Does air quality improve with elevation?

This article explains how air quality gets affected by the elevation. Furthermore, we enumerate the factors associated with altitude that play an important role in the air quality of the region.

Does air quality improve with elevation

Yes, air quality improves with elevation. Air quality provides an insight about the concentration of pollutants in the ambient air. A good air quality signifies low concentration levels of pollutants in the ambient air, and vice versa.

However, under such conditions, air quality is more susceptible  for degradation than it would be for a region with the same settings, but at a lower elevation.

We shall see what factors affect the air quality at higher altitudes.

Factors affecting air quality in higher altitudes

As the elevation increases, several factors come into play which help to regulate and rejuvenate the air quality. These factors include:

  • Wind speed
  • Density of pollutants
  • Temperature and climate conditions
  • Sources of pollution

We shall discuss these in more detail

Wind speed

With the rapid rise of urbanisation, there has been an increased development of housing over the previous years. In large cities, skyscrapers are constructed in order to accommodate more people in a smaller land space.

However, one of the biggest disadvantages of construction of high-rise buildings is that in the ground level, particularly in the streets, wind can’t enter due to the impedance to wind flow offered by the buildings which act as obstacles.

This phenomenon is known as the ‘urban canyon’ effect, since the buildings act as anyon through which no air can flow through. 

However, as the elevation increases, the obstacles in the path of wind decrease, therefore allowing the winds to flow more freely. 

This is an important factor, since the winds bring in fresh air that dilutes the ambient pollution, and carry away the pollutants from the region as well.

Density of pollutants


The maximum height a pollutant can go up in the air is usually determined by its density. For pollutants denser than the average density of air, they will more likely be near the surface level, and vice-versa.

This is an important factor that determines the air quality as elevation increases. One of the most common pollutants present in the atmosphere is particulate matter, or simply PM.

Description of particulate matter (PM)

The rate of particulate matter in the ambient air has been gradually increasing since the dawn of industrialization. It has even reached levels where it may have major health consequences in some areas.

Particulate matter is made up of solid or liquid particles that are combined and suspended in the air. They contribute significantly to air pollution and come from a number of natural and manmade sources. They normally vary from 2.5 to 10 microns in size (PM10 and PM2.5).

Particulate matter plays a significant role in both long-term and short-term ailments. Many well-documented research have linked an increase in PM concentration with the course of diseases.

Inhalable particulate matter is defined as particles with a diameter of 2.5 to 10 microns. These are captured by the cilia found in the inside linings of the throat.

They do, however, include particles with a variety of chemical compositions that can serve as irritants and trigger exacerbations, particularly in those with allergic asthma.

PM2.5 is a major source of worry since it can readily penetrate the lower respiratory tract, resulting in more serious cardiac and respiratory problems.

According to studies, every 5-6 g/m3 increase in PM2.5 results in a significant increase in the risk of cardiovascular disease, ranging from 0.5 to 1.5 percent.

PM2.5 is a cause for worry, particularly in developing nations, where ambient air quality for PM2.5 concentrations is ten times greater than the US National Ambient Air Quality Standards.

How elevation regulates particulate matter concentration

Generally, the average density of particulate matter is more than that of the ambient air. This is an important factor, since it implies that the concentration of PM mass will decrease with height.

A study carried out in the urban regions in the Central Amazon basin found out that as the height increased, the concentration of PM mass in the ambient air decreased.

In the study, it was found out that the concentration of particulate matter that is generated from vehicular emissions was decreasing as the elevation increased.

However, the rate of decline in concentration with the rise in elevation was the same for both PM2.5 as well as PM10.  

Temperature and climate conditions


Temperature and climate play an important role in regulation of air quality. Temperature and climate mainly regulate the mixing height of the atmosphere.

Mixing height refers to the vertical height at which the atmosphere is unstable due to the heating effect of the sun, which allows the vertical movement of particles and gaseous species within the air.

This implies that pollutants can only move up to a certain height, beyond which they stop. The mixing height varies depending on the temperature.

On colder days, the mixing height drops down, and vice-versa. Furthermore, climate regulates the mixing height too. Regions that have a warm and humid climate have a higher mixing height than that for cold and dry regions.

This implies that after a certain elevation, there will be a point where the air quality will be much better than the corresponding near surface air. 

An example can be taken of winter nights in a large city. As the ground starts cooling itself by releasing radiation, the near surface air gets warmed and moves above.

At a certain point, the vertical temperature profile of the air gets ‘inverted’ i.e., instead of the temperature decreasing as the height increases, the temperature starts to get warmer as the elevation increases.

This reduces the height of the mixing layer even further, causing the pollutants to get trapped in the near surface air. As a result, the near surface air has very poor air quality, while it starts increasing with the increase in height.

Sources of pollution

At ground level, the various sources of pollution, from both natural processes as well as human activities, is staggering in number. 

The biggest contributors to pollution near surface level, and how they fail to affect air quality at higher elevations, are discussed below.

