Reply on RC1

Abstract

With the advent of spaceborne instruments in geostationary constellation, measuring high-spectral ultraviolet–visible (UV–Vis) and selected near-/shortwave-infrared (NIR/SWIR) radiances can enable probing the lifecycle of key atmospheric trace gases and aerosols at higher temporal resolutions over the globe. The UV-Vis measurements are important for retrieving several key trace gases (e.g., O3, SO2, NO2, HCHO) and particularly for deriving aerosol characteristics (e.g., aerosol absorption and vertical profile). This study examines the merit of simultaneous retrievals of trace gases and aerosols using a ground-based spectroradiometer covering the UV–NIR to monitor their physicochemical processes, and to obtain reliable aerosol information for various applications. During the 2019 pre-monsoon season over northern Thailand, we deployed a ground-based SMART–s (Spectral Measurements for Atmospheric Radiative Transfer–spectroradiometer) instrument, which is an extended-range Pandora with reliable radiometric calibration in 330–820 nm range, to retrieve remotely sensed chemical and aerosol properties for the first time near biomass-burning sources. The high spectral-resolution of Sun and sky measurements from SMART–s provides several key trace gases (e.g., O3, NO2, and H2O) as well as aerosol properties covering the UV where significant light-absorption occurs by the carbonaceous particles. During the measurement period, highly correlated total column amounts of NO2 and aerosol optical thickness (𝜏aer) retrieved from the SMART–s (correlation coefficient, R = 0.74) indicated their common emissions from biomass-burning events. The SMART-s retrievals of spectral single-scattering albedo (ω0) of smoke aerosols showed an abrupt decrease in the UV, which is an important parameter dictating photochemical processes in the atmosphere. The values of ω0 and column precipitable water vapor (H2O) gradually increase with the mixing of biomass-burning smoke particles and higher water vapor when approaching the monsoon season. The retrieved ω0 and weighted-mean-radius of fine-mode aerosols from the SMART–s showed positive correlations with the H2O (R = 0.81 for ω0 at 330 nm and 0.56 for volume-weighted-mean-radius), whereas the real-part of the refractive-index of fine-mode aerosol (nf) showed negative correlations (R = −0.61 at 330 nm), which suggest that aerosol aging processes including hygroscopic growth (e.g., humidification and cloud processing) can be a major factor affecting temporal trends of aerosol optical properties. Retrieved nf and ω0 were closer to those of the water droplet (i.e., nf of about 1.33 and ω0 of about 1.0) under lower amounts of NO2 during the measurement period; considering that the NO2 amounts in the smoke may indicate aging of the plume after emission due to its short lifetime, the tendency is also consistent with active hygroscopic processes of the aerosols over this area. Retrieved UV aerosol properties from the SMART–s generally support the assumed smoke aerosol models (i.e., spectral shape of aerosol absorption) used in current NASA’s satellite algorithms, and their spectral ω0 retrievals from ground and satellites showed good agreements (R = 0.73–0.79). However, temporal and spectral variabilities of the aerosol absorption properties in the UV emphasize the importance of realistic optical model of aerosols for further improvements of satellite retrievals.