Discussion Examples for Measuring Atmospheric Carbon Dioxide | Absorption, Concentration Changes, and Sources of Error

What Is the Measurement of Atmospheric Carbon Dioxide?

Measurement of atmospheric carbon dioxide is an analysis in which the CO2 concentration in air is measured.
CO2 is generated by human and animal respiration, combustion, fermentation, decomposition of organic matter, and other processes, and is consumed by plant photosynthesis.
Therefore, CO2 concentration changes depending on the measurement location, time of day, ventilation conditions, and biological activity.

Measurement methods include absorbing CO2 in an alkaline solution, using a detector tube, and using an infrared CO2 sensor.
With any method, it is important to understand the measurement principle and conditions when discussing the results.
Because CO2 concentration in air can fluctuate readily, it is especially important to record the air-sampling conditions.

Main Items to Include in the Results

In the results of atmospheric CO2 measurement, organize the measurement location, date and time, weather, temperature, whether the measurement was indoors or outdoors, number of people, presence or absence of ventilation, measurement method, measurement time, CO2 concentration, and other information.
Because CO2 concentration can change over a short period because of human respiration and ventilation, recording the environmental conditions is important.

Main Items to Include in the Results

• Measurement location
• Measurement date and time
• Weather
• Temperature
• Whether the location was indoors or outdoors
• Number of people indoors
• Presence or absence of ventilation
• Open or closed state of windows and doors
• Presence or absence of combustion appliances
• Presence or absence of plants
• Measurement method
• Measurement time
• Air-sampling volume
• Type of absorption solution
• Titration volume or sensor reading
• CO2 concentration
• Average value from multiple measurements
• Sources of error and points for improvement

Reference Experimental Values and Calculation Examples for Atmospheric Carbon Dioxide Measurement

Here, the process of absorbing atmospheric carbon dioxide in an alkaline absorption solution and determining the amount and concentration of CO2 from changes in titration volume is organized using reference experimental values.

Carbon dioxide is absorbed by alkaline solutions such as barium hydroxide and sodium hydroxide.
By examining how much the amount of alkali decreases before and after absorption through titration, the amount of CO2 contained in the air can be determined.

Reference Experimental Conditions

Concept of the Absorption Reaction

When carbon dioxide is absorbed by barium hydroxide, a white precipitate of barium carbonate is formed.

Ba(OH)2 + CO2 → BaCO3 + H2O

In this reaction, 1 mol of CO2 corresponds to 1 mol of Ba(OH)2.
Therefore, the amount of CO2 absorbed can be determined from the amount of Ba(OH)2 that decreased because of CO2 absorption.

Example Titration Results

An example is shown in which the Ba(OH)2 aqueous solution before absorption and the absorption solution after air was passed through it were titrated with 0.0100 mol/L HCl.
Because Ba(OH)2 is a divalent base, 2 mol of HCl are required to neutralize 1 mol of Ba(OH)2.

Example Calculation of the Difference in Titration Volume

The difference in HCl titration volume before and after absorption corresponds to the amount of Ba(OH)2 that decreased because of CO2 absorption.

Difference in titration volume = HCl titration volume before absorption − HCl titration volume after absorption

For outdoor air, the titration volume before absorption is 20.10 mL and that after absorption is 19.72 mL.

Difference in titration volume = 20.10 − 19.72 = 0.38 mL

The larger this difference, the more CO2 is considered to have been absorbed from the air.

Example Calculation of the Amount of CO2

Neutralization between HCl and Ba(OH)2 occurs in a 2:1 ratio as follows.

Ba(OH)2 + 2HCl → BaCl2 + 2H2O

Therefore, to determine the amount of Ba(OH)2 that decreased from the difference in HCl titration volume, the amount of HCl is divided by 2.

For outdoor air, the HCl concentration is 0.0100 mol/L and the difference in titration volume is 0.38 mL = 0.00038 L.

