I once had the pleasure of travelling to Beijing and staying for a couple of weeks. Just a few hours before my plane landed there had been a considerable amount of rain in the city. Not only was I lucky enough to arrive just after it had finished but the precipitation had cleaned out the air which is notorious for its pollution. On this first day I got to my student room and looked out the window to see some massive sky scrapers in the distance. By the end of the week they were gone, consumed by the smog. The impact of aerosols in a range of 0.05 to 1 μm is controlled by their constant removal from the atmosphere by rain. It makes sense that the rain would remove some dust from the atmosphere but the exact method is much harder to determine as such precise observations are hard to perform on such a massive scale.
The two methods that are commonly suggested are nucleation scavenging and impaction scavenging. Nucleation scavenging is when, much like a normal droplet, the water forms around a core particle. In this case the core particle is an aerosol or pollutant which is dragged out of the air by the rain. Of course as the rain falls it can activate the second method. Impaction scavenging is when a water droplet combines with an aerosol in a collision and then precedes to fall. Once the ground is reached these pollutants are permanently removed.
In order to gather data parcels containing aerosols were sent up into the atmosphere on convection currents. The pollutant would be absorbed by a cloud and the amount of aerosols could be recorded from the cloud’s precipitation. This may sound like environmental vandalism but it is more controlled than that. With each sample there was an amount of black carbon (BC) added as a tracer. Black carbon is inert and insoluble and this means it would be perfect as a control element. Since the rate of removal of BC should never alter, it is possible to compare the different families of pollutants and how well they are individually removed using BC as a baseline. Data was collected for 20 days in Tokyo and it was found that nucleation appears to have an overwhelming effect on air cleaning. This method of atmospheric experimentation will certainly be helpful in future atmospheric and oceanographic studies.