Does Air Quality change during the Day?

In this article, we will discuss whether air quality for a region changes during the day. Also, we will elaborate on factors that contribute to the variation in air quality, and species which show variation.

Does Air Quality change during the Day

Yes, air quality changes during the day. Air quality refers to the measure of cleanliness of ambient air in a given region. Air quality is affected by both natural activities, as well as human activities.

Natural factors that contribute to the phenomenon

Natural activities refer to processes that take place in nature. These can include factors such as dust being suspended in the air due to wind, release of pollen by flowering plants, and emission of gases from natural processes.

Certain processes occur on a daily basis, such as suspension of dust from the ground, while some are dependent on seasons, such as release of pollen into the air.

Lastly, there are extreme events which, although occur rarely, still have a huge impact on the ambient air quality. This includes forest fires, volcanic explosions, and so on.

Human factors that contribute to variation in air quality

Over the previous decades, the air quality has been deteriorating, which is more pronounced in urban areas. This degradation is mainly due to human activities, duee to the rise of emissions and dependence on fossil fuels.

Human activities give rise to multiple types of pollutants, such as traffic-related pollution, emissions from industries, deforestation, change in land usage, and livestock grazing.

These activities not only cause change in the air quality on a daily basis, but also give rise to pollutants that have negative implications on the climate as well.

Chemical species that play an important role in diurnal variation of air quality

As discussed above, there are a myriad of sources, both natural as well as anthropogenic, that give rise to various types of chemical species that cause a diurnal (i.e. during a day) variation in the air quality.

The major contributors that cause variation in the air quality are listed below:

  • Particulate matter (PM)
  • Sulphur dioxide (SO2)
  • Nitrous oxides (NOx)
  • Methane (CH4)
  • Volatile organic compounds (VOCs)
  • Ozone (O3)
  • Carbon monoxide (CO)

We shall discuss these pollutants in detail below.

Particulate matter (PM)

Particulate matter refers to particles in the sub-micron range that are suspended in the ambient air. They are emitted from both natural as well as human activities.

Natural processes include suspension of dust from the ground by wind, release of pollen from flowering plants, or release of soot from wildfires. However, the biggest contributor to PM is human-based activities.

Over the previous decades, there has been a significant rise in PM, which can solely be attributed to human activities such as combustion of fossil fuels and wood, emissions from industries, traffic-related pollution, change in land usage pattern, and so on.

Particulate matter are of two types – inhalable particulate matter, or PM10, which are particles equal to or less than 10μm, and fine particulate matter, or PM2.5, which are particles equal to or lesser than 2.5μm.

Other than the implication on climate, PM, especially PM2.5, have serious implications on human health, as they can enter the alveoli, where oxygen exchange takes place.

In urban areas, the PM levels rise in peak traffic hours. Other sources such as emissions from industries too contribute to the variation. In many parts of the world, such as Beijing and Delhi, PM2.5 is majorly responsible for deterioration of air quality.

Furthermore, there is a seasonal variation seen as well. In winters, especially in cold countries, people light fires using fuels and firewood to keep themselves warm. These activities contribute to the change in PM levels, which is also noticed at nighttime.

Sulphur Dioxide (SO2)

Sulphur dioxide is a pungent-smelling gas that is released from natural sources such as volcanic activities, and from human activities such as combustion of fossil fuels, particularly from diesel vehicles, and industrial emissions. 

Being a traffic-related pollutant, this too shows an increase in concentration in the ambient air during peak traffic hours. It is an irritant, and can cause issues such as wheezing, asthma, irritation of the eyes, nose, and throat, and so on.

Other than traffic, the other source responsible for SO2 emissions are coal-based thermal power plants (CTPPs). The combustion of coal gives rise to SO2, along with PM, which in turn are released into the air, thereby increasing the SO2 concentrations.

Nitrous oxides (NOx)

NOx species refers to various compounds that contain nitrogen and oxygen atoms in varying numbers. These are a pollutant of concern, as they increase the risk of respiratory infection, since they can reach the lower respiratory tract with ease.

NOx species are formed as a byproduct of burning of fuels, and can also occur naturally during lightning. Therefore, these too show a change in concentration levels similar to those of SO2.


Methane is a greenhouse gas, meaning that it has implications to global warming. Other than that, it can act as a pollutant, since it can displace oxygen from the ambient air.