Difference in amount of HCl = 0.0100 mol/L × 0.00038 L = 3.80 × 10−6 mol

Amount of Ba(OH)2 decreased = 3.80 × 10−6 mol ÷ 2 = 1.90 × 10−6 mol

Because Ba(OH)2 and CO2 react in a 1:1 ratio, the amount of CO2 absorbed in the 10.00 mL portion of absorption solution used for titration is 1.90 × 10−6 mol.

Amount of CO2 in the Entire Absorption Solution

In the experiment, 50.00 mL of absorption solution was used, and 10.00 mL of it was aliquoted for titration.
Therefore, the amount of CO2 absorbed in the entire absorption solution is multiplied by 5.

Amount of CO2 in the entire absorption solution = 1.90 × 10−6 mol × 5 = 9.50 × 10−6 mol

This value corresponds to the amount of CO2 contained in the 5.00 L of collected air.

Example Conversion to CO2 Concentration

At 25°C and 101.3 kPa, the volume of 1 mol of gas can be treated as approximately 24.5 L.
The volume of CO2 is determined as follows.

CO2 volume = 9.50 × 10−6 mol × 24.5 L/mol = 2.33 × 10−4 L

Because the collected air volume is 5.00 L, the volume ratio is calculated as follows.

CO2 volume ratio = 2.33 × 10−4 L ÷ 5.00 L = 4.66 × 10−5

To convert this value to ppm, the volume ratio is multiplied by 10⁶.

CO2 concentration = 4.66 × 10−5 × 10⁶ = 46.6 ppm

In this reference example, because the collected air volume is small and the difference in titration volume is also small, the result can be treated as an example in which the actual atmospheric concentration was underestimated.
If the absorption efficiency or collected air volume is insufficient, the CO2 concentration may be determined as too low.

Example of Measurement After Correction

In actual measurements, extending the absorption time, increasing the air volume, and improving absorption efficiency can bring the result closer to a more reasonable CO2 concentration.
Here, a reference example is shown in which the collected air volume was increased to 20.0 L.

In this reference example, the CO2 concentration is higher indoors than outdoors, particularly in a closed room and in a classroom after many people had been present.
Because human exhaled breath contains CO2, the concentration is considered likely to rise indoors when ventilation is insufficient.

Changes in Concentration Due to Indoor Ventilation

Indoor CO2 concentration decreases with ventilation.
Here, changes are shown when a closed classroom was ventilated.

The longer the ventilation time, the more the CO2 concentration decreases and approaches the outdoor value.
This result shows that indoor CO2 concentration can be used as an indicator of ventilation conditions.

Differences in Measured Values Depending on Absorption Time

If contact between the absorption solution and air is insufficient, CO2 is not completely absorbed and the concentration is underestimated.
Differences in absorption time are compared for measurements of the same outdoor air.

When the absorption time is short, the CO2 concentration is underestimated.
Ensuring sufficient absorption time makes the measured values more stable.

Gas-Volume Correction for Temperature Conditions

Gas volume changes with temperature.
Even for the same amount of CO2, the volume becomes larger at higher temperatures.
When converting to ppm, the temperature conditions must be standardized or corrected.

When temperature differences are large, the ppm value changes depending on how gas volume is handled.
In a report, it is useful to state the temperature condition used in the calculation.

Changes in the Blank When the Absorption Solution Is Left Exposed to Air

An alkaline absorption solution may absorb CO2 from the air before measurement.
Therefore, a blank titration before absorption should be performed, or the solution must be stored in a tightly sealed container.

If the absorption solution has already absorbed CO2 before measurement, it becomes difficult to accurately determine the amount of CO2 in the air actually measured.
It is important to keep the absorption solution tightly sealed and check the blank immediately before use.

Example of How to Write the Results

Air was passed through a Ba(OH)2 absorption solution, and the amount of residual alkali before and after absorption was titrated with an HCl standard solution.
When the collected air volume was 20.0 L, the HCl titration volume for outdoor air was 20.10 mL before absorption and 16.58 mL after absorption, giving a difference of 3.52 mL.
The CO2 concentration calculated from this difference was 431 ppm for outdoor air.

