient particles into raindrops is the collision among the particles below the cloud base. The efficiency of the collision depends on the size distributions of particles and the raindrops [3, 5]. Therefore, the exposure amount of pollutants by rain is variable depend on rainfall properties (e.g., rainfall amount, rainfall duration, rainfall intensity, and raindrop size distribution). More than all, as mentioned above, raindrop size distribution is crucially important because it plays an important role in capturing pollutants.

The collection of raindrops as a function of their size has not been generalized because it is technically difficult to capture and a high degree of skill in handling of raindrops is demanded [6, 7].

This study has been carried out to improve our understanding of particle scavenging properties of size-resolved raindrops from the chemical properties of the residuals in individual raindrops.

## **2. Experimental methods**

#### **2.1 Collection and handling of size-resolved raindrops**

For the sampling of single raindrops as a function of their size, the raindrop collector devised by our oneself in previous research [8] was applied. It was located on the rooftop of a four-story building (a height of 20 m above ground level) (33.40 N 130.26 E) at Fukuoka Women's University in Fukuoka City, Japan at a time when it was raining. The rain event lasted 14 h, with rain intensity of 0.2–1.2 mm h<sup>−</sup><sup>1</sup> , and air temperature between 9.6 and 16.4°C. The concentration of PM2.5 in that time was higher than the Japanese central government's safety standard for PM2.5 (i.e., a mean of 35 μg m<sup>−</sup><sup>3</sup> over a 24-hour period).

Although more details can be found in our previous papers [6, 8, 9], the principle of our own raindrop capture device can be summarized as follows:

Fallen raindrops into the liquid nitrogen are frozen and they sink to lower sieves owing to their higher density as illustrated at the top in **Figure 1**. As shown in **Figure 2**, the raindrops kept their spherical shape during the freezing process, and consequently by using stainless steel sieves of different mesh widths (1.7 mm, 0.17 mm, and back-up) it is possible to separate the frozen raindrops according to their sizes.

Our previous study [8] has been clearly presented that there was no meaningful size change of raindrop when it was freezing. In other words, it can be said that diameter change of frozen raindrops by liquid nitrogen did not affect the size segregation of our raindrop collector.

**97**

*The Chemical Nature of Individual Size-Resolved Raindrops and Their Residual Particles…*

After sampling, the sieves were pulled out from the dewar vacuum flask and each frozen raindrop on each sieve was placed onto the non-hole Nucleopore® filter and Ag thin film (99.99% purity) by using a vacuum pipette (HAKO 392). The frozen raindrops were melted and dried under an infrared lamp for 5 min. Because every process was performed in the clean air system filled with the cooling nitrogen gas, the raindrop handling could be done successfully without any loss of some

*Flow of collection, pretreatment, and analysis of individual size-resolved raindrops and their residual particles.*

Moreover, in order to measure the rainfall intensity, the standard rain gauge (260-2510, NovaLynx Co.) consisting of a funnel that empties into a graduated

Elemental analyses of solid residues and individual residual particles in raindrops were subsequently analyzed by particle induced X-ray emission (PIXE) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX), respectively. The PIXE installed at the Cyclotron Research Center of Iwate Medical University was applied and it has the great advantages such as an excellent sensitivity, a nondestructive technique for multielement with a wide range of elements (Z > 10). The sensitivity, if defined by the ratio between PIXE yield per unit dose and mass thickness, can be determined for all objective elements both

*DOI: http://dx.doi.org/10.5772/intechopen.84227*

residues, evaporation, and contamination.

**2.2 Elemental analyses of residues in raindrops**

cylinder was applied.

**Figure 1.**

*The Chemical Nature of Individual Size-Resolved Raindrops and Their Residual Particles… DOI: http://dx.doi.org/10.5772/intechopen.84227*

**Figure 1.**

*Rainfall - Extremes, Distribution and Properties*

them consider that the level of air pollution is serious.

measures like providing the real-time hourly PM2.5 data on website.

consider air pollution to be the most imminent environmental issue, and 68% of

In the case of Japan, in recent years, high levels of PM2.5 were recorded in parts of western Japan, especially Fukuoka Prefecture [2]. Under the growing concern to this pollution crisis, many regional organizations in Japan are planning various

