Abstract
Abstract. The purpose of this study is to demonstrate the role of aerosol layer height (ALH) in quantifying the single scattering albedo (SSA) from ultraviolet satellite observations for biomass burning aerosols. In the first experiment, we retrieve SSA by minimizing the near-ultraviolet (near-UV) absorbing aerosol index (UVAI) difference between observed values and those simulated by a radiative transfer model. With the recently released S-5P TROPOMI ALH product constraining forward simulations, a significant gap in the retrieved SSA (0.25) is found between radiative transfer simulations with spectral flat aerosols and those with strong spectrally dependent aerosols, implying that inappropriate assumptions regarding aerosol absorption spectral dependence may cause severe misinterpretations of the aerosol absorption. In the second part of this paper, we propose an alternative method to retrieve SSA based on a long-term record of co-located satellite and ground-based measurements using the support vector regression (SVR) approach. This empirical method is free from the uncertainties due to the imperfection of a priori assumptions on aerosol microphysics seen in the first experiment. We present the potential capabilities of SVR using several fire events that have occurred in recent years. For all cases, the difference between SVR-retrieved SSA and AERONET are generally within ±0.05, and over half of the samples are within ±0.03. The results are encouraging, although in the current phase the model tends to overestimate the SSA for relatively absorbing cases and fails to predict SSA for some extreme situations. The spatial contrast in SSA retrieved by radiative transfer simulations is significantly higher than that retrieved by SVR, and the latter better agrees with SSA from MERRA-2 reanalysis. In the future, more sophisticated feature selection procedures and kernel functions should be taken into consideration to improve the SVR model accuracy. Moreover, the high-resolution TROPOMI UVAI and co-located ALH products will guide us to more reliable training data sets and more powerful algorithms to quantify aerosol absorption from UVAI records.
Highlights
The concept of the near-ultraviolet absorbing aerosol index (UVAI) initially came along with the ozone product of the Total Ozone Mapping Spectrometer (TOMS) on board Nimbus 7
The purpose of this paper is to demonstrate the role of the aerosol layer height (ALH) in quantifying aerosol absorption from UVAI using the newly released TROPOspheric Monitoring Instrument (TROPOMI) Level 2 ALH product
The long-term record of global UVAI data is a treasure with respect to deriving aerosol optical properties such as single scattering albedo (SSA), which is important for aerosol radiative forcing assessments
Summary
The concept of the near-ultraviolet (near-UV) absorbing aerosol index (UVAI) initially came along with the ozone product of the Total Ozone Mapping Spectrometer (TOMS) on board Nimbus 7. It detects elevated UV-absorbing aerosol layers by measuring the spectral contrast difference between a satellite observed radiance in a real atmosphere and a model simulated radiance in a Rayleigh atmosphere (Herman et al, 1997): UVAI = −100 log Iλ Iλ0 obs − log Ray (1). Aerosols are considered to be the largest error source in radiative forcing assessments (IPCC, 2014), and SSA is one of the key parameters to reduce this uncertainty (Haywood and Shine, 1995)
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