Below‐cloud scavenging of fine and coarse aerosol particles by rain: The role of raindrop size

Abstract

Below‐cloud aerosol scavenging is an important process to remove particles from the atmosphere. In this study, both precipitation (measured with a laser disdrometer registering drops between 0.125 and 8 mm) and atmospheric aerosol particles (measured with a Passive Cavity Aerosol Spectrometer Probe registering aerosol sizes between 0.1 and 24 µm) have been sampled on a 1 min basis. Based on 6 months of outdoor measurements in León (northwest Iberian Peninsula), the scavenging coefficients were calculated for the aerosol particles measured, with a mean value of 2.6 × 10−5 ± 6.0 × 10−5 s−1 for the fine mode and 5.8 × 10−5 ± 9.6 × 10−5 s−1 for the coarse mode in 54 rain events. Likewise, the particle concentration decreased during rain by 10%, and by 18% between the time before and after the rain. In the Greenfield Gap (sizes between 0.3 and 1 µm) the scavenging is less effective, mainly between 0.6 and 1 µm, than with smaller particles. The number of drops between 0.75 and 3 mm shows statistically significant (p < 0.05) negative correlations with the number of particles between 0.16 and 1.76 µm, thus illustrating the potential of those raindrops to favour the scavenging of particles of that size. In addition, there were statistically significant negative correlations between the total volume removed by falling drops (mm3/m3) and the variation of the number of particles in channels with diameters between 0.12 and 0.19 µm.

Aerosol Populations, Processes, and Ages in Bulk Deposition: Insights From a 9‐Year Study of 7Be, 210Pb, Sulfate, and Major/Trace Elements

“…First, aerosol composition varies with altitude (Dibb, Talbot, Scheuer, Seid, Avery, et al., 2003; Holecek et al., 2007) and, undoubtedly, precipitation samples a different air mass than ground level sampling for ambient aerosol (Dibb & Jaffrezo, 1993). Second, wet scavenging could discriminate aerosol populations based on their composition, size, density, electrical charge, and so on (Blanco‐Allegre et al., 2018; Bourcier et al., 2014; Seinfeld & Pandis, 2016). Discrimination through aerosol scavenging and formation of condensation nuclei may cause precipitation to be preferentially enriched in plant waxes, biogenic components, oxidized polar and aromatic compounds, and mineral dust, for example (Holecek et al., 2007; Simoneit & Mazurek, 1989).…”

Mitigating air pollution strategies based on solar chimneys

“…It was claimed the rain can clean up 40% of the PM2.5 in the study area due to the scavenging effect (Lu et al, 2019). It was observed that the fine particles with a diameter between 0.2 and 0.4 μm were mainly scavenged by rain droplets with a diameter ranging from 0.3 to 1 mm, while the coarse particles (>1 μm) could be efficiently collected by all drop sizes, mostly from raindrops larger than 1.5 mm (Blanco-Alegre et al, 2018). A geoengineering approach by the means of spraying water into the polluted air on the top of high buildings on the scale of a whole city, which aimed to mimic the rainfall, was proposed to scavenge the aerosols in the air (Yu, 2014).…”

Chemical composition of rainwater under two events of aerosol transport: A Saharan dust outbreak and wildfires

“…The volume swept by raindrops during this period was mainly due to drops with sizes N2 mm, while for the lower precipitation intensity (about 0.3 mm h −1 ) it was due to drop sizes b2 mm. Blanco-Alegre et al (2018 showed that higher volume swept by drops caused a more effective scavenging on particles between 0.12 and 0.19 μm, and there was a significant correlation between the scavenging efficiency of black carbon particles and the number of drops with diameters between 0.375 and 2.5 nm. Therefore, this precipitation event could have been responsible for removing pollutants from the atmosphere through the washout process.…”

“…The BCS process of different chemical species is affected by the intensity and volume of rainfall (Custódio et al, 2014;Luan et al, 2019;Pan and Wang, 2015). Furthermore, the physical parameters of precipitation (raindrop size distribution and volume swept by raindrops) are also important factors influencing the scavenging process and can also determine the chemical composition of the rainwater (Blanco-Alegre et al, 2018Fredericks and Saylor, 2019). Why is the raindrop size important?…”