This fiendish PM2.5 are removed from the atmosphere by wet and/or dry depositions, the former proceeds more efficiently in the form of precipitation such as rain, fog, and snow [3–5]. Most people have probably experienced a clear view of the sky and landscapes after rainfall. This is a good example to understand that the rain is a great cleaner of PM2.5. Therefore, in the assessment of public health risks associated with PM2.5, a study of the chemical nature of the rain have fallen during high atmospheric loading for PM2.5 is no less important than that of the ambient PM2.5 itself. One of major mechanisms of the incorporation of ambient particles into raindrops is the collision among the particles below the cloud base. The efficiency of the collision depends on the size distributions of particles and the raindrops [3, 5]. Therefore, the exposure amount of pollutants by rain is variable depend on rainfall properties (e.g., rainfall amount, rainfall duration, rainfall intensity, and raindrop size distribution). More than all, as mentioned above, raindrop size distribution is crucially important because it plays an important role in capturing

The collection of raindrops as a function of their size has not been generalized because it is technically difficult to capture and a high degree of skill in handling of

For the sampling of single raindrops as a function of their size, the raindrop collector devised by our oneself in previous research [8] was applied. It was located on the rooftop of a four-story building (a height of 20 m above ground level) (33.40 N 130.26 E) at Fukuoka Women's University in Fukuoka City, Japan at a time when it was raining. The rain event lasted 14 h, with rain intensity of 0.2–1.2 mm h<sup>−</sup><sup>1</sup>

air temperature between 9.6 and 16.4°C. The concentration of PM2.5 in that time was higher than the Japanese central government's safety standard for PM2.5 (i.e., a mean

Although more details can be found in our previous papers [6, 8, 9], the prin-

Our previous study [8] has been clearly presented that there was no meaningful size change of raindrop when it was freezing. In other words, it can be said that diameter change of frozen raindrops by liquid nitrogen did not affect the size

Fallen raindrops into the liquid nitrogen are frozen and they sink to lower sieves owing to their higher density as illustrated at the top in **Figure 1**. As shown in **Figure 2**, the raindrops kept their spherical shape during the freezing process, and consequently by using stainless steel sieves of different mesh widths (1.7 mm, 0.17 mm, and back-up) it is possible to separate the frozen raindrops according to

ciple of our own raindrop capture device can be summarized as follows:

, and

This study has been carried out to improve our understanding of particle scavenging properties of size-resolved raindrops from the chemical properties of

**96**

pollutants.

of 35 μg m<sup>−</sup><sup>3</sup>

their sizes.

raindrops is demanded [6, 7].

**2. Experimental methods**

the residuals in individual raindrops.

**2.1 Collection and handling of size-resolved raindrops**

over a 24-hour period).

segregation of our raindrop collector.

*Flow of collection, pretreatment, and analysis of individual size-resolved raindrops and their residual particles.*

After sampling, the sieves were pulled out from the dewar vacuum flask and each frozen raindrop on each sieve was placed onto the non-hole Nucleopore® filter and Ag thin film (99.99% purity) by using a vacuum pipette (HAKO 392). The frozen raindrops were melted and dried under an infrared lamp for 5 min. Because every process was performed in the clean air system filled with the cooling nitrogen gas, the raindrop handling could be done successfully without any loss of some residues, evaporation, and contamination.

Moreover, in order to measure the rainfall intensity, the standard rain gauge (260-2510, NovaLynx Co.) consisting of a funnel that empties into a graduated cylinder was applied.

#### **2.2 Elemental analyses of residues in raindrops**

Elemental analyses of solid residues and individual residual particles in raindrops were subsequently analyzed by particle induced X-ray emission (PIXE) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX), respectively. The PIXE installed at the Cyclotron Research Center of Iwate Medical University was applied and it has the great advantages such as an excellent sensitivity, a nondestructive technique for multielement with a wide range of elements (Z > 10). The sensitivity, if defined by the ratio between PIXE yield per unit dose and mass thickness, can be determined for all objective elements both

experimentally and theoretically. For instance, the sensitivity of calcium was calculated to be 1700 (counts cm2 /μC μg) with a detection limit of 9.4 × 10<sup>−</sup><sup>3</sup> (μg/cm2 ). The more detailed analytical procedures and experimental setup for PIXE analysis were described elsewhere [10].

The overall process of collection and handling of size-resolved raindrops, and their elemental analyses is shown in **Figure 1**.
