Abstract. A sensitivity study of aerosol retrievals to the geometrical configuration of the ground-based sky radiometer observations is carried out through inversion tests. Specifically, this study is focused on principal plane and almucantar observations, since these geometries are employed in AERONET (AErosol RObotic NETwork). The following effects have been analyzed with simulated data for both geometries: sensitivity of the retrieval to variability of the observed scattering angle range, uncertainties in the assumptions of the aerosol vertical distribution, surface reflectance, possible instrument pointing errors, and the effects of the finite field of view. The synthetic observations of radiometer in the tests were calculated using a previous climatology data set of retrieved aerosol properties over three AERONET sites: Mongu (Zambia) for biomass burning aerosol, Goddard Space Flight Center (GSFC; Maryland, USA) for urban aerosol and Solar Village (Saudi Arabia) for desert dust aerosol. The results show that almucantar retrievals, in general, are more reliable than principal plane retrievals in presence of the analyzed error sources. This fact partially can be explained by practical advantages of the almucantar geometry: the symmetry between its left and right branches that helps to eliminate some observational uncertainties and the constant value of optical mass during the measurements, that make almucantar observations nearly independent of the vertical variability of aerosol. Nevertheless, almucantar retrievals present instabilities at high sun elevations due to the reduction of the scattering angle range coverage, resulting in decrease of information content. It is in such conditions that principal plane retrievals show a better stability, as shown by the simulation analysis of the three different aerosol models. The last part of the study is devoted to the identification of possible differences between the aerosol retrieval results obtained from real AERONET data using both geometries. In particular, we have compared AERONET retrievals at the same sites used in the simulation analysis: Mongu (biomass burning), GSFC (urban) and Solar Village (desert dust). Overall, this analysis shows robust consistency between the retrievals from simultaneous observations in principle plane and almucantar All identified differences are within the uncertainties estimated for the AERONET operational aerosol retrieval. The differences in the size distribution are generally under 10% for radii between 0.1 μm and 5 μm, and outside this size range, the differences can be as large as 50%. For the absorption parameters, i.e., single scattering albedo and the imaginary part of the refractive index, the differences are typically under 0.01 and 0.003, respectively. The real part of the refractive index showed a difference of 0.01 for biomass burning and urban aerosol, and a difference of around 0.03 for desert dust. Finally, it should be noted that the whole data set includes only 200 pairs, which have been taken under very stable atmospheric conditions; therefore, in a general case, differences between principal plane (PPL) and almucantar (ALM) are expected to be higher. Though the observed differences between ALM and PPL are rather small, it should be noted that this analysis has been conducted using a limited set of 200 observation pairs selected under stable atmospheric conditions.

/pdf/sensitivity-of-aerosol-retrieval-to-geometrical-1vu3xk40vd.pdf

Sensitivity of aerosol retrieval to geometrical configuration of ground-based sun/sky radiometer observations [Discussion paper]

. This paper presents measurements of mineral dust concentration in the diameter range from 0.4 to 14.0  µ m with a novel balloon-borne optical particle counter, the Universal Cloud and Aerosol Sounding System (UCASS). The balloon launches were coordinated with ground-based active and passive remote-sensing observations and airborne in situ measurements with a research aircraft during a Saharan dust outbreak over Cyprus from 20 to 23 April 2017. The aerosol optical depth at 500 nm reached values up to 0.5 during that event over Cyprus, and particle number concentrations were as high as 50 cm −3 for the diameter range between 0.8 and 13.9  µ m. Comparisons of the total particle number concentration and the particle size distribution from two cases of balloon-borne measurements with aircraft observations show reasonable agreement in magnitude and shape despite slight mismatches in time and space. While column-integrated size distributions from balloon-borne measurements and ground-based remote sensing show similar coarse-mode peak concentrations and diameters, they illustrate the ambiguity related to the missing vertical information in passive sun photometer observations. Extinction coefficient inferred from the balloon-borne measurements agrees with those derived from coinciding Raman lidar observations at height levels with particle number concentrations smaller than 10 cm −3 for the diameter range from 0.8 to 13.9  µ m. An overestimation of the UCASS-derived extinction coefficient of a factor of 2 compared to the lidar measurement was found for layers with particle number concentrations that exceed 25 cm −3 , i.e. in the centre of the dust plume where particle concentrations were highest. This is likely the result of a variation in the refractive index and the shape and size dependency of the extinction efficiency of dust particles along the UCASS measurements. In the future, profile measurements of the particle number concentration and particle size distribution with the UCASS could provide a valuable addition to the measurement capabilities generally used in field experiments that are focussed on the observation of coarse aerosols and clouds.

