Wavelength-dependent extinction statistics based on long-term aerosol size distribution measurements

Variations in aerosol particle size distributions introduce a time-varying wavelength-dependent scattering probability for optical radiation. Mie calculations on experimental particle data are used to generate extinction coefficients as a function of time and wavelength. Long-term aerosol and meteorological data were collected from the atmospheric and ecosystem research station Norunda, located north of Uppsala in the southern part of the boreal forest zone, in Sweden. For particle size distribution measurements subjected to an aerosol dryer, a growth factor was introduced to account for the hygroscopic effects in particles as a function of relative humidity. Scattering cross-sections were calculated based on Mie theory and translated to extinction coefficients for different wavelengths from the visible to the long-wavelength infrared regions. Visibility and infrared to visual-ratios based on the extinction coefficients have been calculated. Statistics of the variation with close to a year of data is analyzed.