Estimation of seasonal variability of aerosol radiative forcing obtained from measurements of optical properties of atmospheric aerosols at the ZOTTO station

The aerosol radiative forcing efficiency (RFE) for the atmosphere of central Siberia is estimated from measurements of aerosol scattering and absorption coefficients performed at the background station ZOTTO (60.80°N, 89.35°E) during 2007-2024.  Aerosol scattering coefficient, was measured by a TSI 3563 nephelometer at wavelengths of 450 nm, 550 nm, and 700 nm. The aerosol absorption coefficient was measured with a PSAP photometer at a wavelength of 574 nm. Hourly time series of the aerosol scattering and absorption coefficients integrated over the visible spectral region, as well as the backscatter fraction and the single scattering albedo were obtained by  primary data processing. The upscatter fraction  were calculated from the phase function asymmetry, taking into account the zenith angle of the Sun at the time of measurements. MERRA-2 reanalysis data including  the atmospheric transmittance function, cloudiness and  surface albedo were used to calculate aerosol forcing. The RFE time series obtained in this way for ZOTTO station demonstrates a strong inter-day variability but a clearly pronounced seasonal cyclicity. In spite of the fact that the maximum concentrations of absorbing (soot) aerosol and, correspondingly, the maximum values of the aerosol absorption coefficient are observed in the summer months, the aerosol forcing efficiency during this period is negative, the characteristic RFE value is - 30 W/m².  In winter, when the aerosol concentration and aerosol optical coefficients are significantly lower, the aerosol forcing efficiency is positive and equal to about +25 W/m².  The average RFE over the measurement period is equal to -5 W/m². The change of the sign of aerosol forcing from positive to negative occurs in early May, and the reverse sign occurs in late October.