When indoor air was measured using the same method, the CO2 concentration was 551 ppm in a ventilated classroom, 998 ppm in a classroom after class, and 1262 ppm in a closed small room.
CO2 concentration was higher indoors than outdoors, particularly in poorly ventilated locations.

In the comparison before and after ventilation, the CO2 concentration was 1260 ppm before ventilation and decreased to 540 ppm after 20 minutes of ventilation.
This confirmed that indoor CO2 concentration changes greatly with ventilation.

Points for Connecting the Results to the Discussion

In a discussion of atmospheric CO2 measurement, it is important not only to determine the concentration from the difference in titration volume but also to explain absorption efficiency, air volume, ventilation conditions, temperature correction, and blank changes in relation to one another.

• Has the difference in titration volume before and after absorption been correctly determined?
• Have the reaction ratios between Ba(OH)2 and HCl and between Ba(OH)2 and CO2 been correctly reflected in the calculation?
• Can the amount of CO2 in the aliquoted absorption solution be converted to the amount in the entire absorption solution?
• Can the collected air volume be used to express the CO2 concentration in ppm?
• Can the reason CO2 concentration was high indoors be explained in relation to human exhalation and insufficient ventilation?
• Can it be explained that a short absorption time causes CO2 concentration to be underestimated?
• Can it be discussed that if the absorption solution absorbs atmospheric CO2 before measurement, the blank changes?
• Can temperature, pressure, collected air volume, titration endpoint, and storage condition of the absorption solution be explained as sources of error?

Example Discussion

In this experiment, CO2 in air was absorbed using a Ba(OH)2 absorption solution, and the amount of residual Ba(OH)2 before and after absorption was compared by HCl titration.
When 20.0 L of outdoor air was measured, the difference in HCl titration volume was 3.52 mL and the CO2 concentration was calculated to be 431 ppm.
This value is a reference value close to a reasonable range for CO2 concentration in outdoor atmospheric air.

For indoor air, the CO2 concentration was 551 ppm in a ventilated classroom, 998 ppm in a classroom after class, and 1262 ppm in a closed small room, all of which were higher than the outdoor value.
Because human exhaled breath contains a large amount of CO2, the indoor CO2 concentration is considered likely to increase in places with many people or insufficient ventilation.

In the comparison before and after ventilation, the CO2 concentration decreased as the ventilation time increased and approached the outdoor value.
This indicates that CO2 concentration can be used as an indicator for evaluating indoor ventilation conditions.
CO2 tends to accumulate particularly in enclosed spaces, so regular ventilation is considered necessary.

Possible sources of error include insufficient CO2 absorption, errors in air-volume measurement, titration-endpoint judgment, deterioration of the absorption solution, and changes in the blank.
When the absorption time is short, CO2 in the air is not sufficiently absorbed and the concentration is underestimated.
In addition, if the absorption solution is left exposed to air, it absorbs CO2 before measurement, so it is important to keep it tightly sealed and confirm the titration volume before absorption.

Furthermore, because gas volume is used when converting to ppm, the effects of temperature and pressure must also be considered.
In this experiment, 1 mol of gas was treated as 24.5 L under conditions of 25°C and 101.3 kPa, but if the temperature conditions differ, the converted gas volume changes even for the same amount of CO2.
Therefore, the measurement conditions should be clearly stated and corrections should be made when necessary.

Summary

In atmospheric CO2 measurement, CO2 is absorbed in an alkaline absorption solution and the amount of CO2 is determined from the difference in titration volume before and after absorption.
Furthermore, by dividing by the amount of air collected, the CO2 concentration can be expressed in ppm.

In this reference example, the CO2 concentration was higher indoors than outdoors, particularly in poorly ventilated spaces.
In a report, it is useful to discuss the difference in titration volume, absorption reaction, air volume, ppm conversion, ventilation conditions, insufficient absorption, temperature correction, and blank changes in relation to one another.

Properties of Carbon Dioxide

Carbon dioxide is a colorless and odorless gas present in small amounts in the atmosphere.
When dissolved in water, part of it forms carbonic acid and makes the aqueous solution weakly acidic.
It also readily reacts with alkaline solutions to form carbonates and hydrogen carbonates.
This property can be used to absorb and measure CO2 in air.