/pdf/measurement-report-balloon-borne-in-situ-profiling-of-29ep42o65i.pdf

Measurement report: Balloon-borne in-situ profiling of Saharan dust over Cyprus with the UCASS optical particle counter

Abstract The Boundary layer Air Quality-analysis Using Network of Instruments (BAQUNIN) supersite is presented. The site has been collecting pollutant concentrations and meteorological parameters since 2017. Currently, BAQUNIN consists of three observation sites located in the city center of Rome (Italy), and in the neighboring semirural and rural areas. To the best of our knowledge, BAQUNIN is one of the first observatories in the world to involve several passive and active ground-based instruments installed in multiple locations, managed by different research institutions, in a highly polluted megacity affected by coastal weather regimes. BAQUNIN has been promoted by the European Space Agency to establish an experimental research infrastructure for the validation of present and future satellite atmospheric products and the in-depth investigation of the planetary and urban boundary layers. Here, the main characteristics of the three sites are described, providing information about the complex instrumental suite and the produced data. The supersite adopts a policy of free sharing of its validated dataset with the community. Finally, the BAQUNIN potential is demonstrated with a case study involving a major fire that occurred in a waste treatment plant near the urban center of Rome, and the consequent investigation of the plume properties revealed by different instruments. 

/pdf/the-boundary-layer-air-quality-analysis-using-network-of-3d32vm7j.pdf

The Boundary Layer Air Quality-Analysis Using Network of Instruments (BAQUNIN) Supersite for Atmospheric Research and Satellite Validation over Rome Area

The computational efficiency of solving the radiative transfer equation is an important issue for remote sensing and monitoring of aerosols and radiation. This study developed a novel calculation method for polarized radiance named Pn-IMS (Improved Multiple and Single scattering approximation by n-th order multiple scattering correction of the forward Peak Pn), consisting of P1-IMS, P2-IMS and P3-IMS. The P1- and P2-IMS methods are extended versions of the TMS (Truncated Multiple and Single scattering approximation) and IMS (Improved Multiple and Single scattering approximation) methods of Nakajima and Tanaka (1988), and are, respectively, the first and secondary order scattering corrections of the forward peak of the scattering phase function in the scalar approximation of the radiation field. We extended these methods to include the polarization effect based on the vector radiative transfer theory. We also developed the third order scattering correction named P3-IMS method and generalized the formula for any n-th order scattering. A series of numerical tests were performed to indicate that the P1-IMS method is accurate enough to reconstruct the Stokes parameters to within 0.2% with a hemispheric Gaussian stream number as low as N = 10 in the hemisphere, except for total radiance. The total radiance in the solar aureole region requires a higher order scattering correction by the P2- and P3-IMS methods. Numerical tests indicated that the P2- and P3-IMS methods have an accuracy within 5 and 1%, respectively, with N = 10 in the 340–1020 nm spectral region in a moderately thick atmosphere at an aerosol optical thickness at 500 nm of  20.

