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Measuring Carbon Dioxide in Outdoor Air

Measuring Carbon Dioxide in Outdoor Air

How to Measure CO2 Concentration in Air?

Measuring carbon dioxide levels in outdoor air is both a simple and complex exercise. One of the main problems with talking about regulating global CO2 levels is that scientists aren’t all in agreement on how to measure it.

At the simplest level, the monthly average global CO2 levels has been measured since 1958 at the Mauna Loa observatory in Hawaii. At Mauna Loa, the remote location, undisturbed air, and minimal influences of human activity and vegetation are ideal for monitoring CO2 levels over time.

The data collected at Mauna Loa indicates a continual rise in global CO2 levels from less than 320 ppm (parts per million) when measurements were first recorded to approximately 395 ppm today.

This video shows how scientists measure CO2 in outdoor air at the same place but at different altitudes using a manometer.


However, when it comes to measuring CO2 at the local, regional or country level, the science becomes more complicated. Carbon dioxide mixes quickly in air, which can hide "hot spots" of high levels of CO2. For local levels, grids of dozens of sensors are used to record CO2 concentrations. The challenge is the changing CO2 levels depending on the time of day, the time of year, and the varied weather conditions.

How do you measure CO2 levels outside?

To solve the problem of measuring CO2 levels on a local level, scientists have developed a 2-pronged approach.

The first approach is for researchers to predict all the CO2-creating objects in a given area, then extrapolate the amount of CO2 that is likely to be produced. For example, by adding up all the cars, buildings and power plants in an area, a theoretical model can be made for total CO2 produced over a given time period. This kind of theoretical model was popularized in a video by Dickinson and Tenorio entitled Inventory of New York City Greenhouse Gas Emissions.

The second approach is for researchers to actually measure CO2 levels at the micro scale. This involves laying down grids of dozens of CO2 sensors, then creating a real-time map of CO2 levels. One of the most ambitious projects like this has been undertaken by the Global Institute of Sustainability at Arizona State University.

Projects like this are completed or in process in dozens of major urban areas worldwide where most CO2 emission occurs. Sensors like the K33 ELG 1% Data Logging Sensor or the COZIR Ambient CO2 Sensor have been used in many of these projects due to their ability to record CO2 levels over long periods of time.

To date, the theoretical and measured models show as much as a 15% discrepancy between the results. This is too high an error to claim the CO2 measurement problem “solved.” Yet with continued testing in this area, an accurate model of CO2 levels seems to be within reach. It will then be up to governments to decide what to do with this information. For these reasons, the monitoring of air quality is critical, especially the concentrations of gases that pose a severe health risk. 

What is the normal range for CO2 in the outside air?

Elevated CO2 levels are only one aspect of outside air pollution. Particulates, nitrogen dioxide, hydrocarbons, carbon monoxide, and others all lead to  reduced air quality. The focus on carbon dioxide in the last few years has been the result of CO2's importance as a greenhouse gas. 

When we look at normal ranges of carbon dioxide levels outdoors near ground level, we find numerous studies that show 450-550ppm in concentration - although this may vary depending on different regions. For example, in high traffic metropolitan areas these levels can peak as high as 600-900ppm.

While these are normal outdoor CO2 ranges not harmful to humans, the following CO2 facts should also be considered:

  • The exhaust gas of a combustion engine contains 140,000ppm CO2
  • ASHRAE "unsafe" CO2 levels are recognized at 5,000ppm or higher
  • Locally elevated CO2 levels outdoors occur due to CO2 emissions from transportation, energy production and industrial manufacturing

CO2 effects on the human body

The chart below summarizes carbon dioxide concentrations at specific levels and the direct negative effects too high of concentration can cause to ones overall health.

  •  <1000 ppm
  • Limited or no severe health effects
  • 1000-2500ppm
  • Fatigue, loss of focus, uncomfortable
  • 2500-5000ppm
  • Headaches, drowsiness, fatigue
  • 5000-40,000ppm
  • Violates OSHA requirements, severe headaches, intoxication, nausea
  • 40,000-100,000ppm
  • Dizziness, increased heart rate, asphyxiation, sweating, seizures, loss of consciousness
  • >100,000ppm
  • Loss of consciousness within minutes, coma, risk of fatality

Measuring CO2 levels in Air Outdoors

eSense Outdoor CO2 Sensor

Knowing outdoor CO2 levels helps to assess indoor CO2 levels. If a building is located in an area with an average CO2 level 200ppm higher than normal outdoors, this will change the HVAC air handling requirements indoors. For this reason, most IAQ and LEED requirements have been rewritten to specify indoor CO2 levels as relative to outdoor levels instead of using the old global average of 400ppm as a guide.

In order to measure outdoor CO2 levels  accurately a specialized outdoor CO2 sensor can be used. For example, the SenseAir eSense is designed specifically to measure CO2 concentrations outdoors in any weather condition. The built-in heating element maintains a consistent temperature during air samples. This provides users with a consistent, accurate, and guaranteed measurement over time. 

For more information on sources of carbon dioxide, effects of CO2 on your health, CO2 emissions, indoor air quality or carbon dioxide measurement technologies - visit our blog for more information.

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