Is the air cleaner after rain?

The below article talks about air quality, weather conditions that affect air quality- particularly rainfall, along with some frequently asked questions about air quality after rain. 

Is the air cleaner after rain?

Yes, studies have shown that rainfall can improve air quality, through a phenomenon called scavenging by precipitation or wet deposition. It essentially refers to when water droplets collide and coagulate with airborne pollutants and drag them to the ground, leaving them immobile and grounded. Although humans feel a significant improvement in air quality post-rainfall, various sources have confirmed that rainfall can only attribute to a maximum of 11% improvement in air quality. 

Rainfall and its effects on air pollutants 

With rapid urbanization, the air is full of pollutants and chemical particles that contribute to poor air quality globally. From carbon monoxide, particulate matter, nitrogen oxides to fine dust and other allergens. These pollutants can be attributed to various factors such as the burning of fossil fuels, burning of waste, emissions from factories, extractive industries, etc. These processes emit large quantities of greenhouse gas emissions and other pollutants via varying forms of emissions. The air is a mixture of all of these pollutants, and these are the airborne particles that we are being exposed to on a daily basis. 

As rain starts to fall from the sky in the form of water droplets of varying sizes from a diameter of 0.5 mm to 4 mm, water droplets have to cut through various layers of air to reach the ground. This forces them to collide with the airborne particles present in the air, due to the properties of water, pollutants are absorbed by these water particles and coagulate, causing the water droplet to get heavier and be dragged down into the ground. 

At a ground level, these pollutants are immobile and waterlogged, and therefore not of much harm by then. Within the scientific community, this phenomenon is referred to as “scavenging by precipitation” (LAMI, n.d.). The likelihood of a raindrop colliding and coagulating with an airborne pollutant is referred to as the collection efficiency and collection efficiency of 80% in a certain area means that the rainfall has the ability to clear up to 80% of the airborne pollutants present in that specific area. 

Studies over the years have concluded that we overestimate the benefits of rainfall on air quality (Chu J., 2015), as collection efficiency usually isn’t in the higher quantities that make a significant improvement in air quality. There are various factors that play into increasing collection efficiency including humidity conditions, electric charge of the water droplet, the size of the water droplet, the kind of pollutants present in the air, etc. 

Under various restricted conditions produced artificially, it has been found that water does have the capability to absorb air-borne particulates, however, this isn’t the case in real-life scenarios where there are much more complicated intruding factors that reduce the efficiency of the process. Artificially produced scenarios allow for researchers to modulate several elements including the electric charge of the water droplets that would instigate coagulation, since these charges or conditions wouldn’t be found naturally in the atmosphere unless during heavy thunderstorms, these studies would effectively not amount to much in terms of identifying methods to treat poor air quality. 

What do studies say?

A study conducted by researchers in Lanzhou, China reported the following (Feng X. & Wang S., 2012): 

  • Looking at particulate matter between 1 and 2.5 microns, they looked at how these pollutants were affected under different types of rainfall. For light to medium rainfall, the reduction in the concentration of PM2.5 is almost zero, and for the heaviest rains, the concentration of these pollutants was reduced by just 8.7%. 
  • They have found out that larger particles seemed to collide and coagulate in a higher rate than smaller particles with up ≤10% reduction with moderate rains and close to 30% during heavy rainfall. 

A study conducted in Bogotá, Columbia in 2015, reported the following (Blanco-Becerra L. C. et al., 2015):

  • PM2.5 values taken on rainy days were just 11% lower than those taken otherwise 
  • There was a higher decrease in PM10 values with up to 17% reduction
  • Since values of particulate matter concentrations post percipitations were inconclusive, they could not clearly state that rainfall had a significant impact on particulate matter pollution of lower sizes. 

Why do we feel the air is much better after rain?

There are varying hypotheses as to why humans intuitively feel that the air is much cleaner after rain, and some of them are as follows:

  • Either human senses have been fine-tuned over the years that we are able to detect even the smallest changes in air quality
  • Or our senses do not recognise these changes at all and we are being misled due to years worth of inaccurate researches and studies, into thinking that the rain improves air quality drastically
  • Or the air quality is actually better and can be sensed, but this improvement cannot be attributed to the rainfall alone

Our understanding of rainfall patterns and how it affects the world around us have bee moulded through years of random studies that could have been disapproved later, through our physical intuition purely, or by making connections that may not really be correlated; due to this there is quite often a mismatch of what is expected post-rainfall and what the true reality is. 

