Dense fog events sometimes result in the increase in PM concentration during the events. Hence the PM level on the days coming after the fog event days could be seen relatively lower than the event days. I think you need to check whether the PM concentrations in the fog event days are much higher than usual or not, so you can make sure the PM concentrations in next day are really low. Please find the attached paper showing the changes in meteorology and PM concentration due to a severe fog event. You will be able to find a couple of similar papers and they would be helpful for you to figure out what you want to know. Good luck.

Article Modeling the feedback between aerosol and meteorological var...

Dense fog events sometimes result in the increase in PM concentration during the events. Hence the PM level on the days coming after the fog event days could be seen relatively lower than the event days. I think you need to check whether the PM concentrations in the fog event days are much higher than usual or not, so you can make sure the PM concentrations in next day are really low. Please find the attached paper showing the changes in meteorology and PM concentration due to a severe fog event. You will be able to find a couple of similar papers and they would be helpful for you to figure out what you want to know. Good luck.

Article Modeling the feedback between aerosol and meteorological var...

It is possible. Fog droplets could scavenge gaseous and particulate pollutants, thereby facilitating their deposition to the ground. Meanwhile, fog is normally formed when the ground surface is humid/wet, and the wet surface also helps to lower down particulates.

How would a wet surface enhance particle deposition

Wet surface suppresses the mechanical suspension of dust.

The water vapour uptake of BC depends on the combustion process and fuel, i.e. the BC can be nearly hydrophobic or hygroscopic enough for water vapour uptake already as a fresh emission at low saturation ratios. The transformation of BC water vapour uptake (if the fresh emission is "hydrophobic") can be quite fast in the ambient air, and probably scavenge also the BC particles. But as Wonbae Jeon pointed out, you need to really check the PM concentrations.

Depending on the type of monitor you are using, some may 'see' fog as PM. The same can occur with ice crystals. I saw this when using some of the lower cost sensors that use light scattering tech for PM measurements. However, I would agree with others that you are likely seeing scavenging as you are measuring BC separately. 

Xufai

The discussion is also on BC: that is not suspended dust

It can happen if the BC particles are aged in nature as during aging and transportation the BC particles might have got mixed with water soluble inorganic components externally or internally either. But fresh emissions are largely containing hydrophobic BC emissions ...hope it helps

Hello Vikas ji, It can be because of wet scavenging by the fog. Please find one of my articles published this year in ASEN wherein the data shows that fog scavenging decreases PM conc and it's constituent fractions when monitoring is done during post foggy days. Also, one of our articles in Environmental Pollution in 2018 on submicron aerosols composition change might be also of your interest. But in EP paper we show differences in aerosol composition during foggy and non foggy episodes. But the above discussed paper in ASEN would give you a better picture what happens soon after the fog i.e. in post foggy events.

https://link.springer.com/article/10.1007/s41810-018-0032-6

Thank you Prashantji. I will read the said article.