In this article, we will discuss the effects of rain on the ambient air quality. Furthermore, we shall break down the interaction of rain with certain pollutants.
Does air quality improve after rain?
Yes, the ambient air quality of a region improves after it experiences rainfall. Air quality is a measure of how clean or polluted the ambient air of a region is.
Air quality degrades due to emissions of pollutants in the form of gases or particles from natural as well as anthropogenic (human-based) sources.
What does research say?
Several studies have been carried out in different regions of the world to understand the link between rain and air quality. All of them suggest rainfall has a positive impact on air quality.
Rain interacts with various pollutants, but is most effective against particulate matter. In some cases, although rain removed certain pollutants from the ambient air, there were other adverse effects observed.
What pollutants are affected by rainfall?
Rainfall interacts differently with various pollutant species, but mainly affects the following pollutants:
- Particulate matter (PM)
- Nitrous oxides (NOx)
- Sulphur dioxide (SO2)
- Carbon monoxide (CO)
- Volatile Organic Compounds (VOCs)
- Ozone (O3)
- Heavy metals
We shall discuss in brief about the various pollutants.
Particulate matter (PM)
One of the biggest components of air pollution is particulate matter, which are particles in the sub-micron range which are suspended in the ambient air.
Particulate matter is pretty dangerous for human health, as the smaller particles can get into the alveoli of the lung, where oxygen and carbon dioxide is exchanged with the bloodstream.
Particulate matter originates from natural sources, which include pollen dust, animal dander, and so on. It also originates from anthropogenic (human-based) sources, such as combustion of fossil fuels and firewood, emissions from industries, and so on.
However, in developed regions, anthropogenic activities, such as combustion of fossil fuels and firewood, emissions from industries, etc., are the major sources of particulate matter.
On the basis of size, these particles are classified as:
- PM10 (PM ≤10 µm in aerodynamic diameter) includes the largest inhalable particles. These particles are usually not inhaled past the trachea, get trapped in the nose and throat regions, hence do not get deposited in the lungs. PM10 also includes:
- PM2.5-10, which is also known as coarse particulate matter (PM with an aerodynamic diameter >2.5 µm but ≤10 µm).
- PM2.5, which is also known as fine particulate matter (PM with an aerodynamic diameter ≤2.5 µm). These particles can be inhaled into the deepest recesses of the lung, including to the alveoli sacs, where oxygen exchange occurs.
Nowadays, PM2.5 has increasingly become a major research focus of adverse human health impacts of outdoor air pollution exposure over recent decades.
Particulate matter can stay suspended in the air for a long time, and can travel distances as far as hundreds to thousands of kilometres.
There are two main phenomena responsible for removal of particulate matter from air – sedimentation, which refers to settling down of particles.
The other phenomenon is wet deposition, which refers to removal of particles by process of precipitation, such as rain, hail, and snow.
Raindrops interact with the particulate matter, causing them to either dissolve or simply deposit onto the surface after precipitation.
However, if the source emissions were to remain unchanged, the concentration of respective particulate matter will increase once the rain subsides.
Nitrous oxides (NOx)
NOx species are compounds which are formed by nitrogen and oxygen in varying numbers. These can form naturally in the atmosphere, due to the interaction of light with the ambient nitrogen and oxygen.
NOx species, however, is a prevalent anthropogenic-based pollutant. It is produced as a byproduct when fossil fuels or wood is burned. It is also released in emissions from different industries such as power plants, brick kilns, boilers, and so on.
NOx species are a pollutant of concern, as they increase the risk of respiratory infection. Various studies have shown that NOx species, particularly NO2, are responsible for the onset of asthma in children and young adults.
NOx species are reactive with water, hence are easily scavenged from the atmosphere. However, the biggest problem is that when NOx species react with water, they form nitric acid, which in higher concentrations can cause acid rain.
Acid rain is bad not just for plants and animals, but for humans too. Nitric acid is highly reactive, and can corrode buildings and steel components of structures such as bridges, automobiles, etc.
Sulphur dioxide (SO2)
Sulphur dioxide is a major air pollutant. It is released from natural sources such as volcanic activities, and from human activities such as combustion of fossil fuels and industrial emissions.
Various studies have shown that children, old people, and people with preexisting respiratory illnesses are more prone to skin and lung diseases.
Since SO2 can dissolve in water, its elimination from the ambient air is similar to the way NOx species are scavenged by rainfall.
However, even SO2 reacts with water to form sulphuric acid, which is a highly reactive, corrosive acid. Sulphuric acid is also a component of acid rain, and wrecks havoc on human health too.
Carbon monoxide (CO)
Carbon monoxide is a serious air pollutant, and in higher concentrations, it can prove to be lethal. Carbon monoxide is a colourless, odorless gas, therefore cannot be detected by human senses.
Carbon monoxide is formed from incomplete combustion of fossil fuels and wood. It is also formed from smoking cigarettes.
Carbon monoxide forms a strong bond with haemoglobin, the compound present in red blood cells (RBCs), which is responsible for transportation of oxygen from the lungs to the tissues, and carbon dioxide from the tissues back to the lungs.
When carbon monoxide interacts with water, it gets converted to carbon dioxide. This carbon dioxide, in turn, gets dissolved in water to form carbonic acid, which gives rainwater its characteristic pH value.
Volatile Organic compounds (VOCs)
VOCs refer to a variety of organic compounds that are emitted from certain materials in the form of a gas. These compounds are formed when burning of fuel or wood takes place.
