Physics and Chemistry of the Arctic Atmosphere

The scales of dynamical processes in the Arctic atmosphere range from turbulence in the atmospheric boundary layer (ABL) via interactive mesoscale processes, such as orographic flows and Polar lows, to synoptic-scale cyclones, and further to hemispherical-scale circulations characterized by the Polar front jet stream and planetary waves. Specific boundary conditions for tropospheric dynamics in the Arctic include (a) sea ice and snow, which strongly affect the surface energy budget, (b) large transports of heat and moisture from lower-latitudes, and (c) the wintertime stratospheric Polar vortex, which has a large impact on tropospheric large-scale circulation and synoptic-scale cyclones. Knowledge on dynamics of the Arctic atmosphere is advancing but, compared to mid- and low-latitudes, still limited due to lack of process-level observations from the Arctic. The dynamics of the Arctic atmosphere poses a challenge for numerical weather prediction (NWP) and climate models, in particular in the case of ABL, orographic flows, Polar lows, and troposphere-stratosphere coupling. More research is also needed to better understand how the atmospheric dynamics affects and is affected by climate warming.

In a such changing environment like the Arctic, improving the understanding of the thermodynamic state and processes of the atmosphere is critical for the development of accurate prediction and climatic models. This is fundamental for example for studies on sea-ice development as well as on cloud formation. Taking into account the above remarks, it is very important to know the pressure, temperature and moisture conditions of the Arctic atmosphere throughout the year and over the whole tropospheric and stratospheric altitude range. Furthermore, knowing these data is necessary to realistically evaluate the radiative effects involving both the short-wave and long-wave radiation fluxes, which regulate the energy balance of the Arctic surface-atmosphere system.

In this contribution, continuous measurements of these parameters by means of radiosonde in the Arctic are reviewed, including correction algorithms, in order to obtain detailed climatologies in terms of seasonal, inter-annual and vertical behaviour.

The Arctic atmosphere is coupled to lower latitudes, both as a receptor for global pollution and as a driver for the global climate system. Arctic atmospheric composition is variable and changing, making measurements of trace gas concentrations essential for understanding atmospheric processes.

The atmospheric concentrations of carbon dioxide (CO₂) and methane in the Arctic are increasing in concert with global trends. Meanwhile, the Arctic represents up to 25% of the global land carbon sink and the Arctic Ocean accounts for 10–12% of the global ocean CO₂ sink. The Arctic thus has a strong influence on the global carbon cycle, while also responding more strongly to changes in climate than do mid-latitude regions. However, many processes that lead to carbon emissions and exchange in the Arctic are poorly understood and the region is sparsely sampled, resulting in large uncertainties in the quantification of carbon stocks, sources, and sinks.

The Arctic experiences poor air quality due to local sources and transport from diverse mid-latitude emission sources such as wildfires. The springtime Arctic troposphere frequently experiences ozone depletion episodes that are linked to surface-based production of reactive halogen species that then deplete ozone, particularly associated with bromine explosions. A major source of bromine in the Arctic is sea salt, but the importance of blowing snow and the mechanisms involved in the heterogeneous bromine release are the focus of ongoing research.

In the stratosphere, springtime ozone depletion continues in the Arctic, with significant interannual variability driven by atmospheric dynamics, transport, and temperature. Ozone recovery is anticipated due to reduction of chlorofluorocarbons under the Montreal Protocol and its Amendments, but there are uncertainties due to coupling between stratospheric chemistry and climate.

This chapter provides a review of the trace gas composition of the Arctic atmosphere. It surveys our current knowledge and discusses outstanding questions, with a focus on tropospheric ozone and halogens, greenhouse gases, and the stratosphere.

Aerosols play an important role in the climatic system through their direct and indirect effects on radiation. Beside this, they are also part of the complex chain of chemical reactions taking place in the atmosphere. Indirect effects involve aerosols acting as cloud and ice condensation nuclei, brightening of clouds, modification of precipitation capabilities etc. After deposition, they also change reflectivity properties of bright surfaces, particularly important in polar regions.

