The Aerosol Robotic Network (AERONET) Version 3 (V3) aerosol retrieval algorithm is described, which is based on the Version 2 (V2)
algorithm with numerous updates. Comparisons of V3 aerosol ...retrievals to
those of V2 are presented, along with a new approach to estimate
uncertainties in many of the retrieved aerosol parameters. Changes in the V3 aerosol retrieval algorithm include (1) a new polarized radiative transfer code (RTC), which replaced the scalar RTC of V2, (2) detailed
characterization of gas absorption by adding NO2 and H2O to
specify total gas absorption in the atmospheric column, specification of
vertical profiles of all the atmospheric species, (3) new bidirectional reflectance distribution function (BRDF) parameters for land sites adopted
from the MODIS BRDF/Albedo product, (4) a new version of the extraterrestrial
solar flux spectrum, and (5) a new temperature correction procedure of both direct Sun and sky radiance measurements. The potential effect of each change in V3 on single scattering albedo (SSA) retrievals was analyzed. The
operational almucantar retrievals of V2 versus V3 were compared for four AERONET sites: GSFC, Mezaira, Mongu, and Kanpur. Analysis showed very good
agreement in retrieved parameters of the size distributions. Comparisons of
SSA retrievals for dust aerosols (Mezaira) showed a good agreement in 440 nm
SSA, while for longer wavelengths V3 SSAs are systematically higher than those of V2, with the largest mean difference at 675 nm due to cumulative
effects of both extraterrestrial solar flux and BRDF changes. For non-dust
aerosols, the largest SSA deviation is at 675 nm due to differences in
extraterrestrial solar flux spectrums used in each version. Further, the SSA
675 nm mean differences are very different for weakly (GSFC) and strongly
(Mongu) absorbing aerosols, which is explained by the lower sensitivity to a bias in aerosol scattering optical depth by less absorbing aerosols. A new
hybrid (HYB) sky radiance measurement scan is introduced and discussed. The HYB combines features of scans in two different planes to maximize the range
of scattering angles and achieve scan symmetry, thereby allowing for cloud
screening and spatial averaging, which is an advantage over the principal plane scan that lacks robust symmetry. We show that due to an extended range of scattering angles, HYB SSA retrievals for dust aerosols exhibit smaller
variability with solar zenith angles (SZAs) than those of almucantar (ALM), which allows extension of HYB SSA retrievals to SZAs less than 50∘ to as small as 25∘. The comparison of SSA retrievals from closely
time-matched HYB and ALM scans in the 50 to 75∘ SZA range showed good agreement with the differences below ∼0.005. We also present an approach to estimate retrieval uncertainties which
utilizes the variability in retrieved parameters generated by perturbing
both measurements and auxiliary input parameters as a proxy for retrieval uncertainty. The perturbations in measurements and auxiliary inputs are
assumed as estimated biases in aerosol optical depth (AOD), radiometric
calibration of sky radiances combined with solar spectral irradiance, and
surface reflectance. For each set of Level 2 Sun/sky radiometer
observations, 27 inputs corresponding to 27 combinations of biases were
produced and separately inverted to generate the following statistics of
the inversion results: average, standard deviation, minimum and maximum
values. From these statistics, standard deviation (labeled U27) is used as a proxy for estimated uncertainty, and a lookup table (LUT) approach was implemented to reduce the computational time. The U27 climatological LUT was
generated from the entire AERONET almucantar (1993–2018) and hybrid
(2014–2018) scan databases by binning U27s in AOD (440 nm), Angström exponent (AE, 440–870 nm), and SSA (440, 675, 870, 1020 nm). Using this LUT approach, the uncertainty estimates U27 for each individual V3 Level 2 retrieval can
be obtained by interpolation using the corresponding measured and inverted
combination of AOD, AE, and SSA.
