Arctic air pollution transported from the mid-latitudes
In late winter and early spring, Eurasian pollution can be efficiently transported to the Arctic at low altitudes (Rahn, 1981), causing Arctic Haze. This strong influence of Eurasian emissions is due, in part, to the position of the Arctic front. Air masses traveling from the mid-latitudes to the Arctic usually rise along surfaces of constant potential temperature (isentropic transport). These surfaces form a fictional « dome » called the Arctic front, which isolates the lower Arctic troposphere from the mid-latitudes (Klonecki et al., 2003; Stohl, 2006). In addition, these rising air masses are usually associated with precipitation, which remove pollutants from the atmosphere (“wet removal”) during transport. On the contrary, pollutants emitted North of the Arctic front can be easily transported to the Arctic surface. Figure 1-10 shows the position of the Arctic front during winter and during summer, as well as the main atmospheric pathways from the mid-latitudes to the Arctic.
During winter and spring, the Arctic front can extend south down to 40 ∘N over Europe and Russia due to the extensive snow cover and low temperatures there. Eurasian emissions within the Arctic front can then be transported into the lower Arctic troposphere. In winter and spring, pollution removal processes are also lower in Eurasia and in the Arctic due to strong atmospheric stability and reduced precipitation (Shaw, 1995; Garrett et al., 2011), causing the buildup of Arctic Haze. During summer, the Arctic front is located further north, and removal processes are higher, isolating the Arctic atmosphere from pollution in the mid-latitudes.
The main source regions of Arctic pollution are presented on Figure 1-11. This Figure shows the result of an earlier multi-model analysis (Shindell et al., 2008), estimating the rel-ative contributions of Europe, South Asia, East Asia, and North America to Arctic pollution at the surface and in the upper troposphere (250 hPa). These contributions were estimated for two aerosol components (BC and sulfate) and two trace gases (CO and ozone), by per-forming simulations with 20 % reduction in anthropogenic emissions of pollution precursors from each source region.
Developing local sources of Arctic pollution
Aerosols and ozone can be chemically destroyed or deposited during transport. These re-moval processes (described in Chapter 2) limit the efficiency of long-range transport of pollution from the mid-latitudes. Furthermore, pollution transport from the mid-latitudes occurs along rising isentrops, which tend to bring remote pollution at high altitudes in the Arctic (Stohl, 2006). Local Arctic emissions are, by definition, directly emitted in the Arctic boundary layer and do not experience aging during transport. For this reason, local sources can influence Arctic surface pollution (Sand et al., 2013) and Arctic pollution burdens (Wang et al., 2014a) with much higher per-emission efficiency than remote sources.
However, anthropogenic emissions in the Arctic are thought to be small compared to other regions. There are very few large cities north of the Arctic circle, the most populated being Murmansk in Russia ( ∼ 300,000 inhabitants); other large cities include Norilsk in Russia ( ∼ 175,000 inhabitants), Tromsø and Bodø in Norway ( ∼ 70,000 and 50,000 inhab-itants). There are some industrial sources of pollution north of the Arctic circle, such as mines and metal smelters in Norilsk, Russia (AMAP, 2006), and metal smelters in the Kola Peninsula, Russia (Prank et al., 2010). Oil and gas related activities in northern Russia and Norway (AMAP, 2006; Peters et al., 2011) are thought to be an important local source of Arctic pollution (especially for BC), and recent studies (Stohl et al., 2013; Huang et al., 2015) indicate that these emissions might be higher than previously thought.
Arctic shipping emissions are another noteworthy local source of pollution, emitting NOx, SO2 (forming sulfate) and BC along with other pollutants. The Arctic council’s AMSA report (Arctic Marine Shipping Assessment, Arctic Council, 2009) found that, in 2004, about 6000 ships operated within the Arctic (latitude > 60∘N). This traffic mostly takes place along the Norwegian Coast, in northwestern Russia, around Iceland, in southwestern Greenland and in the Bering Sea. Arctic shipping is made up of a combination of supply ships for Arctic communities, bulk transport of resources extracted within the Arctic region, fishing ships, passenger ships and cruise ships (Arctic Council, 2009).
