*2.3. Analysis Methodology*

Precipitation event: A precipitation process starts in the first hour when precipitation reaches at least 0.1 mm. If precipitation in an hour was zero after the beginning of the precipitation process, that hour was recorded as an interrupted hour, and the end of the process appeared when three consecutive precipitation interruptions occurred. The hour before the interrupted hour was recorded as the last hour of the precipitation process. As a result, we obtained 27,219 precipitation processes in total.

The effects of concentration and precipitation on the removal rate were analyzed by classifying precipitation processes (0–1 mm, 1–5 mm, 5–10 mm, 10–20 mm, 20–30 mm, 30–50 mm, >50 mm).

Scavenging efficiency (*SE*): *SE* is the particle mass concentration change in unit time (*t*). In an hour with a particle mass concentration *CONbef ore* before the rain starts and with a particle mass concentration *CONa f ter* after the rain stops, the *SE* is expressed as

$$SE = \left(\text{CON}\_{after} - \text{CON}\_{before}\right) / t$$

Scavenging rate (*SR*): *SR* is the percentage change of particle mass concentration changes. For a precipitation process with a particle mass concentration *CONbef ore* before the rain starts and a particle mass concentration *CONa f ter* after the rain stops, we defined *SR* as

$$SR = \frac{\text{CON}\_{b\text{before}} - \text{CON}\_{after}}{\text{CON}\_{before}} \times 100\text{\%} \tag{1}$$

In some cases, the rain did not remove the particles, but the concentration continued to increase. Therefore, we made a rule that if the *SR* is positive, it is a positive scavenging process, and if the *SR* is negative, it is a negative scavenging process (which means the precipitation had a very limited scavenge).
