Does air quality worsen at night?

In this article, we shall discuss how air quality varies at nightime. Also, we will discuss factors that influence the nighttime air quality, and the various chemical species that play a role in variation of nighttime air quality.

Does air quality worsen at night

Yes, contrary to popular belief, air quality actually worsens at night. Air quality is the measure of cleanliness of the ambient air. The lower the quantity of pollutants in the ambient air, the better the air quality, and vice versa.

Usually, air quality monitoring done by government based agencies is done with respect to formal and regulated industrial activities.

However, during nighttime, a significant amount of emissions are generated from unregulated activities and informal industries, which also account for pollution.

Air quality is dynamic in nature. Several factors come into play, which include both natural processes as well as anthropogenic activities, and external factors such as meteorological conditions, climate, temperature, and so on.

There is a common belief that air quality improves at night, since most of the processes that contribute towards degradation of air quality have stopped, or reduced significantly.

While it might be true under certain circumstances, it is not the case most of the time. We shall discuss what factors come into play that cause further degradation of air quality at night.

Factors that affect air quality at night

There are certain activities, some of which are dominantly nocturnal in nature, that play an important role in the degradation of air quality at night. Some of the major factors that do so include:

  • Temperature conditions
  • Urbanisation
  • Combustion of solid fuels
  • Central heating processes

Let us discuss these in more detail.

Temperature conditions

In the nighttime, the Earth gives off radiation that it had absorbed throughout the day in order to cool itself. Furthermore, even the atmosphere radiates its heat towards outer space, in order to get cooler.

However, the cooling of the air causes a decrease in the mixing height, which is the height at which pollutants can get dispersed. 

Furthermore, the heat emanating from the surface starts to heat up the near surface air. This can give rise to a special condition, called ‘inversion layer’.

In a typical atmosphere, as the altitude increases, the temperature starts to decrease. This is important as the process helps to carry away the pollutants from near the surface to higher altitudes, from where the wind can disperse it.

However, in an inversion layer, as we go up, the temperature starts to increase. This causes a decrease in the mixing height, and also prevents the pollutants from dispersing upwards.

Therefore, the pollutants get trapped near the surface layer. Due to this, the concentration of the pollutants does not decrease. 

Moreover, if the sources of pollution are still prevalent, the conditions can get even worse, and can have serious implications on the health of people.


Ever since the onset of urbanisation, the air quality has deteriorated steadily with time. Especially in areas where urbanisation is rapid and unregulated.

Urbanisation refers to the shift of population from rural areas to urban cities. In order to accommodate this, therefore, large scale construction of buildings takes place.

With time, the number of skyscrapers within a city has increased. This gives rise to the formation of ‘urban canyons’, which refers to streets which are flanked by tall buildings on both sides, which creates a canyon-like environment,

The biggest drawback of this is that there is significant impedance to the winds. Wind is important as it brings in fresh air that dilutes the pollution in the ambient air, while at the same time, it disperses the pollutants from the area.

Since the buildings block those winds, the pollution present in the ambient air doesn’t dissipate from the area. Furthermore, when conditions in which inversion layers prevail, this becomes an even bigger problem.

Studies that involved measuring the ambient PM2.5 variations with height found that in lower floors, there is a significantly higher concentration of ambient PM2.5 than it is at the higher floors.

One of the factors that was attributed to this factor was that the wind conditions near the higher floors of the building were much better than that for the lower floors of the same building.

Furthermore, the pollutants mainly emerged from ground level activities, such as traffic, due to which their concentration was higher at lower floors. 

Combustion of solid fuels

Solid fuels refer to solid materials that can be burned to release energy in the form of heat and light. Solid fuels include substances such as coal, firewood, crop residues, and dry dung.

Globally, about 2-3 billion people rely upon solid fuels for their energy needs. The use of biomass as a source of energy is predominant in developing countries, such as India.

In India, one-third (18GW) of the total primary energy used is derived from biomass, and nearly 70% of the total population depends upon it for their cooking and heating purposes.

However, combustion of solid fuels is usually carried out under unfavourable conditions, such as poor ventilation, due to which there’s lack of oxygen available for combustion, and improper drying methods.

Incomplete combustion of these solid fuels generates a lot of particulate matter, in the form of ash, soot, and gases such as nitrous oxides (NOx), sulphur dioxide (SO2), carbon monoxide (CO), volatile organic compounds (VOCs), and much more.

In the nighttime, combustion of solid fuels increases prominently, as it is used for heating purposes, or cooking meals. Due to this, there is a considerable amount of pollution being released in the environment.

