A Twenty-First Century Molar Mass for Dry Air

Abstract

The audience for this paper includes researchers, educators, and engineers in the fields of air conditioning, atmospheric physics, meteorology, psychrometrics, standards, and thermodynamics. This paper provides a brief history of the molar mass of dry air, Mda, followed by the composition of Earth's atmosphere for the year 2008 and the calculation of Mda. A single equation is given to calculate Mda based on the actual abundance of CO2 in the atmosphere, which is currently increasing at an annual rate of 1.9 ìmol·mol−1. This causes an increase in the value of Mda at a rate of 0.0001 kg·kmol−1 (lb·lbmol−1) for every 8.33 ìmol·mol−1 increase in the abundance of CO2. It is practical for many calculations to use the average projected Mda value over a period of a half-century, during which time the value of Mda will increase by approximately 0.0010 kg·kmol−1 (lb·lbmol−1). For most psychrometric calculations, the authors recommend an Mda value of 28.966 kg·kmol−1 (lb·lbmol−1), which is projected for the year 2036. This value will be correct when rounded to three decimal places through 2058 if CO2 increases at its current rate.