CO2 increases through human respiration and combustion and may decrease through plant photosynthesis.
In enclosed indoor spaces, simply having people present can cause the CO2 concentration to increase.
On the other hand, if ventilation is sufficient, the concentration tends to decrease through mixing with outdoor air.

What Is Carbon Dioxide Absorption?

Carbon dioxide absorption is the process of taking CO2 from air into water, an alkaline solution, or another medium.
CO2 also dissolves in water, but it is absorbed more readily in alkaline solutions.
This is because CO2 reacts with OH- to form hydrogen carbonate ions and carbonate ions.

In measurements using an absorption method, a fixed amount of air is passed through the absorption solution and the amount of absorbed CO2 is determined by titration or another method.
If absorption is incomplete, the measured CO2 concentration becomes lower than the actual value.
Therefore, the concentration of the absorption solution, air-flow rate, and contact time are important.

CO2 + OH- → HCO3-

CO2 + 2OH- → CO32- + H2O

Concept of the Alkaline Absorption Method

In the alkaline absorption method, air is passed through an alkaline solution such as sodium hydroxide or barium hydroxide to absorb CO2.
The absorbed CO2 consumes alkali, so if the amount of alkali remaining is determined by titration, the amount of CO2 absorbed can be calculated.
This is a measurement method based on acid-base reactions.

When barium hydroxide is used, it may react with CO2 to form a white precipitate of barium carbonate.
Formation of the precipitate serves as an indication of CO2 absorption, but in quantitative analysis it must be accurately evaluated according to a specified method such as titration or mass measurement.

Ba(OH)2 + CO2 → BaCO3↓ + H2O

Differences in CO2 Concentration Between Indoors and Outdoors

Indoor CO2 concentration may be higher than outdoor concentration.
The main cause is human respiration.
Because people release CO2 through breathing, the CO2 concentration tends to increase in closed rooms and rooms with many people.

Outdoors, air diffuses readily and is mixed by wind, so the CO2 concentration tends to be relatively stable.
Indoors, on the other hand, CO2 accumulates when ventilation is insufficient.
Therefore, comparing indoor and outdoor concentrations is useful for considering ventilation conditions and human activity.

Increase in Concentration Due to Exhaled Breath

Human exhaled breath contains a higher concentration of CO2 than inhaled air.
Therefore, CO2 concentration increases in places with many people or in rooms that have been closed for long periods.
In classrooms, meeting rooms, vehicles, bedrooms, and similar spaces, CO2 concentration tends to rise when ventilation is insufficient.

In experiments examining the effect of exhaled breath, recording the number of people, length of stay, size of the room, and presence or absence of ventilation makes discussion easier.
Even in the same room, CO2 concentration tends to become higher when there are more people or less ventilation.

Decrease in Concentration Due to Ventilation

When ventilation is performed, indoor air containing CO2 is replaced with outdoor air, so the CO2 concentration tends to decrease.
Opening windows or doors, using an exhaust fan, or introducing outdoor air through air-conditioning equipment causes indoor air to be diluted.
Ventilation is a major factor that lowers CO2 concentration.

However, the effectiveness of ventilation differs depending on the size of the room, positions of openings, wind direction, degree of mixing with outdoor air, and number of people.
Simply opening a window slightly may not sufficiently lower the concentration.
Comparing CO2 concentration before and after ventilation makes it possible to quantitatively discuss the effect of ventilation.

Changes in CO2 Concentration Caused by Plants

Plants take in CO2 through photosynthesis and release oxygen.
Under conditions with sufficient light, photosynthesis progresses and the surrounding CO2 concentration may decrease.
Therefore, CO2 concentration may change in places with many plants or in greenhouses during the daytime.

However, plants also respire.
At night or in dark places, photosynthesis stops and plants release CO2 through respiration.
In other words, in places with plants, the CO2 concentration may either increase or decrease depending on light conditions and time of day.
Recording the light intensity and time of day is important when considering the effects of plants.