In addition to this local Arctic shipping, it has long been known that the routes through the Arctic Ocean are the shortest way from northern Europe and northwestern America to Asia. For example, the Arctic route between the Netherlands and Eastern Asia (called the Northern Sea Route, NSR, or Northeast passage, NEP) is 40 % shorter than the corre-sponding route through the Suez Canal (Liu and Kronbak, 2010). The Arctic route from Northeastern America to Eastern Asia (the northwest passage, NWP) is 15 to 30 % shorter than the corresponding route through the Panama canal (Somanathan et al., 2009). These routes (shown in Figure 1-12) could be used to save trip distance and costs and might al-ready be profitable, but they are not widely used yet due to the presence of sea ice, leading to additional costs, additional risks due to potential ice damage, and reduced vessel speeds (Liu and Kronbak, 2010; Somanathan et al., 2009).
The NSR and NWP could become more economically competitive along with Arctic sea ice decline. Models and observations indicate that the number of ice-free days per year along the NSR and NWP increased by 22 and 19 days between 1979–1988 and 1998–2007 (Mokhov and Khon, 2008). As a result, transit along these routes is increasing: a record number of 71 ships transited through the NSR in 2013 (Northern Sea Route information office, 2013). At the same time, decreasing sea ice extent also contributed to a rise in Arctic cruise tourism (Stewart et al., 2009). The ice-free shipping season is expected to continue to lengthen due to climate change (Prowse et al., 2009; Khon et al., 2009). This will allow increased traffic along the NSR and NWP, by opening these routes to ships with no hull ice strengthening (Smith and Stephenson, 2013). As a result, Corbett et al. (2010) estimate that Arctic shipping emissions of NOx and BC could increase by a factor of 10 between 2004 and 2050 (high-growth scenario).
Increased shipping access in the Arctic is also expected to facilitate resource extraction in this region (Prowse et al., 2009). The Arctic contains vast resources of minerals (Lindholt, 2006), oil, and gas (Gautier et al., 2009). Arctic oil and gas resources are already being exploited, and the Arctic is expected to remain an important producer of oil by 2050, while its relative importance in gas production could decrease due to its high extraction prices (Lindholt and Glomsrød, 2012; Peters et al., 2011, projections shown in Figure 1-13). The oil and gas sector is expected to keep contributing to future local pollutant emissions in the Arctic (Peters et al., 2011), although current and future emission inventories from this source remain very uncertain.
Scientific challenges in modeling Arctic aerosols and ozone and their impacts
In this thesis, aerosol and ozone pollution in the Arctic is studied using regional simulations of the Arctic troposphere, and new global and local emission inventories of Arctic pollution. Model results are used to analyze recent aircraft measurements in the Arctic. This approach (details in Chapter 3) was motivated by results from previous studies, which showed that modeling aerosol and ozone pollution in the Arctic was especially challenging.
Modeling aerosol and ozone pollution from long-range transport
Models do not represent aerosols well in the Arctic. Shindell et al. (2008) showed that models often underpredicted sulfate at Arctic surface stations, and greatly underpredicted BC, while several models struggled to reproduce the seasonal cycle of surface aerosol concentrations. Shindell et al. (2008) attributed this poor agreement to the treatment of aerosol aging and removal within models. Koch et al. (2009) and Schwarz et al. (2010) compared global models to different sets of aircraft observations of BC in the Arctic, and found that models underestimated BC at the surface but overestimated it aloft. A more recent intercomparison by Lee et al. (2013) also indicates that most models strongly underestimate surface BC observations in the Arctic, especially during winter and spring. Several studies (Huang et al., 2010; Liu et al., 2012; Browse et al., 2012; Wang et al., 2013) showed that Arctic BC could be improved by the use of more complex wet removal schemes within models. However, implementing these schemes does not fully resolve model disagreement with measurements (Browse et al., 2012; Wang et al., 2013, 2014b; Eckhardt et al., 2015).
Additionally, most models included in the recent intercomparison in the Arctic of Em-mons et al. (2015) underestimate ozone in the middle and high Arctic troposphere by ∼ 10 to 30 %, and exhibit stronger biases for ozone precursors such as NOx, carbon monoxide (CO), peroxyacetyl nitrate (PAN) and several VOCs. Results from another recent model intercomparison in the Arctic performed by AMAP (Arctic Monitoring and Assessment Programme, AMAP, 2015) indicates that models are strongly biased for both O3 and its precursors. These biases are attributed to uncertainties in emissions, errors in stratosphere-troposphere exchange and uncertainties related to the hydroxyl radical OH (these processes are described in Section 2.1).
