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A twenty-first century molar mass for dry air.

Article Excerpt
INTRODUCTION--[M.sub.da] VALUES FROM 1945 TO 2005

Researchers, practitioners, and educators in the fields of agricultural and food science engineering, air conditioning, atmospheric physics, drying and dehumidification, gas turbines, compressors and expanders, meteorology, psychrometrics, and standards make numerous psychrometric (moist air) calculations that are based in part on the molar mass of dry air. Dry air is a mixture of nitrogen, oxygen, argon, [CO.sub.2], and eight or more minor constituents called trace gases. The molar mass of dry air is calculated as the sum of the products of the mole ratio of each gas times its molar mass.

In the last half of the twentieth century, the following changes took place that resulted in an increase in the molar mass of dry air:

* The scientific community changed from the Oxygen-16 to the Carbon-12 reference for the molar mass of elements and compounds in 1960.

* The molar masses of the basic chemical elements were updated by the International Union of Pure and Applied Chemistry (IUPAC) (Wieser 2005).

* [CO.sub.2] in the atmosphere has increased from 314 [[mu]mol.[mol.sup.-1]]] (~1955) to 379 [[mu]mol.[mol.sup.-1]] (Keeling and Whorf 2005a, 2005b). The 65 [[mu]mol.[mol.sup.-1]] increase in [CO.sub.2] in this time span is accompanied by a decrease in [O.sub.2] because combustion and respiration processes combine a carbon atom with [O.sub.2] from the atmosphere to produce [CO.sub.2] (Park et al. 2004).

* The stated argon mole fraction in air has changed from 9340 [[mu]mol.[mol.sup.-1]] at the start of the twentieth century to 9170 [[mu]mol.[mol.sup.-1]] at mid-century to 9332 [[mu]mol.[mol.sup.-1]] (Park et al. 2004). NASA apparently did not accept the mid-century value, as the U.S. Standard Atmosphere 1976 document (NOAA/NASA 1976) used the 9340 [[mu]mol.[mol.sup.-1]] value.

* With more accurate calculations and data, the scientific community has revised their best estimate of the universal gas constant from 8.31441 [J.[mol.sup.-1].[K.sup.-1]] to 8.314510 and finally to [8.314472 (1545.349 ft.lb.[mol.sup.-1].[degrees][R.sup.-1])], which is the value recommended by the Committee on Data for Science and Technology (of the International Council for Science headquartered at Paris, France) (Mohr and Taylor 1999) and is now officially listed by IUPAC and the National Institute of Standards and Technology (NIST).

A sampling of dry-air molar mass values used by researchers over the last 30 years is shown in Figure 1. Different values result from different assumed or measured compositions for atmospheric air. Some models of Earth's atmosphere include [CO.sub.2] and others do not. [CO.sub.2] is the fourth most abundant atmospheric gas and is currently increasing at an annual rate of approximately [1.9 [mu]mol.[mol.sup.-1]/year] (Keeling and Whorf 2005a, 2005b). The current rate of increase in [CO.sub.2] results in an increase in the molar mass of dry air of 0.0001 [kg.[kmol.sup.-1]] (lb.[lbmol.sup.-1]) every four to five years. (Note: Throughout the rest of this paper, the units associated with molar mass values have been omitted.)

[FIGURE 1 OMITTED]

The NIST Boulder (Lemmon et al. 2000; Lemmon and Jacobsen 2004) and the VDI-4670 (VDI 2003) models of dry air use similar but not identical atmospheric air models made up of the three gases: nitrogen, oxygen, and argon. Neither contain [CO.sub.2]. Most of the remaining models include 314 [[mu]mol.[mol.sup.-1]] of [CO.sub.2], which is representative of the middle of the twentieth century.

REASONS FOR UPDATING THE MOLAR MASS OF DRY AIR

The effect of using an [M.sub.da] value of 28.966 [kg.[kmol.sup.-1]] (lb.[lbmol.sup.-1]) for psychrometric calculations will...

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