The variation in the particle number size distribution during the rainfall: wet scavenging and air mass changing

Abstract

Abstract. Below-cloud wet scavenging is an important pathway to remove atmospheric aerosols. The below-cloud wet-scavenging coefficient (BWSC) is the value to describe the ability of rainfall to remove aerosols. The reported BWSCs obtained from the field measurements are much higher than the theory, but the reason for this remains unclear. Based on the long-term field measurements in the Yangtze River Delta of eastern China, we find that 28 % of the rainfall events are high-BWSC events. The high-BWSC events show the sudden decrease in the particle number concentration in all size bins near the end of rainfall. By investigating the simultaneously observed changes in carbon monoxide and aerosol chemical compositions during rainfall events, the circulation patterns, and backward trajectories, we find the cause of the high-BWSC events is the air mass changing but not the wet scavenging. The change in air masses is always followed by the rainfall processes and cannot be screened out by the traditional meteorological criteria, which would cause the overestimation of BWSC. After excluding the high-BWSC events, the observed BWSC is close to the theory and is correlated with the rainfall intensity and particle number concentrations prior to rainfall. This study highlights that the discrepancy between the observed BWSC and the theoretical value may not be as large as is currently believed. To obtain reasonable BWSCs and parameterization from field measurements, the effect of air mass changing during rainfall needs to be carefully considered.

Vertical Profiles of Particle Number Size Distribution and Variation Characteristics at the Eastern Slope of the Tibetan Plateau

“…At the TQ site, the PBLH increased by an average of 86.6 m, and the particle number concentration decreased by 58.76%. The significant decrease in PNC at the TQ site can be attributed to two possible factors: first, the dispersion facilitated by the uplift of the PBL, as depicted in Figure 5; and second, the precipitation removal, as demonstrated in Figure 3, which has been previously discussed [69,70]. Throughout the afternoon and evening, specifically from 13:00 to 21:00 LST, the PNC at the top of the PBL demonstrated a continuous increase due to the decreased PBLH.…”

High Concentration of Atmospheric Sub‐3 nm Particles in Polluted Environment of Eastern China: New Particle Formation and Traffic Emission

“…The differences in the peak values between the warm and cold seasons indicate the different intensities of photochemical processes as well as CS. The CS was low in warm season, which was caused by high planetary boundary layer height, low primary aerosol emissions and high wet scavenging (Niu et al., 2023). At night, when the photochemical formation process ceases and the concentration of sub‐3 nm particles reaches its lowest value of around 10 4 cm −3 in all seasons.…”