COMPARISON OF AIR-MASS COMPUTATIONS

Knowledge of air mass is vital for the interpretation of twilight meas urements of trace gases, as well as the conversion of measured slant c olumn amounts to vertical abundances for comparison with model predict ions. Radiative transfer computations were used to determine NO2 air m ass values for clear skies at 450 and 650 nm using a discrete ordinate (two different formulations), Monte Carlo, and an integral equation m ethod, All four methods yielded agreement to within 6% at a solar zeni th angle of 90 degrees when the absorber was located in the stratosphe re, For a tropospheric absorber, differences as large was 21% occurred at 90 degrees. Since only the Monte Carlo method treats the scattered radiation in spherical geometry, it is more accurate for computing tr opospheric air masses where multiple scattering is significant. The ot her three models use a conceptual approximation by treating the scatte red radiation in plane parallel geometry, However, for absorbers in th e stratosphere, major saving of computing time without any loss of acc uracy is obtained using the discrete ordinate or integral equation met hod as compared to the Monte Carlo method.