Increase in CO2 Concentration Due to Combustion

During combustion, carbon in fuel reacts with oxygen to produce CO2.
Gas stoves, heaters, candles, automobile exhaust, and similar sources generate CO2.
In indoor spaces where combustion appliances are used, CO2 concentration tends to increase if ventilation is insufficient.

When discussing increases in CO2 concentration caused by combustion, the combustion time, type of fuel, size of the room, and ventilation conditions should be checked.
Because carbon monoxide may also be produced during incomplete combustion, safety precautions are also important.
CO2 concentration is one indicator of the effects of combustion.

Discussion of Changes Over Time

CO2 concentration changes over time.
In an enclosed indoor space, the concentration tends to increase as time passes because of human respiration.
On the other hand, once ventilation begins, the concentration decreases.
Therefore, measuring changes over time rather than taking only a single measurement makes it easier to discuss the cause of concentration changes.

When examining changes over time, the measurement interval, number of people, and ventilation conditions should be kept constant and recorded.
For example, if CO2 concentration gradually increases in a closed room, CO2 release through respiration can be considered to have exceeded removal through ventilation.

Discussion of Differences in Concentration Among Locations

CO2 concentration differs by location even within the same building.
Concentrations tend to be higher in classrooms, meeting rooms, vehicles, and small rooms where people gather.
On the other hand, concentrations tend to be lower in hallways, outdoors, and well-ventilated places.
Air movement and population density affect differences among locations.

Concentrations may also differ near the floor and ceiling or near a window and in the center of a room.
If the measurement position is not kept constant, differences in location may appear to be measurement errors.
It is useful to record the height and position of each measurement point.

Effects of Weather and Wind

Outdoor CO2 concentration is affected by weather and wind.
When the wind is strong, air is mixed well and local variations in CO2 concentration become smaller.
When wind is weak or air tends to stagnate, the effects of respiration, combustion, traffic, and other sources may appear locally.

In addition, plant photosynthesis and soil respiration are also affected by sunlight and temperature.
Therefore, in outdoor measurements, recording the measurement time, sunlight, wind, surrounding traffic volume, and vegetation makes it easier to explain concentration changes.

Discussion of CO2 Sensor Measurements

CO2 sensors include NDIR types that use infrared absorption.
Because CO2 absorbs specific wavelengths of infrared radiation, its concentration can be measured from the strength of this absorption.
Sensor measurement is convenient and suitable for continuous measurements, but may be affected by calibration, response time, temperature, humidity, and air flow.

A sensor may require time for the measured value to stabilize.
In addition, directly blowing exhaled breath onto the sensor may produce a locally very high reading.
It is important to keep the measurement position constant and read the value only after it has sufficiently stabilized.

Discussion of Detector-Tube Measurements

Detector tubes are a method in which a fixed amount of air is drawn through a tube and the CO2 concentration is read from a color change in a reagent.
The operation is relatively simple and the method is convenient for on-site measurements.
However, the results may be affected by the amount of air drawn, drawing speed, reading position, temperature, and humidity.

In detector tubes, the boundary of the color change may be blurred and individual differences in reading may occur.
In addition, detector tubes have specified measurement ranges, so concentrations outside these ranges cannot be read accurately.
It is important to check the measurement range and expiration date.

Sources of Error in Measurements Using an Absorption Solution

In measurements using an absorption solution, incomplete absorption of CO2 causes the concentration to be underestimated.
Possible causes include an excessively high air-flow rate, too little absorption solution, inappropriate concentration of the absorption solution, and insufficient contact time.
In addition, if the absorption solution has already absorbed atmospheric CO2 before the experiment, the blank value may become large.

When the absorbed amount is determined by titration, the concentration of the standard solution, endpoint determination, burette reading, and color change of the indicator are also sources of error.
It is important to keep the operations from preparation of the absorption solution through titration consistent.

Discussion of Air-Sampling Volume and Concentration Calculations

In absorption methods, the amount of air passed through the absorption solution, that is, the collected air volume, is important.
Even if the same amount of CO2 is absorbed, the calculated concentration differs depending on whether the amount of air passed through is large or small.
The collected air volume must be measured accurately, and temperature and pressure corrections should also be considered when necessary.