Due to this, the air quality during nighttime worsens even further, and When the aforementioned conditions prevail, the decline of air quality will be accelerated.

Pollutants that are responsible for the degradation of air quality at nights

Amongst the various pollutants, the following species heavily contribute to the worsening of air quality at night.

  • Particulate matter (PM)
  • Sulphur dioxide (SO2)
  • Nitrous oxides (NOx)
  • Ozone (O3)
  • Carbon monoxide (CO)

We shall discuss the effects and sources of these pollutants in detail.

Particulate Matter (PM)

Particulate matter refers to a certain class of air pollutants that comprise particles in the sub-micron range with varying chemical compositions that are suspended in the air.

There are two forms of particulate matter: coarse particulate matter (PM10) and tiny particulate matter (PM2.5). PM2.5 is a prevalent problem among them.

There are many sources of PM in the ambient air. Some of the main sources include factories, power plants, refuse incinerators, fumes from automobiles, construction activities, fires and natural windblown dust.

PM has been linked with many diseases. It has also been shown to have caused premature deaths of people who had pre-existing respiratory and cardiovascular diseases.

Sulphur dioxide (SO2)

Sulphur dioxide is a major air pollutant, Naturally, it is produced from volcanoes, but it is mainly an anthropogenic pollutant, produced by emissions from vehicles, particularly diesel vehicles and ships, and industrial emissions. 

Sulphur dioxide is a deep lung irritant. It also triggers atopic ailments such as asthma, eczema, and so on.

Nitrous oxides (NOx)

Nitrous oxides (NOx) are a class of compounds which are composed of nitrogen and oxygen atoms in varying numbers. 

Amongst them, nitrogen dioxide (NO2) is a more prevalent species, which has been extensively studied and has been linked to several diseases that pertain to the cardiovascular system, as well as the respiratory system.

It is predominantly produced by anthropogenic activities such as vehicle exhaust, industrial chimney emissions, and solid fuel burning. 

Ozone (O3)

Ozone is a highly reactive gaseous molecule which is naturally present in the upper layers of the atmosphere, where it absorbs the sun’s damaging ultraviolet radiation. 

When present in the lower levels of the atmosphere, however, it acts as a pollutant, and can have serious adverse effects on human health, which also includes afflictions related to the cardiovascular and respiratory systems.

Ozone is formed as a secondary pollutant when chemical species such as polycyclic aromatic hydrocarbons (PAHs) and NOx species are emitted from vehicular exhausts and react with each other. 

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.

Case study – Air pollution in Delhi, India

Every year in Delhi, India, particularly in the winter, Delhi experiences harsh conditions in which air quality levels degrade to  hazardous conditions.

According to studies, the biggest contributors to the severe air quality conditions in Delhi are attributed to three major sources – vehicular emissions, dust, and emissions from various industries inside as well as around Delhi.

Furthermore, in India, particularly in the northern states, stubble burning, which refers to the practice of burning the crop residues left on the field after harvest, is commonly practiced.

Agriculture is one of the main occupations in the states around Delhi. Therefore, the emissions generated from stubble burning in those places also reaches Delhi due to wind.

During winters, these conditions prevail, due to which the air quality reaches toxic levels, upto values of 999. An air quality index (AQI) above 500 signifies that the air is toxic in nature.

Due to the cold weather, the mixing layer’s height in the region decreases considerably. Furthermore, in the metropolitan region, due to the impedance to wind flow by obstacles such as buildings, the pollutants cannot dissipate.

At nights, conditions favouring formation of the inversion layer prevails, due to which the pollutants get concentrated near the surface. This even prevents the winds further from dissipating the pollutants.

Therefore, the air quality at night becomes toxic for breathing, and has severely adverse effects on the residents in as well as around the region.

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

Can Air Pollution Cause Asthma?

Can Air Pollution Cause Cancer?

Can air pollution cause autism?


Air quality worsens at night, contrary to popular belief. External factors, such as temperature, scale of urbanisation, and reliance on solid fuels determine the severance of air quality degradation.


Can indoor air quality deteriorate at night as well?

Yes, indoor air quality can deteriorate at night as well. This is because the pollution from outdoors can enter the house through openings such as windows, cracks, doors, and so on.

How can I keep my indoor air clean at nights as well

You can do this in the following ways:


  • Misra, Prakhar; Takeuchi, Wataru  (2016). Analysis of air quality and nighttime light for Indian urban regions. IOP Conference Series: Earth and Environmental Science, 37(), 012077–. doi:10.1088/1755-1315/37/1/012077

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