Retrieval of particle size distribution of polar stratospheric clouds based on Wide-Angle Color and Polarization Analysis
Graphical abstract
Introduction
Polar stratospheric clouds (PSC) were first reported in March 1870 in northern Scandinavia (Kassner, 1895). A few decades later, PSC were also noticed during the Antarctic winter in the Southern hemisphere (Arctowskiy, 1902). Clouds can remain bright during the twilight, which shows that their altitude is about 20 km. Sometimes, the fragments of the cloud had intensive color changing from red to blue. Due to this irisation effect, these clouds are also called “nacreous clouds” or “mother-of-pearl clouds”. This can take place if the particles forming the cloud have similar sizes that are significantly larger than the optical wavelength.
The chemical nature of the clouds became clear in the late 20th century. PSC particles are nucleated on stratospheric aerosol consisting of a H2SO4 solution (Rosen, 1971). As the temperature falls below 195 K, nitric acid HNO3 condenses on these particles (Crutzen and Arnold, 1986; Toon et al., 1986). Depending on the physical conditions and cooling rate, these can form liquid supercooled ternary solution H2O/H2SO4/HNO3 (STS, Molina et al., 1993; Carslaw et al., 1994) or solid particles of nitric acid trihydrate (NAT) (Tabazadeh et al., 1994). These particles form PSC of types Ib and Ia, respectively.
Solid NAT particles have a size of more than 1 μm, that is, several times larger than liquid STS particles (Voigt et al., 2000); however, the numeric density of solid particles is 30–100 times lower. Taking into account that the optical scattering coefficient increases with the mean particle radius r0 slower than ~ r02 for r0 from 0.2 to 2 μm, liquid particles have a stronger mass-equivalent optical effect compared with solid particles. While STS particles are spherical, solid NAT particles are not, and light scattering is not exactly described by the Mie theory. The basic observational effect of non-sphericity is the depolarization of backscattering measured by lidar sounding (Browell et al., 1990).
If the temperature falls below the ice frost level (about 185 K in lower stratosphere conditions), the nucleation of water ice onto a particle begins. Ice crystals can reach a size of more than 10 μm forming bright and color-variable clouds of type II (Poole and McCormick, 1988). They can be seen as bright fragments against the background of type I PSC.
PSC have been an object of special interest during the last few decades due to heterogeneous reactions releasing active chlorine-containing molecules and radicals that destroy stratospheric ozone (Toon et al., 1986; Solomon et al., 1986; Solomon, 1990). PSC particles are studied directly from balloons (Deshler et al., 2000). Lidar measurements are effective to distinguish between different types of PSC particles using a cross-polarization scheme (Browell et al., 1990) and also to find the particle size distribution (Jumelet et al., 2009). Lidar backscattering analysis is effective in combination with balloon measurements (Deshler et al., 2000) and for sounding of PSC from space (Noel et al., 2008).
The ground-based passive optical sounding of PSC particles seems difficult owing to the rare occurrence of bright clouds with a high S/N ratio against the twilight background, weather restrictions, and a low number of observational sites in high latitudes. However, anomalies of the Arctic stratosphere polar vortex during the winter of 2019–2020 led to a number of bright events over northern Russia. In this paper, the authors suggest a method of PSC particle size distribution retrieval based on all-sky measurements and compare the results with existing data.
Section snippets
Observations
Polar stratospheric clouds were detected during the winter of 2019–2020 by all-sky cameras for regular sky surveys in the Lovozero station (68.0°N, 35.1°E) of the Polar Geophysical Institute. This work is based on the measurements of color and polarization cameras. Both devices had a field size diameter of 180°; the sky area with zenith angles of up to 70° was analyzed. The cameras worked during both the twilight and night; star images were used to fix the camera position and the field
Cloud field separation
Fixing the cloud’s scattering field against the twilight background and measuring its observational characteristics (polarization or the intensity ratio in different color bands) are the most difficult problems of cloud study by all-sky analysis. Clouds do not have sharp borders, and the twilight background is also spatially variable. Its brightness significantly changes from date to date, and one can not simply subtract the background measured during another twilight without PSC.
Solving this
Polarization analysis
The polarization of PSC light scattering is a function of the angle θ. Each sky arc with (θ = const) can cross a number of different cloud spots. The simplest way is to find the average value as it was done for NLC by Ugolnikov et al. (2016), and Ugolnikov and Maslov (2019). However, these data can be also used to check the influence of the incompletely eliminated sky background. It can be assumed that polarization is measured for the sum of the cloud field j1(τ) and background admixture αI1
Color analysis
Multi-wavelength effects in PSC are considered in a way similar to NLC analysis (Ugolnikov et al., 2017). The basic difference is that PSC particles are significantly larger than NLC particles. Small particles approximation is applicable for NLC; in this case, the color ratio of Mie differential scattering cross sections SG,R(θ)/SB(θ) is a linear function of cosθ, the coefficient is simply related to the mean particle size. For the particle radius above 0.1 μm, this dependency becomes more
PSC size distribution retrieval
The basic difference between polarization and color analysis is that the polarization p is estimated directly and compared with theoretical calculations for different particle ensembles, while the color ratio of PSC scattering remains unknown, only its gradient W by the scattering angle θ can be found. Polarization is measured in a short interval of θ, its variations over this interval are not significantly stronger than errors for its measurements. In this case it is enough to determine the
Discussion and conclusion
Polar stratospheric clouds play a significant role in stratosphere chemistry, by activating chlorine species that destroy ozone. Negative temperature trends in the stratosphere caused by greenhouse gases (Thompson et al., 2012) can increase the rate of PSC formation and expand it to the lower latitudes. Stratospheric cooling and PSC visual frequency in northern Russia in the winter of 2019–2020 were maximal over the last dozens of years, and one can expect an increase in the PSC occurrence rate
Authors statement
We had revised the paper “Retrieval of Particle Size Distribution of Polar Stratospheric CloudsBased on Wide-Angle Color and Polarization Analysis ", Ref: PSS_2020_140, according to the reviewers’ notes. We confirm that paper meets all the rules of Privacy and Ethics of Publishing. This is the research paper in atmospheric physics and it is not related with studies in humans and animals.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are thankful to Valery I. Demin (Polar Geophysical Institute, Apatity, Russia) for his help during the work.
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