Particle number size distribution measurements of PM1 and PM10 in fogs and implications on fog droplet evolutions

Abstract

<p>CCN number concentration (N<sub>CCN</sub>), particle size-resolved activation ratio at supersaturation (SS) of 0.10% and particle number size distribution (PNSD) in dry state of both ambient PM1 and PM10 particles were measured in the North China Plain in November in 2018. Two fog events were observed during nighttime of 12nd and 13rd Nov. During fog events, the dry particle concentrations sampled from the PM10 inlet were much higher than those from PM1 inlet for particles (particle size bins) with diameter larger than ~200 nm. Additional sub-micron particles sampled by PM10 inlet but not been sampled by PM1 inlet indicates that these particles have grown into droplets with diameter larger than 1um. The growth of particle size by over 5 times can be resulted from not only the activation to form fog droplets but also the hygroscopic growth at RH higher than 99%. There was no significant decrease of particle number concentration larger than ~200 nm during fog periods compared with those beyond the fog periods, suggesting that the fog droplets may be generally smaller than 10um and can be sampled by PM10 inlet. The size-resolved activation ratio curve showed that the critical diameter was about 160-180nm and there was significant difference (>50%) of N<sub>CCN</sub> at SS of 0.1% between PM1 and PM10, mainly due to the difference of PNSD between PM1 and PM10 in fogs. The measured PNSD and CCN-activity might be applied on the analysis of the relationship between fog droplets and the corresponding ambient supersaturations.</p>