**2.3 Monitoring of sulfate, PM2.5, and particle number concentration**

In addition to the raindrop collection, the highly time-resolved (10-minute cycle) PM2.5 sulfate was monitored during a whole research period by the ambient particulate sulfate monitor (8400S, Rupprecht & Patashnick Co.). A detailed description of design, operation, and data reduction and processing can be found elsewhere [11]. In order to measure PM2.5 mass concentration, a light scattering PM2.5 monitors (Dust Scan Scouts 3020, Rupprecht & Patashnick Co.) was simultaneously operated. Details on this PM2.5 monitoring system was previously described [12].

The number concentrations of size-selective particulate matters (i.e., 0.3–0.5, 0.5–1.0, 1.0–2.0, and 2.0–5.0 μm) were also monitored by an optical particle counters (OPC) (RION, KC-01D).

**99**

**Figure 3.**

*Raindrop number size distribution at the beginning (0.2 mm h<sup>−</sup><sup>1</sup>*

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

The number size distribution of raindrops collected at two different rainfall intensities was estimated by the volume of melted raindrops and that of calculated single raindrop from its average size. **Figure 3** shows the raindrop number

falling per square meter of surface per hour. Raindrop number tended to drastically decrease as the drop size goes up at both hourly rain rates (i.e., rain intensity). Needless to say, it showed higher number concentration when the higher rainfall intensity. This result indicates that the increase in the rain rate stemmed mainly from the increase in the number concentration of raindrops with drop

**3.2 Calculation of particle scavenging efficiency of size-resolved raindrops**

particles as a function of raindrop size was theoretically calculated.

Prior to the interpretation of the actual measurement data about particle scavenging properties of size-resolved rain drops, the collection efficiency of ambient

Slinn and Hales [13] proposed three particle collection efficiencies (*E*), that is, *E* by Brownian diffusion (*Edif.*), *E* by interception (*Eint.*), and *E* by inertial impaction processes (*Eimp.*). Subsequently, Strauss [14] suggested a more advanced equation for particle collection efficiency (*Einteg.*) of raindrops that integrated three kinds of

h<sup>−</sup><sup>1</sup>

) and subsequent (1.2 mm h<sup>−</sup><sup>1</sup>

*) and subsequent (1.2 mm h<sup>−</sup><sup>1</sup>*

*) rainfalls.*

) is the total number of drops

)

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

**3.1 Number size distribution of raindrops**

size distribution at the beginning (0.2 mm h<sup>−</sup><sup>1</sup>

rainfalls. The raindrop concentration Nr (m<sup>−</sup><sup>2</sup>

**3. Results and discussion**

diameter < 0.94 mm.

efficiencies as follows:

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