A REVIEW OF THE MOLECULAR DIFFUSIVITIES OF H2O, CO2, CH4, CO, O-3, SO2, NH3, N2O, NO, AND NO2 IN AIR, O-2 AND N-2 NEAR STP

Accurate knowledge of the coefficients of molecular diffusivity (D) of trace gases has important application to global change; atmospheric c hemistry, combustion science, studies of indoor air pollution and atmo sphere-biosphere interactions. This study reviews and re-analyzes the historical data and some modeling results for D for H2O, CO2, CH4, CO, O-3, SO2, NH3, N2O, NO, and NO2 in air, N-2 and O-2. Because CO and N -2 are isosteric molecules the self-diffusion coefficient for N-2 and D for N-2 in O-2 are also evaluated as an independent check on the det ermination of D for CO. Also included is the coefficient of self-diffu sion of O-2 (D-O2,D-O2) because an earlier model parameterized D for O -3 in O-2 in terms of D-O2,D-O2. Because D is a function of temperatur e and pressure, all modeling results and data are corrected to 1 atm p ressure and then used with a one-and two-parameter regression model to determine optimal values for the temperature exponent and the value o f D at 0 degrees C. For most experimental data the loess regression te chnique is used to identify and eliminate outliers. To simplify compar isons and usage all results and recommendations are standardized to on e regression curve. For some gases like H2O and CO2 present results ar e similar to some (but not all) values cited in current sources. Howev er, the coefficient of diffusivity in air for some very important gase s, such as O-3, NO and NO2, have never been measured and therefore, re main relatively uncertain. Based on present recommendations it is sugg ested that the ratio of the diffusivities of water vapor and ozone sho uld be 1.51 rather than the often cited 1.65. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.