Methane is largely produced from marshes and bogs, and is also produced by ruminant animals. Therefore, livestock is a prevalent contributor to methane emissions.

As stated before, methane can displace oxygen from the ambient air. With a rise in concentration, methane can induce symptoms such as rapid breathing, rapid heart rate, and increased fatigue.

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 naturally emitted by vegetation – such as black gum, poplar, oak, and willow trees – which emit significant amounts of volatile organic compounds (VOCs) on warmer days.

From human sources, formed when burning of fuel or wood takes place. Therefore, VOCs too are traffic-related pollutants. 

Other than this, certain chemicals such as paints, solvents, dyes, etc., and their respective manufacturing industries as well emit VOCs.

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.

Ozone (O3)

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.

As stated above, ozone formation in the ground level air relies upon sunlight as well. Therefore, the increase in ozone concentrations are seen during the afternoon, when photolytic reactions have maximum energy available.

Carbon monoxide (CO)

Carbon monoxide is a serious air pollutant, and in higher concentrations, it can prove to be lethal. It is a colorless, odourless gas, therefore it is hard to detect its presence.

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.

Thus, carbon monoxide hampers the ability of oxygen transportation. This causes short term effects such as rapid breathing, susceptibility to fatigue, confusion, headache, arrhythmia, and so on.

Being a traffic-related pollutant, carbon monoxide shows a typical pattern of variation similar to that of other traffic-related pollutants such as PM, SO2, and NOx.

Natural events such as wildfires also produce a lot of CO. This can spread to nearby areas and therefore affect the air quality.

There have been studies to show how forest fires are linked to cardiovascular and respiratory issues in nearby occupants, with CO being one of the main contributing factors.

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

Does air quality affect running?

Can air pollution cause lung cancer?

How air pollution affects the lungs


Based on various studies  in different regions of the world, it was found out that air quality changes with the duration of the day.

This variability depends upon factors such as number of sources contributing, the intensity of emission, and other factors, such as season, temperatures, and so on. 

It is important to note, however, that human-based activities, especially traffic-related pollution, was the major contributor to the change in diurnal concentration of various pollutants.

This is evident from the variation of concentration of certain pollutants with the traffic. Furthermore, the concentration increases even more on weekends, when vehicular traffic is relatively higher than other days.


Is air quality better in the morning or afternoon?

Air quality is better in the mornings than in afternoons. This is because throughout the day, the concentration of pollutants starts to rise steadily due to traffic, industrial emissions, and other sources.

Moreover, during the afternoon, when the sun is closest to the surface, the near surface air gets heated up. 

This in turn triggers the formation of secondary pollutants from reaction of other pollutants (eg.: Ozone formation due to reaction between nitrous oxides and PAHs).

On the contrary, in the mornings, human activities are at the lowest. Furthermore, since there is a low concentration of pollutants, the formation of secondary pollutants gets limited.

Is the nighttime air quality better?

Contrary to popular belief, nighttime air is actually worse for your health. At night, the warm air traps the cool air and forms a kind of atmospheric lid, which is known as inversion. 

The vertical mixing of air happens within this layer, as a result of which the pollutants disperse back in the atmosphere. 

The effects of inversion are strong at night, due to which the pollutants do not disperse, and start concentrating in the near surface air.

It is due to this phenomenon due to which the air quality deteriorates at night.

How to tell whether it is safe to step outside?

You can tell whether it is safe by checking the air quality of your area. In case it is polluted, you should wait until favourable conditions prevail.

However, in circumstances where it is necessary to go, you can plan an alternative route which would be less polluted. Furthermore, you can use a mask, particularly an N95 mask, as that would help to block any PM from entering your airways.


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  • Singh V, Singh S, Biswal A, Kesarkar AP, Mor S, Ravindra K. Diurnal and temporal changes in air pollution during COVID-19 strict lockdown over different regions of India. Environ Pollut. 2020;266(Pt 3):115368. doi:10.1016/j.envpol.2020.115368
  • Wei Chen, Hongzhao Tang, Haimeng Zhao, Diurnal, weekly and monthly spatial variations of air pollutants and air quality of Beijing, Atmospheric Environment, Volume 119, 2015, Pages 21-34, ISSN 1352-2310,

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