Systematic characterization of aerosol over the oceans is needed to understand the aerosol effect on climate and on transport of pollutants between continents. Reported are the results of a ...comprehensive optical and physical characterization of ambient aerosol in five key island locations of the Aerosol Robotic Network (AERONET) of sun and sky radiometers, spanning over 2-5 yr.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The diurnal variability of aerosol optical depth (AOD) can be significant, depending on location and dominant aerosol type. However, these diurnal cycles have rarely been taken into account in ...measurement-based estimates of aerosol direct radiative forcing (ADRF) or aerosol direct radiative effect (ADRE). The objective of our study was to estimate the influence of diurnal aerosol variability at the top of the atmosphere ADRE estimates. By including all the possible AERONET sites, we wanted to assess the influence on global ADRE estimates. While focusing also in more detail on some selected sites of strongest impact, our goal was to also see the possible impact regionally. We calculated ADRE with different assumptions about the daily AOD variability: taking the observed daily AOD cycle into account and assuming diurnally constant AOD. Moreover, we estimated the corresponding differences in ADREs, if the single AOD value for the daily mean was taken from the the Moderate Resolution Imaging Spectroradiometer (MODIS) Terra or Aqua overpass times, instead of accounting for the true observed daily variability. The mean impact of diurnal AOD variability on 24 h ADRE estimates, averaged over all AERONET sites, was rather small and it was relatively small even for the cases when AOD was chosen to correspond to the Terra or Aqua overpass time. This was true on average over all AERONET sites, while clearly there can be much stronger impact in individual sites. Examples of some selected sites demonstrated that the strongest observed AOD variability (the strongest morning afternoon contrast) does not typically result in a significant impact on 24 h ADRE. In those cases, the morning and afternoon AOD patterns are opposite and thus the impact on 24 h ADRE, when integrated over all solar zenith angles, is reduced. The most significant effect on daily ADRE was induced by AOD cycles with either maximum or minimum AOD close to local noon. In these cases, the impact on 24 h ADRE was typically around 0.1–0.2 W m−2 (both positive and negative) in absolute values, 5–10% in relative ones.
A nearly 20‐year global data set (1979–1994 and 1996 to the present) of tropospheric absorbing aerosols has been developed from total ozone mapping spectrometer (TOMS) backscattered radiance ...measurements in the range from 331 to 380 nm. The occurrence of aerosols is derived directly from measured backscattered radiances and is represented by a quantity known as the aerosol index. Previous theoretical model simulations have demonstrated that the aerosol index depends on aerosol optical thickness (AOT), single scattering albedo, and aerosol height and that the AOT can be determined provided that the microphysical properties and height of aerosols are known. In this paper we show that the TOMS aerosol index measurements are linearly proportional to the AOT derived independently from ground‐based Sun‐photometer instruments over regions of biomass burning and regions covered by African dust. We also show how this linear relationship can be used to directly convert the aerosol index into AOT for smoke and dust aerosols for the regions near the Sun‐photometer sites and how information about aerosol height can be inferred from the results. Finally, we apply this method to the TOMS data over the last two decades and find a significant increase in the amount of biomass burning smoke in the African savanna regions during the 1990s in addition to the more obvious increase in South America.
Aerosol optical properties over Midway Island in the central Pacific Ocean are considered in conjunction with the information on surface wind speed. In general, optical conditions over Midway ...resemble aerosol found over other maritime locations in the Pacific Ocean (Lanai, Tahiti, and Nauru). The most frequently occurring values of aerosol optical depth at 500‐nm wavelength and Angstrom parameter are 0.06 and ∼0.40, respectively. Empirical relationships are established between columnar aerosol optical properties and surface wind speed. Increased emission of sea‐salt aerosols at greater wind speeds primarily influenced aerosol optical depth at infrared wavelengths. The correlation coefficient between 24 hour average surface wind speed and aerosol optical depth, although not high (0.52 at a 1020 nm wavelength), is statistically significant at a 99% confidence level. Wind speed anticorrelates with the Angstrom parameter owing to an influx of large particles from the surface. Wind speed influences primarily the coarse fraction (radius > 0.5 μm) concentration of the retrieved columnar size distribution (correlation coefficient 0.56). Effective radii of the retrieved fine and coarse modes are found to be independent of wind speed. Average size distributions for various wind speed bins can be very well simulated with the maritime aerosol component model.
As a representative site of the southern African biomass‐burning region, sun‐sky data from the 15 year Aerosol Robotic Network (AERONET) deployment at Mongu, Zambia, was analyzed. For the ...biomass‐burning season months (July–November), we investigate seasonal trends in aerosol single scattering albedo (SSA), aerosol size distributions, and refractive indices from almucantar sky scan retrievals. The monthly mean single scattering albedo at 440 nm in Mongu was found to increase significantly from ~0.84 in July to ~0.93 in November (from 0.78 to 0.90 at 675 nm in these same months). There was no significant change in particle size, in either the dominant accumulation or secondary coarse modes during these months, nor any significant trend in the Ångström exponent (440–870 nm; r2 = 0.02). A significant downward seasonal trend in imaginary refractive index (r2 = 0.43) suggests a trend of decreasing black carbon content in the aerosol composition as the burning season progresses. Similarly, burning season SSA retrievals for the Etosha Pan, Namibia AERONET site also show very similar increasing single scattering albedo values and decreasing imaginary refractive index as the season progresses. Furthermore, retrievals of SSA at 388 nm from the Ozone Monitoring Instrument satellite sensor show similar seasonal trends as observed by AERONET and suggest that this seasonal shift is widespread throughout much of southern Africa. A seasonal shift in the satellite retrieval bias of aerosol optical depth from the Moderate Resolution Imaging Spectroradiometer collection 5 dark target algorithm is consistent with this seasonal SSA trend since the algorithm assumes a constant value of SSA. Multi‐angle Imaging Spectroradiometer, however, appears less sensitive to the absorption‐induced bias.
