Prediction of atmospheric extinction coefficient and comparison of transmission measurement methods: Black target contrast vs. aerosol scattering calculations

Path integrated atmospheric transmittance over a 5.5 km horizontal path is measured using black target contrast. Measurements of on-line particulate distributions by Particulate Measuring System instrumentation (PMS) and of meteorological parameters are also made. The extinction coefficients, primarily scattering, of aerosols are calculated using the PMS data, and those arising from molecular absorption are calculated by LOWTRAN7. Both extinction coefficients, the directly measured path integrated and those calculated from particulate distribution and meteorological parameters near the receiver, are compared. Good agreement exists especially when relative humidity is low, despite the fact that the second method involves aerosol size distribution by data collected from a single point along the atmospheric path. Disagreement between both methods under high values of relative humidity can be explained by classification errors of the PMS instrumentation because of changes in the index of refraction of particles in a humid environment. Statistical regression analysis is made, relating the measured transmittance values to on-line meteorological data. A simple and accurate model is obtained to predict dependence of extinction coefficient on weather. Observation of the regression coefficients shows that also other meteorological parameters in addition to humidity are responsible for the changes in the atmospheric scattering coefficients. This contradicts the LOWTRAN models for a continental atmosphere.