Despite the important role that aerosols play in modulating the radiation budget of high latitudes, our knowledge of their physical and radiative properties, horizontal and vertical distributions, and temporal variability is inadequate. Monitoring sites are sparse, and satellite retrievals of aerosol properties require validation through ‘ground-truth’ measurements. To improve our knowledge of aerosols, a bipolar network of Sun, Moon, and Star photometers has been established to monitor aerosol optical depth (AOD), from which properties of a variety of aerosols can be inferred.  

Origin 

The origins of aerosol particles in polar regions are diverse and complex (Fig. 1). Some particles are naturally occurring, such as sea salt from ocean spray or dust from desert regions. However, human activities, such as transportation and industrial processes, also play a significant role in aerosol particle formation. These particles can travel long distances from their source regions, including highly polluted areas, and enter the Arctic and Antarctic atmospheres. In addition, volcano eruptions, wildfires and biomass burning events can also contribute to the presence of aerosol particles.

In the Arctic, during the winter period, the polar front tends to expand towards mid-latitudes, reaching approximately 50°N. This expansion allows particles from the most heavily polluted regions of Europe, Asia, and America to enter the Arctic atmosphere, leading to the phenomenon known as Arctic Haze. Aerosols lofted into the atmosphere at their source are carried northward by air flow along isentropic levels and typically remain aloft above a surface-based temperature inversion layer. As a consequence, in situ measurments made at/near the surface are not generally representative of the atmospheric column, making AOD measurements vital for assessing affects on the surface radiation balance.

Fig. 1 A schematic shows the pathways for transporting air pollution into the Arctic. According to Stohl (2006), there are three main routes: (i) low-level transport from midlatitude emission regions, followed by uplift at the Arctic front; (ii) pollutants lifted at lower latitudes, then transported in the upper troposphere, with eventual slow descent due to radiational cooling or mixing into the polar dome. This route is common from North America and Asia but is subject to significant wet scavenging; (iii) wintertime low-level transport of already cold air into the polar dome, mainly from northern Eurasia (Law et al., 2014).

Due to the presence of particles, solar radiation is attenuated or weakened along its path through the Earth’s atmosphere. This occurs due to various physical processes, including scattering and absorption (see Fig. 2). Scattering refers to the redirection of sunlight in different directions as it interacts with molecules and particles in the atmosphere. This leads to the diffusion of light and the creation of a diffused sky glow. Particles can scatter light differently, depending on their size. Rayleigh scattering occurs when particles are much smaller than the wavelength of the incident light beam (this type of scattering is responsible for the blue color of the sky and the reddening of the sun during sunrise and sunset), while Mie scattering occurs when particles are similar in size to the wavelength of the light. In general, the scattering phenomenon is dominated by the Rayleigh regime when the ratio of the particle diameter to the wavelength in micrometers is less than 0.6 divided by the refractive index (n), and by the Mie regime when the ratio is between 0.6/n and 5. 

Absorption, on the other hand, occurs when certain molecules in the atmosphere, such as water vapor, ozone, and carbon dioxide, absorb specific wavelengths of light, leading to their removal from the solar spectrum. This absorption can also result in the creation of a thermal gradient in the atmosphere, with warmer temperatures closer to the surface and cooler temperatures at higher altitudes. Understanding how sunlight is attenuated through the atmosphere is essential for predicting and modeling the impacts of solar radiation on climate, weather patterns, and human health.

Fig. 2 Absorbing aerosols (dark grey dots) and scattering aerosols (light grey dots) affect radiative forcing in different ways. Scattering aerosols cause negative forcing (−) by reflecting sunlight directly and by interacting with clouds. Absorbing aerosols generally cause warming (+) by absorbing radiation, though their interaction with clouds might cause slight cooling. When scattering and absorbing aerosols interact, the absorption and warming effect increase. Incident sunlight is shown with light orange arrows, scattered radiation by scattering aerosols with dark orange arrows, re-emitted radiation by absorbing aerosols with red arrows, and scattered sunlight with dark blue arrows. CCN stands for cloud condensation nuclei, and INPs stands for ice-nucleating particles (Li et al., 2022).

Properties

AOD is a measure of atmospheric opacity due to the columnar aerosol load from the observing site to the top-of-the-atmosphere (TOA). Composition can be made up of a variety of particles; e.g., urban pollution, wildfire smoke, dust, sea salt, volcanic emissions, etc. Although direct line-of-sight Sun, Moon or Star observations are made, the measure is normalized to one atmosphere yielding a non-dimensional quantification of vertical opacity. Fundamentally, voltages (V) are measured by a photometer at specific wavelengths, which are proportional to spectral irradiance (I) reaching the instrument, whether at the surface or at some altitude level, e.g., from an aircraft. It is essential to know the TOA irradiances (I0) in terms of voltage (V0), obtained via careful calibration using the Langley Plot methodology (Schmid & Wehrli, 1995). This involves making repeated measurements over a range of path lengths (air masses) under pristine, stable conditions and plotting the results to extrapolate an extraterrestrial value for each wavelength. The total optical depth is then obtained using the Beer-Lambert-Bouguer law:

V(λ) = (V0(λ) / d2) exp[-τ(λ)TOT * m]     


where V(λ) is the voltage measured at wavelength λ, V0(λ) is the extraterrestrial voltage, d is the ratio of the average to the actual Earth-Sun distance, τTOT is the total optical depth, and m is the optical air mass (Holben et al. 1998). In order to derive AOD, all other atmospheric constituents that scatter or absorb light must be evaluated and subtracted from the totals, by wavelength. The primary attenuation results from Rayleigh scattering, and absorption by ozone, water vapor, NO2, CO2 and CH4. Thus,


τ(λ)Aerosol= τ(λ)TOT – τ(λ)Rayleigh -τ(λ)H2O – τ(λ)O3 – τ(λ)NO2 – τ(λ)CO2 – τ(λ)CH4


Typical background values of AOD at 500 nm for pristine Polar atmospheres range from about 0.03 to 0.06 depending on location and season (Tomasi et al. 2015). Highly turbid conditions, for instance due to wildfire smoke, can be two orders of magnitude above background. Major volcanic eruptions can increase AOD by a factor of 10 at stratospheric levels. To obtain AOD measurements with acceptable uncertainty, accurate instrument calibration is necessary, especially when operating in polar regions. The calibration constant V0(λ) of a photometer indicates the value that the instrument would record at the top of the atmosphere at a specific Earth-Sun distance of 1 astronomical unit. The Langley method is the most widely used and accepted technique in photometry for determining the calibration constant (expressed in mV). 

Effect on climate 

Due to the ability of aerosols to scatter and absorb solar radiation, the presence of aerosols in the Polar atmosphere generally leads to a cooling effect. However, the magnitude of this cooling effect depends greatly on the solar incident angle and the albedo of the surface. One of the major concerns related to aerosols in Polar regions is the deposition of black carbon on snow and ice. Black carbon, which is a type of aerosol produced from incomplete combustion of fossil fuels and biomass, can lower the albedo of snow and ice by absorbing more solar radiation. This, in turn, can accelerate melting and promote an amplification of warming. However, the relative effects of various aerosols on a regional and seasonal basis in Polar regions have not been fully assessed. More research, including case studies, is needed to better quantify their influence on climate. Additionally, the sources and transport of aerosols to Polar regions are also important areas of study, as they can greatly affect the composition and concentration of aerosols in the Polar atmosphere. Overall, aerosols have complex and region-specific effects on the climate of Polar regions, and further research is needed to fully understand their influence.