Modeling aerosol and ozone pollution from local Arctic sources
Emissions from local Arctic sources are not well quantified, which makes investigating their impacts difficult. There are very few specific emission inventories focused on local Arctic sources, and existing inventories are known to be incomplete. The current and future Arctic shipping inventories of Corbett et al. (2010) do not include fishing ships, which constitute a significant proportion of Arctic shipping (McKuin and Campbell, 2016). In addition, these inventories are based on the AMSA shipping dataset, which might underestimate Arctic marine traffic (Arctic Council, 2009). Other shipping inventories (Dalsøren et al., 2007, 2009; Peters et al., 2011) are also known to be biased towards specific ship types (i.e. container ships and large ships). Furthermore, Arctic shipping inventories can quickly become out of date as the local traffic increases and new emission control regulations are implemented (e.g., (Jonson et al., 2015)). A new Arctic shipping inventory based on ship positioning by satellite was developed recently by Winther et al. (2014), but it has not yet been validated against measurements.
Emissions from the oil and gas sector are also very uncertain, as most oil and gas activity in the Arctic is located in northern Russia, where very few observations are available to validate inventories. Peters et al. (2011) estimated current and future emissions from Arctic oil and gas activities, but recent inventories (Huang et al., 2015; Klimont et al., 2015) indicate that this earlier estimate might be too low, especially in terms of BC emissions (Stohl et al., 2013).
For these reasons, earlier studies based on the inventories of Corbett et al. (2010), Dal-søren et al. (2007) and Peters et al. (2011), could be underestimating the impacts of Arctic shipping emissions. Furthermore, models do not represent well aerosol pollution in the Arc-tic. This could have a strong impact on results when studies report relative impacts of local emissions over this uncertain background.
Until recently, there was also no specific field measurements focused on Arctic shipping or Arctic resource extraction that could be used to study the impacts of local Arctic emissions, to assess model performance, and to validate inventories. Such a dataset is now available from the ACCESS aircraft campaign (Roiger et al., 2015), which took place in northern Norway in summer 2012 and specifically targeted ships and oil and gas platforms in the Norwegian and Barents seas.
Global emissions of NOx, CO, CH4 and non-methane VOC (NMVOC)
Tropospheric O3 is produced from NOx, CO, CH4, and other (non-methane) VOC. At the global scale, the main source of NOx and CO is human activity (Table 2.1). Natural emis-sions of NOx are due to soils, lightning and wildfires. VOC emissions are mostly natural, due to methane sources from wetlands, and emissions of isoprene and terpenes by vegeta-tion. Human VOC emissions include methane, several alkanes and alkenes, and aromatic compounds such as benzene and toluene. The non-methane VOC category represents a large variety of compounds with different reactivities and different impacts on ozone production.
Peroxyacetyl nitrate (PAN) as a NOx reservoir in the troposphere
NOx have a relatively short lifetime of 0.5–2 days in the troposphere and therefore cannot be transported over long distances. However, research has shown that NOx could be transported at the hemispheric scale through the formation of a reservoir species, peroxyacetyl nitrate (PAN, CH3C(O)OONO2) (e.g. Singh et al., 1986). PAN is formed by the oxidation of carbonyl compounds (e.g. acetaldehyde, CH3CHO) by NO2, and its main sink is thermal decomposition: PAN −−→ CH3C(O)OO + NO2 (2.24) The lifetime of PAN against thermolysis is 1 h at 295 K, and several months at 240 K (Jacob, 2000). As a result, PAN can be formed during high-altitude pollution transport, and can be decomposed to release NO2 in remote regions when reaching lower altitudes. PAN is thought to be an important source of surface and lower tropospheric ozone in the Arctic during summer (e.g. Mauzerall et al., 1996).