Key Points
Seasonal trend in particle absorption observed with AERONET retrievals
Trend in imaginary refractive index observed but no trend in particle size
OMI satellite retrievals also observed seasonal SSA trend in southern Africa
Aerosol optical depth (AOD) measurements were acquired at six Arctic sunphotometer sites during the ARCTAS‐A (April, 2008) campaign. Numerous smoke events were identified and related to extensive ...forest and agricultural fires in eastern Russia and northern Kazakhstan/southwestern Russia respectively. An analysis of the fine (sub‐micron) optical depths from the six stations indicated the presence of underlying low frequency trends which were coherent with general meteorological considerations, source information, model estimates and remote sensing information. Low frequency (diurnal) coarse‐mode optical depth events were observed at a number of the stations; these singular events are likely due to ice particles whose nucleation may have been associated with the presence of smoke, or possibly dust.
The spectral deconvolution algorithm (SDA) and SDA+ (extended SDA) methodologies can be employed to separate the fine and coarse mode extinction coefficients from measured total aerosol extinction ...coefficients, but their common use is currently limited to AERONET (AErosol RObotic NETwork) aerosol optical depth (AOD). Here we provide the verification of the SDA+ methodology on a non-AERONET aerosol product, by applying it to fine and coarse mode nephelometer and particle soot absorption photometer (PSAP) data sets collected in the marine boundary layer. Using data sets collected on research vessels by NOAA-PMEL(National Oceanic and Atmospheric Administration - Pacific Marine Environmental Laboratory), we demonstrate that with accurate input, SDA+ is able to predict the fine and coarse mode scattering and extinction coefficient partition in global data sets representing a range of aerosol regimes. However, in low-extinction regimes commonly found in the clean marine boundary layer, SDA+ output accuracy is sensitive to instrumental calibration errors. This work was extended to the calculation of coarse and fine mode scattering coefficients with similar success. This effort not only verifies the application of the SDA+ method to in situ data, but by inference verifies the method as a whole for a host of applications, including AERONET. Study results open the door to much more extensive use of nephelometers and PSAPs, with the ability to calculate fine and coarse mode scattering and extinction coefficients in field campaigns that do not have the resources to explicitly measure these values.
Aerosol optical properties above the oceans vary considerably, depending on contributions of major aerosol components, i.e., urban/industrial pollution, desert dust, biomass burning, and maritime. ...The optical characterization of these aerosols is fundamental to the parameterization of radiative forcing models as well as to the atmospheric correction of ocean color imagery. We present a model of the maritime aerosol component derived using Aerosol Robotic Network (AERONET) data from three island locations: Bermuda (Atlantic Ocean), Lanai, Hawaii (Pacific Ocean), and Kaashidhoo, Maldives (Indian Ocean). To retrieve the maritime component, we have considered the data set with aerosol optical depth at a wavelength 500 nm less than 0.15 and Angstrom parameter α less than 1. The inferred maritime component in the columnar size distribution, which was found to be very similar for the three study sites, is bimodal with a fine mode at an effective radius (reff) ∼ 0.11–0.14 μm and a coarse mode reff of ∼1.8–2.1 μm. The results are comparable with size distributions reported in the literature. The refractive index is spectrally independent and estimated to be 1.37‐0.001i (single‐scattering albedo is about 0.98), based on the single‐component homogenous particle composition assumption. Fractional contributions of the fine and coarse modes to the computed τa (500 nm) are within the range of τfine ∼ 0.03–0.05 and τcoarse ∼ 0.05–0.06 correspondingly. Angstrom parameters vary from ∼0.8 to 1.0 computed in the UV‐visible (340–670 nm) and from 0.4 to 0.5 estimated in the near IR (870–2130 nm) spectral ranges. Aerosol phase functions are very similar for all three sites considered. The maritime aerosol component presented in this paper can serve as a candidate model in atmospheric correction algorithms.
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