Classification of aerosol properties derived from AERONET direct sun data

Abstract

Abstract. Aerosol spectral measurements by sunphotometers can be characterized by three independent pieces of information: 1) the optical thickness (AOT), a measure of the column aerosol concentration, 2) the optical thickness average spectral dependence, given by the Angstrom exponent (α), and 3) the spectral curvature of α (δα). We propose a simple graphical method to visually convert (α, δα) to the contribution of fine aerosol to the AOT and the size of the fine aerosols. This information can be used to track mixtures of pollution aerosol with dust, to distinguish aerosol growth from cloud contamination and to observe aerosol humidification. The graphical method is applied to the analysis of yearly records at 8 sites in 3 continents, characterized by different levels of pollution, biomass burning and mineral dust concentrations. Results depict the dominance of fine mode aerosols in driving the AOT at polluted sites. In stable meteorological conditions, we see an increase in the size of the fine aerosol as the pollution stagnates and increases in optical thickness. Coexistence of coarse and fine particles is evidenced at the polluted sites downwind of arid regions.

Aerosol characterization at the sub‐Arctic site Andenes (69°N, 16°E), by the analysis of columnar optical properties

“…The parameters α and and α are the first and second spectral derivatives of the total aerosol optical depth τ a (in ln τ a and ln λ space). These were used to identify aerosol types following the representations described by O'Neill (2010) and Gobbi et al (2007). The (500 nm) SDA outputs for the Cimel data are provided routinely by AERONET.…”

Investigation of the Optical Properties of Aerosols over the Coastal Region at Dalian, Northeast China

“…This graphic method can be used to separate the coarse mode aerosol growth from cloud contamination and the dominance of fine mode aerosols to the total AOD440nm according to the spectral variation of Å ngström exponent as Gobbi [54] and Schuster [68] depicted. The spectral difference in the Å ngström exponent (α), defined as δα = α440-675nm − α675-870nm, was used to distinguish aerosol growth from cloud contamination and to examine aerosol humidification using instantaneous observational data.…”

“…The black solid lines represent a fixed size of the fine mode Rf, and the dashed blue lines represent a fixed fraction contribution η of the fine mode to the total AOD440nm. The Å ngström difference is presented as a function of the Å ngström exponent (440-870 nm) and the AOD440nm values for bimodal log-normal size distributions with refractive indices of m = 1.40-0.001i based on Gobbi [54].The climatology of Dubovik [15] also indicates that this refractive index (m = 1.40-0.001i) is typical of urban and industrial aerosols compared with the mineral dust aerosols (m = 1.53-0.003i) and plus water droplets (m = 1.33-0.000i). Gobbi [54] illustrates that there is a weaker sensitivity of the η curves than the Rf with the refractive index in the different classification scheme, and the scheme is robust enough to provide an operational classification of the aerosol properties or size distribution parameters within this level of indetermination.…”

“…Each year was divided into four seasons (spring, March to May; summer, June to August; autumn, September to November; and winter, December to February) to investigate the seasonal variation in the optical properties of aerosols in this region. The spectral difference in Ångström exponent wavelength pairs between 440-675 and 675-870 nm was used to indicate the effective radius of the fine aerosols [54]. value (0.42˘0.31) was recorded in January.…”

Two-Wavelength Method for Calibration of High-Precision Sun Photometer

“…Развитие спутниковой измерительной техники привело, в свою очередь, к созданию более совершенных наземных сетей и систем атмосферных измере-ний, в частности сети AERONET. Подробное описание солнечных радиометров сети AERONET приведено в работе [1]. Автоматизированная сеть AERONET охватывает более 200 измери-тельных пунктов, распределенных по всему миру.…”