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Carbon dioxide as an outdoor air quality monitor?

The term air quality includes most aspects concerning health and comfort of ambient air. When it comes to outdoor air it is often unhealthy substances such as particles, nitrogen dioxide, hydrocarbons, carbon monoxide or ozone that are considered. Most of these originate from combustion sources that also generates large amounts of carbon dioxide.

The local air quality is dependent not only on momentary local emissions, but also on current ventilation, i.e. metrological conditions. In conditions with little air exchange such as low winds, low convection or inversion layers “putting a lid” on top of an area allows for accumulation of polluting agents. Under such conditions also carbon dioxide accumulates and hence there is a direct correlation between local carbon dioxide level and pollution originating from combustion sources such as traffic or industry. 

One can of course measure each polluting substance one by one using dedicated sensors. However, these are often not maintenance free, low-cost and reliable. For instance, particle sensors need regular cleaning of the optics and are not known for being consistent in their reading. Nitrogen dioxide sensors are known to drift over time and need to be calibrated and replaced on a regular basis. Further on, you miss all the polluting substances you do not install a specific sensor for.

Carbon dioxide sensors are known to be reliable and accurate, maintenance free and they do not require calibration. They can be used to effectively measure the ratio between combustion related emission and local ventilation condition, and thus monitor the local air quality.

The correlation factor between the local carbon dioxide value and the specific pollutants are of course dependent on the source of pollution. For instance diesel engines produce more nitrogen dioxide compared to gasoline engines. The correlation will thus be different in different cities and somewhat different from season to season. The exact correlation coefficients can of course be locally monitored at one site in a city and be statistically used for estimates of polluting agents at all other places where only carbon dioxide is monitored. However, a high carbon dioxide reading is always a sign of high emissions in relation to the momentary air exchange and thus an indication of high risk for pollutants. Similarly, a low carbon dioxide level, close to the background level, is a proof that pollutants are not accumulated and thus a guarantee for fresh air. 

 

Using carbon dioxide as a tracer gas is not new. It is the standard for indoor air quality measurements. Actually it is viruses, volatile organic compounds, bacteria, odors etc. that are problematic with indoor air. Sensors for all these different substances are not available as maintenance free, reliable and low cost. Instead carbon dioxide is used as the traces gas since it is a well-defined portion of peoples exhaled breath. High indoor carbon dioxide level is a measure of insufficient ventilation and an indication of Sick-Building Syndrome. 

Carbon dioxide level can be used in the same way as an effective and reliable instant measure of the local outdoor air quality. 

Carbon dioxide level monitored during a walk close to Gullmarsplan in Stockholm, Sweden. In the low traffic street the carbon dioxide level is close to the fresh-air background level of 400 ppm. Close to a traffic light crossing the levels exceed 700 ppm and close to Nynäsvägen the level is about 600 ppm. 

 

Facts:

  • Fresh-air contains about 400 ppm CO2. 
  • The exhaust gas of combustion engine contains about 140 000 ppm CO2.
  • Human breath contains about 50 000 ppm CO2. 
  • Indoor ventilation is well-balanced at a level of 1 000 ppm CO2. Levels above 1 500 ppm is a result of insufficient ventilation.

 

Science:

http://www.who.int/mediacentre/factsheets/fs313/en/

http://researchprofiles.herts.ac.uk/portal/en/publications/high-density-air-quality-network-at-cambridge-and-london-heathrow-airport(48db1317-3ef0-496d-8346-f483e0afdef4)/export.html

http://adsabs.harvard.edu/abs/2016EGUGA..1816213O

 

For more information please contact:

Henrik Rödjegård SENIOR SCIENTIST, PH.D., M.SC. E.E. 
henrik.rodjegard@senseair.com