Estimating Cloud and Aerosol UV Modification Factors Based on Spectral Measurement from the Brewer Spectrophotometer

Sang Park,Yun Lee,Hana Lee,Hanlim Lee,Migyoung Kim,Sang-Min Kim

doi:10.3390/atmos8060109

Abstract

Cloud and aerosol modification factors are investigated in the spectral range of ultraviolet (UV) to correct for cloud and aerosol extinction effects from clear sky irradiance. The cloud modification factor (CMF) and aerosol modification factor (AMF) are estimated using radiative transfer model (RTM) simulations and ground-based observations in Seoul, Korea. The modification factors show a spectral dependence within the spectral range of 300 to 360 nm, which is the range used to estimate erythemal UV. The CMF and AMF values are estimated with high spectral resolution with considerations of solar zenith angle (SZA), cloud/aerosol amount, and total ozone variation. From the simulation studies, the variation in the CMFs within the spectral range of 300–360 nm is estimated to be 0.031–0.055, which is twice as large as the decrease in CMFs resulting from a SZA increase of 10°. In addition, the CMFs estimated from observational data show significant spectral dependence, varying from 2.5% to 10.0%. Because of the small aerosol optical depth (AOD) value, however, the variation in the AMF calculated from simulations is estimated to be between 0.007 and 0.045, indicating lower spectral dependence than the CMF. Furthermore, the spectral difference in the AMF calculated from observational data is insignificant relative to the daily-averaged total ozone error and uncertainties related to the reference irradiance spectrum under aerosol-free conditions.

Highlights

From the top of the atmosphere (TOA) to the terrestrial surface, ultraviolet (UV) radiance is diminished by Rayleigh scattering from air molecules, stratospheric ozone absorption, and Mie scattering from large particles in the troposphere
UV radiation is transmitted to the troposphere, and is affected by the strong ozone
Because the extinction intensity for direct radiation is affected by the spectral dependence of Rayleigh scattering, decreasing rates of cloud modification factor (CMF) with solar zenith angle (SZA) are larger at longer wavelengths

Summary

Introduction

From the top of the atmosphere (TOA) to the terrestrial surface, ultraviolet (UV) radiance is diminished by Rayleigh scattering from air molecules, stratospheric ozone absorption, and Mie scattering from large particles in the troposphere (i.e., clouds and aerosols). To quantify the intensity of UV irradiance variation from clear-sky conditions to non-clear conditions, several previous studies empirically estimated the values for UV irradiance modification by cloud cover change [25,26,27] or cloud optical depth [28], based on UV observational data. UV attenuation by aerosols is related to its single scattering albedo (SSA), size distribution, and aerosol optical depth (AOD) For this reason, the modification factors for clouds and aerosols have spatial and temporal dependencies. A previous study calculated correction factors for the EUV by considering the spectral dependence [30], these factors are estimated using only broadband-based classification (UV-A and UV-B) based on the climatology value of aerosol and cloud optical properties as an input parameter for the radiative transfer model. The aerosol and cloud modification factors from observation are compared with those calculated using the radiative transfer model simulation

Observation Data

Model Simulation

Spectral Modification Factor

CorrectionBefore for Ozone

Spectral dependenceofofCMF

Discussion and Conclusions