Table of contents :
Introduction en français
1 Climate change and air pollution in the Arctic
1.1 Global air pollution and climate change
1.1.1 Air pollution
1.1.2 Global climate change
1.2 Arctic climate change: causes and future projections
1.2.1 What is the Arctic?
1.2.2 Current Arctic warming
1.2.3 Causes of Arctic warming
1.2.4 Future projections
1.3 Arctic air pollution
1.3.1 Arctic Haze
1.3.2 Arctic air pollution transported from the mid-latitudes
1.3.3 Developing local sources of Arctic pollution
1.4 Scientific challenges in modeling Arctic aerosols and ozone and their impacts
1.4.1 Modeling aerosol and ozone pollution from long-range transport
1.4.2 Modeling aerosol and ozone pollution from local Arctic sources
2 Tropospheric ozone and tropospheric aerosols in the Arctic
2.1 Tropospheric ozone
2.1.1 Introduction: stratospheric and tropospheric ozone
2.1.2 Chemical O3 production in the troposphere from NOx and VOC
2.1.3 Photochemical sinks of ozone, HOx and NOx in the troposphere
2.1.4 Dry deposition of NOx and O3
2.1.5 Peroxyacetyl nitrate (PAN) as a NOx reservoir in the troposphere
2.1.6 The global budget of tropospheric ozone
2.1.7 Radiative effects of tropospheric ozone
2.1.8 Tropospheric ozone in the Arctic
2.2 Tropospheric aerosols
2.2.1 Global aerosol sources
2.2.2 Aerosol properties: chemical composition, mixing state, size
2.2.3 Aerosol processes: from nucleation to removal
2.2.4 Aerosol optical properties
2.2.5 Aerosol radiative effects
3 Methods: modeling tools, emission inventories and Arctic measurements
3.1 Modeling the air quality and radiative impacts of short-lived pollutants in the Arctic.
3.1.1 Regional meteorology-chemistry-aerosol modeling with WRF-Chem
3.1.2 Lagrangian modeling with FLEXPART-WRF
3.2 Air pollutant emissions from global and local Arctic pollution sources
3.2.1 Global anthropogenic emissions from ECLIPSEv5 and HTAPv2
3.2.2 Biomass burning emissions
3.2.3 Natural emisssions calculated online within WRF-Chem
3.2.4 Local Arctic pollutant emissions from oil and gas extraction
3.2.5 Local Arctic emissions from shipping
3.3 Aerosol and ozone measurements in the Arctic
3.3.1 Surface measurements
3.3.2 POLARCAT-France and ACCESS aircraft measurement campaigns in the Arctic
4 Transport of pollution from the mid-latitudes to the Arctic during POLARCATFrance
4.2 Transport of anthropogenic and biomass burning aerosols from Europe to the Arctic during spring 2008 (Marelle et al., 2015).
4.2.4 Meteorological context during the spring POLARCAT-France campaign
4.2.5 Model validation
4.2.6 The origin and properties of springtime aerosols during POLARCATFrance
4.2.7 Impacts of European aerosol transport on the Arctic
4.2.8 Summary and conclusions
4.3 Main insights from the study
4.3.1 Aerosol transport to the Arctic
4.3.2 Ozone transport to the Arctic in these simulations, and in the related work of Thomas et al. (2013)
5 Current impacts of Arctic shipping in Northern Norway
5.2 Air quality and radiative impacts of Arctic shipping emissions in the summertime in northern Norway: from the local to the regional scale (Marelle et al., 2016).
5.2.3 The ACCESS aircraft campaign
5.2.4 Modeling tools
5.2.5 Ship emission evaluation
5.2.6 Modeling the impacts of ship emissions along the Norwegian coast
5.3 Main insights from the study
6 Current and future impacts of local Arctic sources of aerosols and ozone
6.1 Introduction and motivation
6.3 Model updates for quasi-hemispheric Arctic simulations
6.4 Model validation
6.5 Model internal variability and noise: issues when quantifying sensitivities to small emission perturbations with WRF-Chem
6.6 Local and distant contributions to surface concentrations and BC deposition in the Arctic
6.6.1 Surface concentrations and BC deposition in spring and summer 2012
6.6.2 Surface concentrations and BC deposition in spring and summer 2050 (2050 emissions)
6.7 Vertical distribution of Arctic aerosol and ozone pollution from remote and local sources
6.8 Radiative effects of aerosols and ozone in the Arctic.
6.8.1 Direct radiative effects of pollution aerosols and ozone in the Arctic.
6.8.2 Semi-direct and indirect radiative effects.
6.9 Conclusions and perspectives
Conclusion en français