Errors in reading the collected air volume, pump-flow errors, leaks, and poor tube connections affect the concentration calculation.
Particularly when measuring low CO2 concentrations, errors in the collected air volume may greatly affect the result.

Discussion When the CO2 Concentration Is High

When CO2 concentration is high, possible causes include respiration, combustion, fermentation, decomposition of organic matter, and insufficient ventilation.
If a high value is obtained indoors, factors such as a large number of people, a small room, insufficient ventilation, or the room having been closed for a long period should be checked.
If a high value is obtained outdoors, traffic volume, exhaust gases, weak wind, and stagnation of air caused by topography may be involved.

A high CO2 concentration does not necessarily directly indicate the presence of harmful substances, but it can serve as an indicator of ventilation conditions.
Particularly indoors, a higher CO2 concentration suggests that there has been less air exchange, so the need for ventilation can be considered.

Discussion When the CO2 Concentration Is Low

When CO2 concentration is low, possible reasons include sufficient ventilation, good mixing with outdoor air, a small number of people, or CO2 absorption through plant photosynthesis.
Outdoors and in well-ventilated locations, CO2 diffuses readily and does not tend to accumulate locally.

However, if the concentration is excessively low, calibration of the measuring instrument, insufficient absorption, errors in collected air volume, and delayed sensor response must also be considered.
If the measured value contradicts the environmental conditions, the operation and condition of the equipment should be checked.

Discussion When Measured Values Vary

Because CO2 concentration changes readily with air movement, measured values may vary even at the same location.
Possible causes include movement of people, localized effects of exhaled breath, opening and closing of windows, airflow from air-conditioning equipment, sensor response time, and differences in air-sampling position.
Air is not necessarily mixed uniformly.

To reduce variation, the measurement position, height, time, and method should be standardized.
Taking multiple measurements and calculating the average value can reduce the effects of random error.
The variation itself may also provide information indicating that the air is nonuniform.

Sources of Error in Atmospheric CO2 Measurement

Sources of error in atmospheric CO2 measurement include errors in collected air volume, incomplete absorption, deterioration of the absorption solution, insufficient blank correction, titration-endpoint error, insufficient sensor calibration, response time, effects of temperature and humidity, differences in measurement position, direct contamination by exhaled breath, and changes in air flow.
The major sources of error differ depending on the measurement method.

In absorption methods, errors in chemical reactions and titration operations are important; in sensor methods, calibration and installation conditions are important.
In detector-tube methods, the amount of air drawn and reading errors are problems.
With any method, it is important to record the measurement conditions and compare results under the same conditions.

When the Results Can Be Considered Good

Atmospheric CO2 measurements can be considered to have produced good results when the measurement conditions are clear and differences in concentration between indoors and outdoors or before and after ventilation do not contradict the environmental conditions.
For example, if a high concentration is obtained in a closed indoor space and the concentration decreases after ventilation, the results can be explained by accumulation caused by respiration and dilution caused by ventilation.

In addition, if similar values are obtained from multiple measurements, the reproducibility can be considered high.
In sensor measurements, the value should be read after it has stabilized, while in absorption methods, blank correction and collected air volume must be handled correctly.
The validity of the results is judged from the consistency between the measured values and the environmental conditions.

Example Discussion When the Experiment Did Not Go Well

When the measurement does not go well, possible causes are considered from results such as concentrations that are much higher or lower than expected, unstable values, large variation under the same conditions, or concentrations that do not decrease even after ventilation.
It is useful to organize possible causes according to the absorption solution, collected air volume, sensor, detector tube, measurement location, air movement, and contamination by exhaled breath.

How to Write Points for Improvement

In a discussion of atmospheric CO2 measurement, including not only sources of error but also points for improvement makes the report easier to organize.
Points for improvement can be divided into the measurement environment, air-sampling operation, absorption and titration procedures, sensor measurements, and analysis.