In the Artic few natural aerosol sources exist, except oceans for sea-salt and soil for dust, both of them increasing in magnitude due to global warming. Beside this, anthropogenic aerosols are easily transported to the Arctic by atmospheric transport from middle latitudes, in particular during winter and early spring, forming the so-called Artic Haze.

In this contribution the processes causing the development of Arctic Haze and its characteristics are introduced. Following, a review of the physical and optical properties as well as chemical composition of Arctic aerosols are reviewed using data obtained from numerous monitoring stations in the Arctic.

The Arctic climate system is complex and clouds are one of its least understood components. Since cloud processes occur from micrometer to synoptic scales, their couplings with the other components of the Arctic climate system and their overall role in modulating the energy budget at different spatio-temporal scales is challenging to quantify. The in-situ measurements, as limited in space and time as they are, still reveal the complex nature of cloud microphysical and thermodynamical processes in the Arctic. However, the synoptic scale variability of cloud systems can only be obtained from the satellite observations. A considerable progress has been made in the last decade in understanding cloud processes in the Arctic due to the availability of valuable data from the multiple campaigns in the Central Arctic and due to the advances in the satellite remote sensing. This chapter provides an overview of this progress.

First an overview of the lessons learned from the recent in-situ measurement campaigns in the Arctic is provided. In particular, the importance of supercooled liquid water clouds, their role in the radiation budget and their interaction with the vertical thermodynamical structure is discussed. In the second part of the chapter, a climatological overview of cloud properties using the state-of-the-art satellite based cloud climate datasets is provided. The agreements and disagreements in these datasets are highlighted. The third and the fourth parts of the chapter highlight two most important processes that are currently being researched, namely cloud response to the rapidly changing sea-ice extent and the role of moisture transport in to the Arctic in governing cloud variability. Both of these processes have implications for the cloud feedback in the Arctic.

Ice fog consists of suspended small ice crystals with maximum sizes less than about 200 μm, having similar fall velocities as fog droplets, and that often reduces visibility to less than 1 km. Its formation is strongly dependent on high number concentrations of available heterogeneous ice nuclei (IN) at temperatures (T) > −40 ºC, homogeneous nucleation below −40 ºC, and available moisture in the air. Radiative cooling, advective cooling, and cold air subsidence, particularly over the Polar region or high elevation mountainous geographical regions, play an important role in its formation and development. Ice fog crystals form at cold T when the relative humidity with respect to ice (RH_i) is ≥100%. Favorable ice nucleation conditions typically occur at T < −15 ºC and its microphysical characteristics and their evolution needs to be better understood for a physically based representation in numerical forecast models. This is likely to be of growing societal importance due to the known sensitivity of the Arctic environment to climate change. Accidents related to low visibility over the northern latitudes may increase tenfold over the Arctic regions because of increasing population and traffic. This suggests that ice fog conditions can have major impacts on aviation and ground/water-based transportation, as well as on climate change and ecosystem. These open issues, as well as challenges related to ice fog measurements and predictions, are discussed in detail, and its importance for evaluating weather and climate conditions over cold environments are emphasized.

Polar ozone depletion is a major environmental issue, for the alterations induced on the chemical-physical equilibrium of the stratosphere and their impact on planetary climate and ecosystem.

Early studies recognized that Polar Stratospheric Clouds (PSCs) play a crucial role in this process, for being the primary surfaces where heterogeneous chemical reactions promote the formation of species responsible for ozone removal. In addition, PSC particle formation scavenges nitric acid and water vapor from the gas phase, causing denitrification and dehydration. As denitrification enhances chlorine radical lifetime, PSCs become a significant element for the chemical balance of the polar stratosphere. A brief survey of the chemistry of the stratospheric ozone and of stratospheric particles is here given, together with an outline of the dynamic of the stratosphere with emphasis on the polar regions. It is then presented the effect of the CFCs in the perturbation of the ozone chemistry in the winter polar stratosphere, and the twofold role of Polar Stratospheric Clouds, promoting the release of active chlorine from its reservoir species and denitrifying the stratosphere upon condensation of nitric acid and further removal by particle gravitational settling, is discussed. A survey of the main scientific activities carried out in the arctic to study the processes in which PSC are involved is then provided. An illustration on PSC formation microphysics theories, and related open issues, ends the chapter.

