Impacts of the large increase in international ship traffic 2000-2007 on tropospheric ozone and methane

Ozone source apportionment during peak summer events over southwestern Europe

It is well established that in Europe, high O₃ concentrations are most pronounced in southern/Mediterranean countries due to the more favourable climatological conditions for its formation. However, the contribution of the different sources of precursors to O₃ formation within each country relative to the imported (regional and hemispheric) O₃ is poorly quantified. This lack of quantitative knowledge prevents local authorities from effectively designing plans that reduce the exceedances of the O₃ target value set by the European air quality directive. O₃ source attribution is a challenge because the concentration at each location and time results not only from local biogenic and anthropogenic precursors, but also from the transport of O₃ and precursors from neighbouring regions, O₃ regional and hemispheric transport and stratospheric O₃ injections. The main goal of this study is to provide a first quantitative estimation of the contribution of the main anthropogenic activity sectors to peak O₃ events in Spain relative to the contribution of imported (regional and hemispheric) O₃. We also assess the potential of our source apportionment method to improve O₃ modelling. Our study applies and thoroughly evaluates a countrywide O₃ source apportionment method implemented in the CALIOPE air quality forecast system for Spain at high resolution (4 × 4 km²) over a 10-day period characterized by typical summer conditions in the Iberian Peninsula (IP). The method tags both O₃ and its gas precursor emissions from source sectors within one simulation, and each tagged species is subject to the typical physico-chemical processes (advection, vertical mixing, deposition, emission and chemistry) as the actual conditions remain unperturbed. We quantify the individual contributions of the largest NO _x local sources to high O₃ concentrations compared with the contribution of imported O₃. We show, for the first time, that imported O₃ is the largest input to the ground-level O₃ concentration in the IP, accounting for 46 %-68 % of the daily mean O₃ concentration during exceedances of the European target value. The hourly imported O₃ increases during typical northwestern advections (70 %-90 %, 60-80 μg m^-3), and decreases during typical stagnant conditions (30 %-40 %, 30-60 μg m^-3) due to the local NO titration. During stagnant conditions, the local anthropogenic precursors control the O₃ peaks in areas downwind of the main urban and industrial regions (up to 40 % in hourly peaks). We also show that ground-level O₃ concentrations are strongly affected by vertical mixing of O₃-rich layers present in the free troposphere, which result from local/regional layering and accumulation, and continental/hemispheric transport. Indeed, vertical mixing largely explains the presence of imported O₃ at ground level in the IP. Our results demonstrate the need for detailed quantification of the local and remote contributions to high O₃ concentrations for local O₃ management, and show O₃ source apportionment to be an essential analysis prior to the design of O₃ mitigation plans in any non-attainment area. Achieving the European O₃ objectives in southern Europe requires not only ad hoc local actions but also decided national and European-wide strategies.

Impact of shipping emissions on ozone levels over Europe: assessing the relative importance of the Standard Nomenclature for Air Pollution (SNAP) categories

The impact of shipping emissions on ozone mixing ratio over Europe is assessed for July 2006 using the Community Multiscale Air Quality modeling system and the Netherlands Organization for Applied Scientific Research anthropogenic emission inventory. Results suggest that ship-induced ozone contribution to the total surface ozone exceeds 5% over the sea and near the coastline, while an increase up to 5% is simulated over a large portion of the European land. The largest impact (i.e., an increase up to 30%) is simulated over the Mediterranean Sea. In addition, shipping emissions are simulated to increase NO₂ mixing ratio more than 90%, locally, and to modify the oxidizing capacity of the atmosphere through hydroxyl radical formation (increase by 20-60% over the sea along the European coasts and near the coastal zone). Therefore, emissions from ships may counteract the benefits derived from the anthropogenic emissions reduction strategies over the continent. Simulations suggest regions where shipping emissions have a major impact on ozone mixing ratio compared to individual anthropogenic emission sector categories. Shipping emissions are estimated to play an important role on ozone levels compared to road transport sector near the coastal zone. The impact of shipping emissions on ozone formation is also profound over a great part of the European land compared to the rest of anthropogenic emission categories.

Climate penalty for shifting shipping to the Arctic

The changing climate in the Arctic opens new shipping routes. A shift to shorter Arctic transit will, however, incur a climate penalty over the first one and a half centuries. We investigate the net climate effect of diverting a segment of Europe-Asia container traffic from the Suez to an Arctic transit route. We find an initial net warming for the first one-and-a-half centuries, which gradually declines and transitions to net cooling as the effects of CO2 reductions become dominant, resulting in climate mitigation only in the long term. Thus, the possibilities for shifting shipping to the Arctic confront policymakers with the question of how to weigh a century-scale warming with large uncertainties versus a long-term climate benefit from CO2 reductions.

Global-mean temperature change from shipping toward 2050: improved representation of the indirect aerosol effect in simple climate models

We utilize a range of emission scenarios for shipping to determine the induced global-mean radiative forcing and temperature change. Ship emission scenarios consistent with the new regulations on nitrogen oxides (NO(x)) and sulfur dioxide (SO(2)) from the International Maritime Organization and two of the Representative Concentration Pathways are used as input to a simple climate model (SCM). Based on a complex aerosol-climate model we develop and test new parametrizations of the indirect aerosol effect (IAE) in the SCM that account for nonlinearities in radiative forcing of ship-induced IAE. We find that shipping causes a net global cooling impact throughout the period 1900-2050 across all parametrizations and scenarios. However, calculated total net global-mean temperature change in 2050 ranges from -0.03[-0.07,-0.002]°C to -0.3[-0.6,-0.2]°C in the A1B scenario. This wide range across parametrizations emphasizes the importance of properly representing the IAE in SCMs and to reflect the uncertainties from complex global models. Furthermore, our calculations show that the future ship-induced temperature response is likely a continued cooling if SO(2) and NO(x) emissions continue to increase due to a strong increase in activity, despite current emission regulations. However, such cooling does not negate the need for continued efforts to reduce CO(2) emissions, since residual warming from CO(2) is long-lived.