Multi-instrument approach for measuring spectral aerosol absorption properties in UV and VIS wavelengths

The spectral dependence of light absorption by atmospheric particulate matter (PM) has major implications for air quality, surface ultraviolet (UV) radiation, and tropospheric oxidation capacity, but remains highly uncertain. Quantifying the spectral dependence of aerosol absorption at UV and visible wavelengths is important for the accurate air pollution characterization using current (e.g., Aura/OMI) and future (e.g., TROPOMI, TEMPO, GEMS) satellite measurements, photolysis rates calculations in chemical and aerosol transport models and surface radiation modeling. Measurements of column atmospheric absorption and its spectral dependence remain the most difficult part of atmospheric radiation measurements. Currently available ground measurements of spectral aerosol absorption properties (e.g., column effective imaginary refractive index (k), single scattering albedo, (SSA), and aerosol absorption optical depth (AAOD)) are limited to the cloud free conditions and few discrete wavelength bands in the visible spectral region by AERONET almucantar inversions. To address the lack of spectral aerosol and gaseous absorption measurements in the UV, a suite of complementary ground-based instruments, modified UV Multifilter Rotating Shadowband Radiometer (UV-MFRSR) was established in 2002 and is currently in use at NASA Goddard Space Flight Center (NASA/GSFC) in Greenbelt, Maryland. In addition, several field campaigns have been carried out to measure aerosol absorption properties in UV and VIS from different sources in different locations. In September-October 2007 biomass-burning season in the Amazon basin (Santa Cruz, Bolivia), light absorbing (chromophoric) organic or “brown” carbon (BrC) is studied with surface and space-based remote sensing. It is found that BrC has negligible absorption at visible wavelengths, but significant absorption and strong spectral dependence at UV wavelengths. Using the ground-based inversion of column effective imaginary refractive index (k) at UV wavelengths down to 305 nm, a strong spectral dependence of specific BrC absorption is quantified in the UV implying more strongly reduced ultraviolet B (UV-B) radiation reaching the surface. Reduced UV-B means less erythema, plant damage, but also a slower ozone photolysis rate. A photochemical box model is used to show that relative to black carbon (BC) alone, the combined optical properties of BrC and BC slow the net rate of production of ozone by up to 18% and lead to reduced concentrations of radicals OH, HO2, and RO2 by up to 17%, 15%, and 14%, respectively. The optical properties of BrC aerosol change in subtle ways the generally adverse effects of smoke from biomass burning. The objective of this thesis is to develop a new method to infer column effective spectral absorption properties (k, SSA, and AAOD) of PM using the ground-based measurements from AERONET in the visible wavelengths and UV-MFRSR in the UV and ozone and NO2 from ground-based (Pandora and Brewer) or satellite spectrometers, such as Ozone Monitoring Instrument (OMI) on NASA EOS Aura satellite. This represents the first effort to separate effects of gaseous (ozone and NO2) and aerosol absorption and partition black and brown (light-absorbing organic) carbon absorption in the short UV-B wavelengths. These measurements are essential to answer key science questions of the atmospheric composition and improve data products from the current and future satellite atmospheric composition missions

Comparison of PM2.5 in Seoul, Korea Estimated from the Various Ground-Based and Satellite AOD

Based on multiple linear regression (MLR) models, we estimated the PM2.5 at Seoul using a number of aerosol optical depth (AOD) values obtained from ground-based and satellite remote sensing observations. To construct the MLR model, we consider various parameters related to the ambient meteorology and air quality. In general, all AOD values resulted in the high quality of PM2.5 estimation through the MLR method: mostly correlation coefficients >~0.8. Among various polar-orbit satellite AODs, AOD values from the MODIS measurement contribute to better PM2.5 estimation. We also found that the quality of estimated PM2.5 shows some seasonal variation; the estimated PM2.5 values consistently have the highest correlation with in situ PM2.5 in autumn, but are not well established in winter, probably due to the difficulty of AOD retrieval in the winter condition. MLR modeling using spectral AOD values from the ground-based measurements revealed that the accuracy of PM2.5 estimation does not depend on the selected wavelength. Although all AOD values used in this study resulted in a reasonable accuracy range of PM2.5 estimation, our analyses of the difference in estimated PM2.5 reveal the importance of utilizing the proper AOD for the best quality of PM2.5 estimation

Impacts of Brown Carbon from Biomass Burning on Surface UV and Ozone Photochemistry in the Amazon Basin

The spectral dependence of light absorption by atmospheric particulate matter has major implications for air quality and climate forcing, but remains uncertain especially in tropical areas with extensive biomass burning. In the September-October 2007 biomass-burning season in Santa Cruz, Bolivia, we studied light absorbing (chromophoric) organic or brown carbon (BrC) with surface and space-based remote sensing. We found that BrC has negligible absorption at visible wavelengths, but significant absorption and strong spectral dependence at UV wavelengths. Using the ground-based inversion of column effective imaginary refractive index in the range 305368nm, we quantified a strong spectral dependence of absorption by BrC in the UV and diminished ultraviolet B (UV-B) radiation reaching the surface. Reduced UV-B means less erythema, plant damage, and slower photolysis rates. We use a photochemical box model to show that relative to black carbon (BC) alone, the combined optical properties of BrC and BC slow the net rate of production of ozone by up to 18 and lead to reduced concentrations of radicals OH, HO2, and RO2 by up to 17, 15, and 14, respectively. The optical properties of BrC aerosol change in subtle ways the generally adverse effects of smoke from biomass burning

Comparisons of Spectral Aerosol Single Scattering Albedo in Seoul, South Korea

Quantifying aerosol absorption at ultraviolet (UV) wavelengths is important for monitoring air pollution and aerosol amounts using current (e.g., Aura/OMI (Ozone Monitoring Instrument)) and future (e.g., TROPOMI (TROPOspheric Monitoring Instrument), TEMPO (Tropospheric Emissions: Monitoring of POllution), GEMS (Geostationary Environment Monitoring Spectrometer) and Sentinel-4) satellite measurements. Measurements of column average atmospheric aerosol single scattering albedo (SSA) are performed on the ground by the NASA AERONET (AEROsol robotic NETwork) in the visible (VIS) and near-infrared (NIR) wavelengths and in the UV-VIS-NIR by the SKYNET (SKY radiometer NETwork) networks. Previous comparison studies have focused on VIS and NIR wavelengths due to the lack of co-incident measurements of aerosol and gaseous absorption properties in the UV. This study compares the SKYNET-retrieved SSA in the UV with the SSA derived from a combination of AERONET, MFRSR (MultiFilter Rotating Shadowband Radiometer), and Pandora (AMP) retrievals in Seoul, South Korea, in spring and summer 2016. The results show that the spectrally invariant surface albedo assumed in the SKYNET SSA retrievals leads to underestimated SSA compared to AMP values at near UV wavelengths. Re-processed SKYNET inversions using spectrally varying surface albedo, consistent with the AERONET retrieval improve agreement with AMP SSA. The combined AMP inversions allow for separating aerosol and gaseous (NO2 and O3) absorption and provide aerosol retrievals from the shortest UVB (305 nanometers) through VIS to NIR wavelengths (870 nanometers)