VOCs are harmful for human health, since they can cause a variety of issues, such as palpitations, dizziness, etc., while some species of VOCs have also been linked with increased chances of cancer.
VOCs can easily dissolve in water. Therefore, they get scavenged out from the ambient air into the rainwater. However, if this VOC-bearing rainwater reaches the groundwater, it can act as a persistent pollutant.
Ozone is a gas that is formed by interaction of light with oxygen molecules. It is a vital gas in the upper atmosphere, where it blocks the harmful UV rays from entering the atmosphere.
However, when present at the ground level, it has negative implications on human health. In the ground level, it is formed when NOx species react with polycyclic aromatic hydrocarbons (PAHs), which are released from vehicle exhausts.
The characteristic pungent odor of ozone can irritate the inner linings of the airways and can trigger an asthma attack. However, ozone is an unstable molecule, and it can attack the DNA, causing impairment of cellular function.
When ozone reacts with water, it gives rise to hydroxyl (OH–) ions. Hydroxyl ions are known as “atmospheric bleach”, as they can react and neutralise almost any pollutant present in the atmosphere.
Therefore, not only does the process reduce the ozone levels in the atmosphere, it can also help remove pollutants that do not interact with water.
Heavy metals refers to metals whose density is more than 4g/cm. These elements are required in trace amounts for the adequate functioning of various processes in the human body.
However, after a certain limit, they act as toxins, and can affect our health adversely. Some of the commonly occuring heavy metals are lead (Pb), zinc (Zn), cadmium (Cd), copper (Cu), and so on.
Some of the main sources of lead in the air are industrial emissions, ores, and from aircrafts that use leaded jet fuel. Other metals come from dust, traffic-related pollution, industrial emissions, chemicals, and so on.
A study done in Australia by Chubaka et al. in 2018 found elevated concentrations of lead, zinc, cadmium, and copper in rainwater samples, suggesting that the elements were leached out by rain after they were emitted from various sources.
Other FAQs about Air Quality that you may be interested in.
We discussed in this article how rain is beneficial for the air quality. Several studies corroborated to this, showing how precipitation leads to the removal of particulate matter, certain gaseous pollutants, and heavy metals.
However, some pollutants when combined with rainwater can cause harm in other ways, in the form of acid rain, or by contamination of groundwater and other water bodies, especially in polluted regions.
FAQs: Does air quality improve after rain?
Is it safe to step outside when it’s raining if I live in a polluted area?
No, it is unsafe to step outside when it’s raining if you live in a polluted area. In polluted areas, pollutants such as NOx species and sulphur dioxide exist.
When it starts to rain, the water droplets interact with the NOx species to form nitric acid (HNO3). Nitric acid is extremely corrosive in nature, and can harm you.
Similarly, when water droplets interact with the sulphur dioxide, they react to form sulphuric acid (H2SO4). Like nitric acid, sulphuric acid also is corrosive in nature and can cause serious harm.
Is humidity due to rain good for the indoor environment?
No, humidity after rain is not good for the indoor environment. Humidity provides favourable conditions for pollutants such as dust mites, mold spores, and other allergens.
These can have adverse effects on the health, and if left unchecked, can even cause harm to the house as well.
In order to deal with the humidity associated with rainfall, you should consider investing in an air dehumidifier. Using an air dehumidifier to reduce the relative humidity will help to keep a check on the above mentioned issues as well.
How long does the effect of rain on air quality last?
For large cities, where traffic is high and industries are present, the effect would be temporary. After the rain has subsided, the pollution levels will start rising once again as normal traffic resumes and industries continue their operation.
However, for suburban and rural areas, the effects will last for a relatively longer time. This is because in these areas, natural processes such as suspension of dust in the air by wind, pollination, etc. are dominant.
With rainfall, the particulates associated with these settle down, leaving the air cleaner than it was before the rains.
- Ha-Young Kwak, Joonho Ko, Seungho Lee, Chang-Hyeon Joh. Identifying the correlation between rainfall, traffic flow performance and air pollution concentration in Seoul using a path analysis, Transportation Research Procedia, Volume 25, 2017, Pages 3552-3563, ISSN 2352-1465, https://doi.org/10.1016/j.trpro.2017.05.288.
- Hyo-Bang Moon, Su-Jeong Lee, Hee-Gu Choi, Gon Ok, Atmospheric deposition of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in urban and suburban areas of Korea, Chemosphere, Volume 58, Issue 11, 2005, Pages 1525-1534, ISSN 0045-6535, https://doi.org/10.1016/j.chemosphere.2004.11.014.
- Tian, X., Cui, K., Sheu, H.L., Hsieh, Y.K., Yu, F. (2021). Effects of Rain and Snow on the Air Quality Index, PM2.5 Levels, and Dry Deposition Flux of PCDD/Fs. Aerosol Air Qual. Res. 21, 210158. https://doi.org/10.4209/aaqr.210158
- Chubaka CE, Whiley H, Edwards JW, Ross KE. Lead, Zinc, Copper, and Cadmium Content of Water from South Australian Rainwater Tanks. Int J Environ Res Public Health. 2018;15(7):1551. Published 2018 Jul 23. doi:10.3390/ijerph15071551
- MIT News. Can Rain Clean The Atmosphere?
- US EPA. Air Quality and Climate Change Research.