Aerosol optical depth determination in the UV using a four-channel precision filter radiometer

Thomas Carlund,Stelios Kazadzis,Julian Gröbner,Natalia Kouremeti

doi:10.5194/amt-10-905-2017

Abstract

Abstract. The determination of aerosol properties, especially the aerosol optical depth (AOD) in the ultraviolet (UV) wavelength region, is of great importance for understanding the climatological variability of UV radiation. However, operational retrievals of AOD at the biologically most harmful wavelengths in the UVB are currently only made at very few places. This paper reports on the UVPFR (UV precision filter radiometer) sunphotometer, a stable and robust instrument that can be used for AOD retrievals at four UV wavelengths. Instrument characteristics and results of Langley calibrations at a high-altitude site were presented. It was shown that due to the relatively wide spectral response functions of the UVPFR, the calibration constants (V0) derived from Langley plot calibrations underestimate the true extraterrestrial signals. Accordingly, correction factors were introduced. In addition, the instrument's spectral response functions also result in an apparent air-mass-dependent decrease in ozone optical depth used in the AOD determinations. An adjusted formula for the calculation of AOD, with a correction term dependent on total column ozone amount and ozone air mass, was therefore introduced. Langley calibrations performed 13–14 months apart resulted in sensitivity changes of ≤ 1.1 %, indicating good instrument stability. Comparison with a high-accuracy standard precision filter radiometer, measuring AOD at 368–862 nm wavelengths, showed consistent results. Also, very good agreement was achieved by comparing the UVPFR with AOD at UVB wavelengths derived with a Brewer spectrophotometer, which was calibrated against the UVPFR at an earlier date. Mainly due to non-instrumental uncertainties connected with ozone optical depth, the total uncertainty of AOD in the UVB is higher than that reported from AOD instruments measuring in UVA and visible ranges. However, the precision can be high among instruments using harmonized algorithms for ozone and Rayleigh optical depth as well as for air mass terms. For 4 months of comparison measurements with the UVPFR and a Brewer, the root mean squared AOD differences were found < 0.01 at all the 306–320 nm Brewer wavelengths.

Highlights

One of the most important atmospheric processes related to solar ultraviolet (UV) attenuation is the absorption and scattering of solar radiation by aerosols (IPCC, 2013; Madronich et al, 2015; UNEP, 2010)
This paper reports on the UVPFR (UV precision filter radiometer) sunphotometer, a stable and robust instrument that can be used for aerosol optical depth (AOD) retrievals at four UV wavelengths
Very good agreement was achieved by comparing the UVPFR with AOD at UVB wavelengths derived with a Brewer spectrophotometer, which was calibrated against the UVPFR at an earlier date

Summary

Introduction

One of the most important atmospheric processes related to solar ultraviolet (UV) attenuation is the absorption and scattering of solar radiation by aerosols (IPCC, 2013; Madronich et al, 2015; UNEP, 2010). The effect of aerosols on solar UV radiation is important as it is linked with the impact on UV radiation on human health (Rieder et al, 2008; Cordero et al, 2009), atmospheric chemistry (e.g., Gerasopoulos et al, 2012) and the biosphere (Diffey, 1991). In heavily polluted areas, analysis of past data series shows that the decrease of UVB (wavelength range 280–315 nm) radiation due to aerosol attenuation can become larger than the expected increase of UVB radiation due to the declining ozone levels (e.g., Meleti et al, 2009; Zerefos et al, 2012; De Bock et al, 2014). T. Carlund et al.: Aerosol optical depth determination in the UV

Methods

Results

Conclusion