Traffic

Traffic is one of the biggest contributors to near surface air pollution. Traffic related air pollution (TRAP) includes emissions from vehicular exhausts, such as nitrous oxides (NOx), sulphur dioxide (SO2), carbon monoxide (CO), PM, and so  on.

Furthermore, the secondary pollutants formed due to reaction between the various pollutants are also part of TRAP, and include gases such as ozone (O3), other forms of PM, and so on.

Finally, road dust that gets suspended due to vehicular activities is also a part of TRAP.

However, most of these pollutants originate near the surface, and by the time they attain a certain height based on their densities, they get carried away by winds at higher elevation.

Industrial emissions

Ever since its inception, industrialisation has introduced a variety of pollutants in the environment. Industries manufacture a variety of products, and certain products have different pollutants with varying potency.

Industries include steel and iron factories, coal-based thermal power plants (CTPPs), brick kilns, chemical and dye manufacturing factories, and much more.

These industries generate a significant volume of air pollutants, which include species such as SO2, NOx species, particulate matter with soot being one of the highest produced pollutant, VOCs, and so on.

The chimneys of industries are required to be at a certain height from the ground surface, in order to prevent ground air from getting contaminated.

However, these pollutants are relatively denser than air, hence moving to the surface. Particulate matter is also pulled towards the surface due to gravity.

Furthermore, the winds enhance the dissipation of these pollutants from the source i.e., chimneys. Therefore, under these factors, the dissipation of pollutants from higher elevations is done with relative ease.

A typical pollutant distribution profile will look like a bell-shaped curve, with the highest elevation being at the chimneys.

Solid fuel combustion

Globally, 2-3 billion people depend upon solid fuel for their energy requirements, mainly for heating purposes and cooking.

Solid fuels include coal, crop residue, firewood, dried dung, and so on.

Combustion of solid fuels generates a lot of PM, as well as gases such as CO, SO2, NOx, etc. However, these pollutants are restricted near the surface due to factors such as temperature, concentration, and density.

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

Does air quality affect blood pressure?

Does air quality affect asthma?

Does air quality improve after rain?

Conclusion

As elevation increases, the air quality increases. This is because of factors such as wind speed, density of pollutants, and temperature and climate conditions.

Lastly, we discussed how air quality for increase in elevation is regulated amidst the presence of primary sources of pollution.

FAQs

Is air quality in hilly areas better than that in areas at sea level?

In hilly areas, the extent of human activities determine the air quality. Naturally, there are no sources of pollution, hence the air quality in itself is clean.

However, in hilly areas, due to the lower air density, there is relatively less oxygen available. Therefore, for combustion, there would be a higher degree of inefficiency observed in the hilly area than in areas at sea level.

This in turn would increase the amount of PM and CO being generated from the process, therefore degrading the ambient air quality.

Hence, if we were to compare both regions, we would need to first see the extent of human activities. However, if we assume there is no human activity, then air quality of a hilly area will be better than the area near sea level.

Is it better to live near the ground or on top floors in a high rise building


While most people would choose the top floors from a purely aesthetic point of view, there is an attached advantage in terms of air quality as well.

At higher floors, there is less obstruction to the flow of natural wind. This will keep the house well-ventilated. 

Whereas in the ground floor, they are more susceptible to indoor air pollution, which can be caused by indoor sources as well as infiltration from outdoor sources, if they live near the road or an industrial complex.

References

  • A. Samad, U. Vogt, A. Panta, D. Uprety, Vertical distribution of particulate matter, black carbon and ultra-fine particles in Stuttgart, Germany, Atmospheric Pollution Research, Volume 11, Issue 8, 2020, Pages 1441-1450, ISSN 1309-1042, https://doi.org/10.1016/j.apr.2020.05.017.
  • Bisht, D.S.; Tiwari, S.; Dumka, U.C.; Srivastava, A.K.; Safai, P.D.; Ghude, S.D.; Chate, D.M.; Rao, P.S.P.; Ali, K.; Prabhakaran, T.; Panickar, A.S.; Soni, V.K.; Attri, S.D.; Tunved, P.; Chakrabarty, R.K.; Hopke, P.K. (2016). Tethered balloon-born and ground-based measurements of black carbon and particulate profiles within the lower troposphere during the foggy period in Delhi, India. Science of The Total Environment, 573(), 894–905. doi:10.1016/j.scitotenv.2016.08.185
  • Guimarães, Patrícia, et al. “Vertical Profiles of Atmospheric Species Concentrations and Nighttime Boundary Layer Structure in the Dry Season over an Urban Environment in Central Amazon Collected by an Unmanned Aerial Vehicle.” Atmosphere 11.12 (2020): 1371.
  • Cao, R.; Li, B.; Wang, H.-W.; Tao, S.; Peng, Z.-R.; He, H.-d. Vertical and Horizontal Profiles of Particulate Matter and Black Carbon Near Elevated Highways Based on Unmanned Aerial Vehicle Monitoring. Sustainability 2020, 12, 1204. https://doi.org/10.3390/su12031204

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