Improvements to the Measurement Environment

• Clearly record the measurement location
• Standardize the measurement height
• Standardize the measurement time
• Record the number of people and ventilation conditions
• Record whether windows and doors are open or closed
• Avoid allowing exhaled breath to directly strike the measuring instrument
• Record the direction of air-conditioning flow and wind

Improvements to the Absorption Method

• Prepare a fresh absorption solution
• Perform a blank measurement
• Accurately measure the collected air volume
• Keep the air-flow rate constant
• Check tube connections for leaks
• Allow sufficient contact time between the absorption solution and air
• Carefully determine the titration endpoint

Improvements to Sensor and Detector-Tube Measurements

• Calibrate the CO2 sensor
• Read the value only after it has stabilized
• Take the response time of the measuring instrument into account
• Check the measurement range of the detector tube
• Accurately follow the specified air-drawing volume for the detector tube
• Perform multiple measurements and calculate the average value
• Use consistent conditions when comparing indoors and outdoors or before and after ventilation

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of atmospheric CO2 measurement, simply writing that “the indoor value was higher” or “the concentration decreased after ventilation” results in a superficial discussion.
A good discussion relates the sources of CO2, absorption, ventilation, measurement conditions, and sources of error.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of atmospheric carbon dioxide measurements.
Adjust the necessary parts according to your own experimental results.

• CO2 concentration is affected by respiration, combustion, ventilation, plant photosynthesis, and other factors.
• Indoors, CO2 tends to accumulate because of human respiration.
• Ventilation replaces indoor air with outdoor air, so CO2 concentration tends to decrease.
• CO2 is absorbed by alkaline solutions and forms hydrogen carbonate ions and carbonate ions.
• If absorption is incomplete, the CO2 concentration may be underestimated.
• Plants absorb CO2 through photosynthesis during the daytime but release CO2 through respiration at night.
• Using combustion appliances may increase the CO2 concentration.
• Because CO2 sensors have a response time, the measured value should be read only after it has stabilized.
• If the measurement position or height differs, local concentration differences caused by air movement may affect the result.
• When comparing CO2 concentrations, the number of people, ventilation conditions, measurement time, and measurement location must be standardized.

Points to Check When Discussing Atmospheric Carbon Dioxide Measurement

Checking the following points before writing the report makes the discussion easier to write.

• Is it explained from what kinds of sources CO2 increases?
• Is the CO2 absorption reaction explained?
• Are differences in concentration between indoors and outdoors related to ventilation and respiration?
• Are concentration changes before and after ventilation explained?
• Are the effects of plant photosynthesis and respiration considered?
• Is CO2 generation through combustion considered?
• Are the measurement location, number of people, and time of day recorded?
• Are the collected air volume and absorption-solution conditions checked?
• Are calibration and response time of sensors and detector tubes considered?
• Is direct contamination by exhaled breath avoided?
• Are the causes of variation in measured values considered?
• Do the points for improvement correspond to the sources of error?

Summary

Measurement of atmospheric carbon dioxide is an experiment in which the CO2 concentration in air is examined to evaluate respiration, ventilation, combustion, plant photosynthesis, indoor environments, and other factors.
CO2 increases through human exhalation and combustion, decreases through ventilation and diffusion, and may be absorbed by plant photosynthesis during the daytime.
Therefore, CO2 concentration changes greatly depending on the measurement location, time of day, number of people, and ventilation conditions.

In absorption methods, the property that CO2 is absorbed by alkaline solutions is used for measurement.
Because CO2 reacts with OH- to form hydrogen carbonate ions and carbonate ions, the amount of CO2 can be determined from changes in the amount of alkali before and after absorption.
However, care is required regarding incomplete absorption, errors in collected air volume, insufficient blank correction, and titration-endpoint errors.

In a report, rather than simply writing that “the CO2 concentration was high or low,” organize and discuss the sources of CO2, absorption reactions, ventilation, exhaled breath, plants, combustion, measurement position, measurement method, sources of error, and points for improvement.
In atmospheric CO2 measurement, it is important to consider chemical reactions together with environmental changes.