INTRODUCTION
The measurement of carbon dioxide in the ambient air is the primary concern of geophysicists, rather than of the air pollution control engineer. The precise measurement of the carbon dioxide content of the atmosphere is of significant concern in determining long-term changes in the composition of the atmosphere. The measurement techniques used by geophysicists are highly precise compared to the techniques used
for air quality- or air pollution-related measurements. Air quality-related measurements can be used to monitor the return air quality from occupied spaces and, based on those measurements, to modulate the rate at which fresh ai  is being introduced. In air pollution-related applications, carbon dioxide is hardly ever measured in the ambient air. It is usually measured at emission points since some combustion equipment regulations are stated in terms of allowable pollutant discharges corrected to 50% excess air.
AMBIENT AIR MEASUREMENT

The precise knowledge of the carbon dioxide concentration in the atmosphere is necessary. An increase in global carbon dioxide concentration of only 1% (or about 3 ppm) has significant consequences on the weather. The instruments used to measure atmospheric carbon dioxide concentrations must, of necessity, be highly precise, such as the gas filter correlation (GFC) units discussed later. Nondispersive Infrared Type These instruments are discussed in detail in Section 8.27  the absorption bands of water and carbon dioxide somewhat overlap, a freeze-out trap  often used in the sample preparation system to remove the  wate When using prepared calibration standard mixtures of carbon dioxide in nitrogen, an inaccuracy of ± 0.2 ppm is attainable. At normal atmospheric CO 2 concentration levels of approximately 314 ppm, this error is equivalent to ± 0.06%. Nondispersive infrared (NDIR) type CO 2 monitors for heating, ventilation, and air conditioning (HVAC) and industrial applications are available in both portable and permanently
installed designs. The portable units are usually battery operated,= and their ambient sample is received by a combination of diffusion and convection effects in the sensor head, without any pumps or filtering.