Efficient calculation of radiative intensity including the polarization effect in moderately thick atmospheres using a truncation approximation

The computational efficiency of solving the radiative transfer equation is an important issue for remote sensing and monitoring of aerosols and radiation. This study developed a novel calculation method for polarized radiance named Pn-IMS (Improved Multiple and Single scattering approximation by n-th order multiple scattering correction of the forward Peak Pn), consisting of P1-IMS, P2-IMS and P3-IMS. The P1- and P2-IMS methods are extended versions of the TMS (Truncated Multiple and Single scattering approximation) and IMS (Improved Multiple and Single scattering approximation) methods of Nakajima and Tanaka (1988), and are, respectively, the first and secondary order scattering corrections of the forward peak of the scattering phase function in the scalar approximation of the radiation field. We extended these methods to include the polarization effect based on the vector radiative transfer theory. We also developed the third order scattering correction named P3-IMS method and generalized the formula for any n-th order scattering. A series of numerical tests were performed to indicate that the P1-IMS method is accurate enough to reconstruct the Stokes parameters to within 0.2% with a hemispheric Gaussian stream number as low as N = 10 in the hemisphere, except for total radiance. The total radiance in the solar aureole region requires a higher order scattering correction by the P2- and P3-IMS methods. Numerical tests indicated that the P2- and P3-IMS methods have an accuracy within 5 and 1%, respectively, with N = 10 in the 340–1020 nm spectral region in a moderately thick atmosphere at an aerosol optical thickness at 500 nm of < 1. Thus, the P3-IMS method is an efficient calculation method more than the single scattering correction method required N > 20. 

 The Prede POM sky radiometer is a filter radiometer
deployed worldwide in the SKYNET international network A new method, called
Skyrad pack MRI version 2 (MRI v2), is presented here to retrieve aerosol
properties (size distribution, real and imaginary parts of the refractive
index, single-scattering albedo, asymmetry factor, lidar ratio, and linear
depolarization ratio), water vapor, and ozone column concentrations from the
sky radiometer measurements MRI v2 overcomes two limitations of previous
methods (Skyrad pack versions 42 and 5, MRI version 1) One is the use
of all the wavelengths of 315, 340, 380, 400, 500, 675, 870, 940, 1020,
1627, and 2200 nm if available from the sky radiometers, for example, in
POM-02 models The previous methods cannot use the wavelengths of 315, 940,
1627, and 2200 nm This enables us to provide improved estimates of the
aerosol optical properties, covering almost all the wavelengths of solar
radiation The other is the use of measurements in the principal plane
geometry in addition to the solar almucantar plane geometry that is used in
the previous versions Measurements in the principal plane are regularly
performed; however, they are currently not exploited despite being useful in
the case of small solar zenith angles when the scattering angle
distribution for almucantars becomes too small to yield useful information
Moreover, in the inversion algorithm, MRI v2 optimizes the smoothness
constraints of the spectral dependencies of the refractive index and size
distribution, and it changes the contribution of the diffuse radiances to the
cost function according to the aerosol optical depth This overcomes issues
with the estimation of the size distribution and single-scattering albedo in
the Skyrad pack version 42 The scattering model used here allows for
non-spherical particles, improving results for mineral dust and permitting
evaluation of the depolarization ratio An assessment of the retrieval uncertainties using synthetic measurements
shows that the best performance is obtained when the aerosol optical depth is
larger than 02 at 500 nm Improvements over the Skyrad pack versions 42
and 5 are obtained for the retrieved size distribution, imaginary part of
the refractive index, single-scattering albedo, and lidar ratio at Tsukuba,
Japan, while yielding comparable retrievals of the aerosol optical depth,
real part of the refractive index, and asymmetry factor A radiative closure
study using surface solar irradiances from the Baseline Surface Radiation
Network and the parameters retrieved from MRI v2 showed consistency, with a
positive bias of the simulated global irradiance of about +1  %
Furthermore, the MRI v2 retrievals of the refractive index,
single-scattering albedo, asymmetry factor, and size distribution have been
found to be in agreement with integrated profiles of aircraft in situ measurements
of two Saharan dust events at the Cape Verde archipelago during the Sunphotometer Airborne Validation Experiment in Dust (SAVEX-D)
2015 field campaign

/pdf/optimal-use-of-the-prede-pom-sky-radiometer-for-aerosol-4dofleo1db.pdf

Kouichi Nakashima

Papers

Optimal use of the Prede POM sky radiometer for aerosol, water vapor, and ozone retrievals