Abstract. A new photometer is described for continuous determination of the aerosol light absorption coefficient, optimized for long-term studies of the climate-forcing properties of aerosols. Measurements of the light attenuation coefficient are made at blue, green, and red wavelengths, with a detection limit of 0.02 Mm −1 and a precision of 4 % for hourly averages. The uncertainty of the light absorption coefficient is primarily determined by the uncertainty of the correction scheme commonly used to convert the measured light attenuation to light absorption coefficient and ranges from about 20 % at sites with high loadings of strongly absorbing aerosols up to 100 % or more at sites with low loadings of weakly absorbing aerosols. Much lower uncertainties (ca. 40 %) for the latter case can be achieved with an advanced correction scheme.

[1] Twenty-eight years of surface solar irradiance observations at five remote climate observatories are analyzed for long-term temporal variations and spatial representativeness. Autocorrelated temporal variations are suggested in the annual mean irradiance records at some of the sites and in normalized anomaly records for various combinations of the surface sites. These variations are illustrated by first-, second-, and third-degree polynomials as well as by a sinusoidal fit, all of which are used as data smoothers not necessarily representative of any related physical processes, although some fits yield statistically significant coefficients. In general, there is an indication of a decreasing tendency in the early portion of the records and then an increasing tendency until toward the very end of the record. Variations among field sites are seen, with the one Arctic site showing the most departure from the others, with primarily a downward trend for most of the record. The spatial representativeness of each site's annual mean record is investigated using spatial cross correlation with satellite-derived global estimates of the surface solar irradiance. These cross correlations show significant correlations with relatively large areas both contiguous and noncontiguous with the sites.

No abstract

Abstract. This study presents the results of the Fourth Filter Radiometer Comparison that was held in Davos, Switzerland, between 28 September and 16 October 2015. Thirty filter radiometers and spectroradiometers from 12 countries participated including reference instruments from global aerosol networks. The absolute differences of all instruments compared to the reference have been based on the World Meteorological Organization (WMO) criterion defined as follows: 95% of the measured data has to be within 0.005 ± 0.001∕m (where m is the air mass). At least 24 out of 29 instruments achieved this goal at both 500 and 865 nm, while 12 out of 17 and 13 out of 21 achieved this at 368 and 412 nm, respectively. While searching for sources of differences among different instruments, it was found that all individual differences linked to Rayleigh, NO2, ozone, water vapor calculations and related optical depths and air mass calculations were smaller than 0.01 in aerosol optical depth (AOD) at 500 and 865 nm. Different cloud-detecting algorithms used have been compared. Ångström exponent calculations showed relatively large differences among different instruments, partly because of the high calculation uncertainty of this parameter in low AOD conditions. The overall low deviations of these AOD results and the high accuracy of reference aerosol network instruments demonstrated a promising framework to achieve homogeneity, compatibility and harmonization among the different spectral AOD networks in the near future.