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

Particle microphysical and chemical properties, including particle number size distribution (PNSD) and chemical composition of both ambient PM1 and PM10 particles were measured in the North China Plain (NCP) during the Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain (McFAN) campaign in November 2018. Focused on the differences of measured PNSD between PM1 and PM10, the variations of aerosol micro physical and chemical properties in fog were analyzed and their corresponding influence on fog droplets evolution was evaluated. Two nighttime fog events were observed on the 12th and 13th November, respectively. During fog events, PNSDs of particles with diameter larger than ~200 nm sampled from the PM10 inlet were similar to those after fogs. But in particle size range larger than 200 nm, particles number concentration sampled from the PM1 inlet were very few and much lower than those after fogs, indicating that some particles larger than 200 nm have grown into droplets with diameter larger than 1 mu m and thus were not detected under the PM1 inlet. No significant decrease in particle number concentration larger than ~200 nm was detected under the PM10 inlet after the dissipation of these fog events, suggesting that fog droplets were generally smaller than 10 mu m and could be sampled by the PM10 inlet. The additional particles sampled in PM10 in fogs demonstrated a normal distri-bution, which indicates that they belonged to an individual particle group externally mixed with those sampled under the PM1 impactor and could outgrow 1 mu m either through hygroscopic growth or through activation. This can lead to uncertainties in droplet number concentration estimations within fog from microphysical and chemical properties measurements of the aerosol with assumption of fully internal mixture. We find that PM1 hygroscopicity derived from particle chemical composition measurements was overestimated, which may result in significant bias in the estimated droplets number concentration in the North China Plain. These results and approaches in this study can improve the understanding of aerosol microphysical properties in fogs and the corresponding influence of aerosol particles on fog microphysics.

Automated summary

This study measured the microphysical and chemical properties of particles in fog, and analyzed their influence on fog droplet evolution. The results showed that after fog dissipates, particles larger than 200 nm were hardly detected under the PM1 inlet, indicating that fog droplets are generally smaller than 10 μm and can be sampled by the PM10 inlet. The study also found that the hygroscopicity of PM1 was overestimated, potentially leading to significant bias in the estimated droplet number concentration.