Noctilucent clouds (NLC) – also known as polar mesospheric clouds (PMC) – occur at mid and high latitudes during the summer months in each hemisphere and are with altitudes of about 83 km the highest clouds in the terrestrial atmosphere. NLC are an optically thin phenomenon. They are known to consist of H₂O ice particles with radii of less than about 100 nm. The first reported sightings of NLC occurred in 1885, two years after the eruption of the volcano Krakatoa in 1883. They exhibit spatial and temporal variability over a large range of scales and react very sensitively to variations in ambient conditions. This high sensitivity makes them highly relevant observables for the investigation of a wide variety of different atmospheric processes including dynamical effects, solar-terrestrial interactions and long-term changes of the Earth’s middle atmosphere. The role of NLC as indicators of long-term changes in the mesopause region in particular has been a topic of intensive debate.

This chapter on NLC serves two purposes. First, it provides an overview of the current understanding of the basic characteristics of NLC. Secondly, it introduces the observational techniques employed by the science community to remotely sense NLC, including both passive and active methods.

In this chapter remote sensing techniques as applied to studies of Arctic aerosol are surveyed. They include the analysis of ground and shipborne observations of atmospheric aerosol using sunphotometers and also airborne/satellite observations using optical instrumentation (lidars, imagers, radiometers).

Arctic surface temperature has been increasing at a rate 2–3 times that of the global average in the last half century. Enhanced warming of the Arctic, or Arctic Amplification, is a climatic response to external forcing. Despite good results obtained by climatic models for the globe, the largest intermodel differences in surface temperature warming are found in the Arctic. The magnitude of this warming drives many different processes and determines the evolution of many climatic parameters such as clouds, sea ice extent, and land ice sheet mass. The Arctic Amplification can be attributed to the peculiar feedback processes that are triggered in the Arctic. Most of these processes include radiation interaction with the atmosphere and with the surface, all of them contributing to the radiation budget. It is then mandatory to correctly evaluate this budget both at the surface and at the top of the atmosphere and in the solar and thermal spectra. This can be done using both direct observations, from ground and from space, and model simulation via radiation transfer codes. This last approach need many observed input parameters anyhow.

In this contribution results on the evaluation of the radiation budget in the Arctic are first reviewed. Follows a detailed description of the effects of the most important atmospheric gases (carbon dioxide, methane, ozone etc.) on both shortwave and longwave radiation ranges. The same is illustrated for aerosol loading in the Arctic, based on a large dataset of aerosol radiative properties measured by means of sun-photometers in numerous Arctic stations. Finally, the effect of the surface reflectivity characteristics on the radiation budget is illustrated by means of albedo models specific for the Arctic.

Observations over the last decades showed large changes in the Arctic regions with a strong warming of the Arctic, which is about twice that of the global mean warming. The largest warming rates with up to 10 K since 1980 are reached near the surface in the Chukchi Sea in autumn and in the Barents Sea in winter. Changes in Arctic climate are a result of complex interactions between the cryosphere, atmosphere, and ocean and different processes contribute to the amplified warming signal such as the ice albedo feedback, changes in clouds and water vapour, enhanced meridional energy transport in the atmosphere and in the ocean, vertical mixing in Arctic winter inversions and temperature feedbacks.

The observed warming is concurrent with a large reduction of the sea ice cover particularly in summer and autumn. The impact of Arctic amplification and sea ice retreat on the atmospheric circulation is still discussed. Positive winter sea level pressure trends along the Siberian Arctic coast have been linked to negative winter temperature trends over Central Asia.

Ocean heat and freshwater transports into and out of the Arctic undergo changes as well with potentially strong consequences for deep water formation in the North Atlantic Ocean and the entire large scale oceanic circulation.

Climate projections indicate that the Arctic will continue to warm faster than the rest of the world in the twenty-first century. Whether summer sea ice is going to melt completely depends on the future emission scenario.

This chapter will review the state of knowledge of mechanisms of the observed changes, the potential consequences of future Arctic warming for sea ice, ocean and atmosphere, and uncertainties due to emission scenarios, model shortcomings and natural variability.