Factors that increase collection efficiency

Electric Charge of the rain droplets 

Rain droplets have small electrical charges – positive or negative. The kind of pollutants they attract depends on their charge as they could only attract oppositly charged pollutants. With the right conditions, a positively charged rain droplet should be able to collide with an a negatively charged air pollutant, or vice-versa to coagulate and thus be removed from the air. This process is called as electro-scavenging. While each particle pollutant cannot be examined to truly understand the nature of electro-scavenging, there are various physical and chemical properties of water droplets and airborne pollutants that determine the effectivity of electro scavenging. An MIT study found that the higer the charge of the water doplet, the higher was the probability for it to collide with an airborne pollutant; as there is a greater force that is exerted between the water droplet and the pollutant (Chu J., 2015) (LAMI, n.d.). 

Size of the rain droplets

The MIT study mentioned above also recognised that the efficiency of the rain droplet to collide and coagulate with airborne pollutants decreased, as the radius of the droplet increased. This fact is an essential factor to collection efficiency as under artificial conditions, the ideal sizes of the rain droplet was 0.0432 mm and in nature, the smallest rain droplet was observed to be 0.5 mm. Therefore, it can be concluded that most rain droplets are not able to collide with air borne particulate matter as they are too large to do so. 

This situation explains why rain fall is able to clear larger particles of pollutants in comparison to smaller particulate matter. Rain droplets are much larger that PM2.5 pollutants, but is comparatively much smaller to PM10 pollutants. Therefore, the rain droplets are more likely to collide with the larger particles causing collision and coagulation.

Prevailing weather conditions

Various studies over the years have tried to find the correlation between weather conditions before and during the rainfall, and how it effects collection efficiency. A study conducted of 10-years worth of meteorological data in Europe showed that there is a high correlation between wind speed and the concentrations of particulate matter. As wind speed rates were higher, the particulate matter concentration was found to be lower, for bigger and smaller airborne pollutants. This can be explained due to the fact that the wind sweeps up pollutants and transports them towards the direction the wind is blowing to, thus reducing the concentration of these pollutants in the focus area. 

A study found that particulate matter pollution decreased air quality during extreme temperature conditions (Barmpadimos et al., 2012). Rainfall under such conditions would not create significant changes and the pollutants tend to remain in the air even after significant rains. The study conducted in Bogotá, did find out that when there was rainfall during mild temperatures and higher windspeeds, there was a higher probability of cleaner air. This leads us to believe that unless there are several factors that support the water droplets during the rainfall, there is no significant change in the air quality around us. Without supporting elements, water droplets will only collide with pollutants that is larger than them, and thus would only neatly coagulate with pollutants of a specific size and nothing else. 

Frequently Asked Questions (FAQs): Does rainfall improve air quality?

Does rain help with reducing dust and other allergens?

Yes, light rain has been found to cause the washing away of major allergens such as dust and pollen. The levels of allergen concentration would reduce if rainfall was also accompanied by slight winds that promoted the sweeping away of these allergens. 

Can rain improve my indoor air quality?

While improvements in outdoor air quality could mean improvements in indoor air quality, this may not always be the case. Especially as rainfalls increase the moisture in the atmosphere, creating an environment where mold and mold spores, mildew, or bacteria may thrive. This could be an impediment to good indoor air quality. Therefore, it is best to take preventative measures to maintain your indoor air quality during rains. 

Other FAQs about Air Quality that you may be interested in.

Is Fresno Air Quality Good?

Is LA Smog good or bad?

How bad is the air quality in Osoyoos?

References 

Barmpadimos I., Keller J., Oderbolz D., Hueglin C., & Prévôt A. S. H. (2012, April 03). Once decade of parallel fine (PM2.5) and coarse (PM10 – PM2.5) particulate matter measurements in Europe: Trends and Variability. Atmospheric Chemistry and Physics Journal. 12. pp. 3189 – 3203. Viewed on 12-15-2021. https://acp.copernicus.org/articles/12/3189/2012/   

Blanco-Becerra L. C., Gáfaro-Rojas A. I., & Rojas-Roa N. Y. (2015). Influence of precipitation scavenging on the PM2.5/PM10 ration at the Kennedy locality of Bogota, Colombia. Revista Facultad De Ingeniería Universidad De Antioquia. 76. pp. 58 – 65. Viewed on 12-15-2021. https://revistas.udea.edu.co/index.php/ingenieria/article/view/21481 

Chu J. (2015, August 28). Can rain clean the atmosphere? Study explains how rain droplets attract aerosols out of the atmosphere. MIT News Office. Massachusetts Institute of Technology. Viewed on 12-15-2021. https://news.mit.edu/2015/rain-drops-attract-aerosols-clean-air-0828  

Feng X. & Wang S. (2012, April 06). Influence of different weather events on concentrations of particulate matter with different sizes in Lanzhou, China. Journal of Environmental Sciences. 24. pp. 665 – 674. Viewed on 12-15-2021. http://www.jesc.ac.cn/jesc_en/ch/reader/view_abstract.aspx?file_no=2012240413 

LAMI. (n.d.). Does Rain clean the Air? Viewed on 12-15-2021. https://www.lamigroup.net/information/water/does-rain-clean-the-air/ 

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