A review of the molecular diffusivities of H2O, CO2, CH4, CO, O3, SO2, NH3, N2O, NO, and NO2 in air, O2 and N2 near STP

Abstract

Accurate knowledge of the coefficients of molecular diffusivity (D) of trace gases has important application to global change, atmospheric chemistry, combustion science, studies of indoor air pollution and atmosphere-biosphere interactions. This study reviews and re-analyzes the historical data and some modeling results for D for H2O, CO2, CH4, CO, O3, SO2, NH3, N2O, NO, and NO2 in air, N2 and O2. Because CO and N2 are isosteric molecules the self-diffusion coefficient for N2 and O for N2 in O2 are also evaluated as an independent check on the determination of D for CO. Also included is the coefficient of self-diffusion of O2 (DO(2), O(2)) because an earlier model parameterized D for O3 in O2 in terms of DO(2),O(2). Because D is a function of temperature and pressure, all modeling results and data are corrected to 1 atm pressure and then used with a one- and two-parameter regression model to determine optimal values for the temperature exponent and the value of O at 0 °C. For most experimental data the loess regression technique is used to identify and eliminate outliers. To simplify comparisons and usage all results and recommendations are standardized to one regression curve. For some gases like H2O and CO2 present results are similar to some (but not all) values cited in current sources. However, the coefficient of diffusivity in air for some very important gases, such as O3, NO and NO2, have never been measured and therefore, remain relatively uncertain. Based on present recommendations it is suggested that the ratio of the diffusivities of water vapor and ozone should be 1.51 rather than the often cited 1.65.