Effect of aerosols on the infrared transmission in Lakiala, Finland

Effects of atmospheric aerosols on radiative transfer are mainly studied because of their role in climate change. Aerosols also affect some technological applications by deteriorating range performance of electro-optical systems. This study focuses on the aerosol-induced attenuation of infrared radiation along a horizontal path. Measured attenuation values are compared with modeled ones and an attempt is made to understand the differences. Similar measurements have been done earlier; however, those measurements have not usually contained any information about the prevailing aerosol size distributions. Both measured and modeled aerosol extinction coefficients are studied as a function of different weather parameters (visibility, relative humidity and temperature). Measured size distributions are also investigated and they are compared with the size distribution assumed in MODTRAN4. Because measurements and extinction calculations contain some error sources, e.g. instrument errors and errors in aerosol growth factors, whose magnitudes are not exactly known, the total uncertainty was difficult to assess. Despite the uncertainty in the measurements, differences between measured size distributions and model size distributions were found. It appears that weather parameters do not offer the most feasible input data to model the aerosol extinction. In addition, aerosol extinction coefficients calculated from the measured size distributions were much lower than the measured and modeled values. The continuation of aerosol attenuation measurements in the future is of vital importance to obtain enough data for the analysis. Moreover, the accuracy of the measuring equipments should be comprehensively assessed.