Long-range transport of pollution to Europe

Ozone (O3) and aerosols are harmful to human health. Long-range transport sources contribute to the background levels upon which local pollution builds. The goal of this thesis is to describe and quantify the respective contribution of local and long-range transported sources to the O3 and aerosol budget in the framework of European air quality management. This issue is examined using a global model of chemistry and transport, the GEOS-Chem model, constrained by several experimental datasets (e.g. trace gases and aerosol distributions, aerosol optical depth, particulate matter concentrations) taken from field experiments, ground-based sites and satellite. The capabilities of the model to reproduce O3 and aerosol patterns is first examined. The model reproduces well in general the distribution of O3 and related species over the North Atlantic/Europe area. The simulation of particulate matter smaller than 2.5 μm (PM2.5) and aerosol optical depth (AOD) over Europe is also satisfying in general, but this may reflect some compensating errors between individual components (e.g. underestimate of organic matter, and overestimate of sulphate and dust). Intercomparison between the GEOS-Chem and another global model (MOZECH) also reveals possible problems in the water vapour distribution associated with the processes which drive the vertical lifting of pollutant over continental boundary layers. Despite these (relative) deficiencies, the model provides anyway insights about the contribution of long-range transport on the O3 and aerosol loads over Europe. The impact of continental outflow on the O3 chemical tendencies over oceanic regions is quantified. Net O3 photochemical production in polluted air masses travelling over oceanic areas under the influence of continental outflow is enhanced all-year round compared to the background marine environment by 2 to 6 ppbv/day in the boundary layer and by 1 to 3 ppbv/day in the upper troposphere. The origin of O3 long-range sources and their impact on the European O3 budget, as well as the role of changing emissions on this budget over the pasts decades is further investigated. Import of North American O3 into Europe is mainly controlled by meteorological patterns and by photochemical production over the North Atlantic and thus reaches a maximum in summer. In addition, Asia contributes to long-range transported O3 pollution by the Indian summer monsoon easterly winds during summer. During the monsoon period, convection is strong and associated NOx lightning emissions also contribute to high O3 levels, especially in the Mediterranean basin. North American and Asian anthropogenic pollution contribute substantially to the annual O3 burden (integrated over the whole tropospheric column) over Europe, accounting for 11% and 8%, respectively while the European contribution only accounts for 9%. The increase in Asian emissions from 1980 to 1997 have compensated the local reduction of O3 precursors, especially in the free troposphere. Finally, the aerosol load over Europe and the contribution of long and medium-range sources including anthropogenic and biomass burning pollution from North America and mineral dust from North Africa is examined. The model was used to interpret MODIS (Moderate Resolution Imaging Spectroradiometer) AOD and observations provided by the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign over the North Atlantic and European area. Trans-Atlantic transport of aerosol is controlled by meteorological patterns and scavenging processes and reaches a maximum in spring. North American uxes of aerosols reach Europe at flower altitudes than that of O3 because a larger fraction of aerosols are scavenged in the venting from the boundary layer by frontal passages and deep convection. During high vegetation fire events in the boreal forest of Alaska and Canada, biomass burning pollution could also reach Europe at high altitudes, because a fraction of fire emissions are emitted directly in the free troposphere. Finally, the Saharan region also contributes significantly to the aerosol load over Europe. European sources contribute by 58% and 48% to the surface PM2.5 and column AOD over Europe. The second main sources are the mineral dust which represent in average 20% of the surface PM2.5 and AOD. In summer, North American anthropogenic and biomass burning emissions represent between 2 to 5% of the surface PM2.5 and AOD. The PM2.5 daily standard levels of the World Health organization (WHO) (10 μg/m3) and of the U.S. Environmental Protection Agency (EPA) (65 μg/m3) are often exceeded during dust outbreaks, especially in southern Europe. Long-range transport of anthropogenic and natural pollution is thus an important issue which should be considered in the definition of air quality standards and regulation treaties.

Impact of transatlantic transport episodes on summertime ozone in Europe

Isabelle Bey

This paper reports on the transport of ozone

{O}_{3}

and related species over the North Atlantic ocean and its impact on Europe. Measurements of nitrogen dioxide (

{NO}_{2}

)and carbon monoxide (CO) columns from the GOME and MOPITT satellite instruments, respectively, are used in conjunction with the GEOS-CHEM global model of transport and tropospheric chemistry to identify the major events of long range transport that reach Europe over the course of summer 2000. Sensitivity model simulations are used to analyse observed

{O}_{3}

distributions with respect to the impact of long range transport events. For that purpose, we used in-situ

{O}_{3}

observations taken at the mountain site of Jungfraujoch as well as O3 vertical profiles taken in the vicinity of central European cities. Over the course of summer 2000, we identified 9 major episodes of transatlantic pollution transport; 7 events are associated with transient cyclones while 2 events occur through zonal transport (e.g. by advection in the strong low-level westerly winds established in summer between the Azores anticyclone and transient cyclones). We find that on average three episodes occur per month with the strongest ones being in June. The number and frequency of long range transport events that reach Europe are driven by the position and strength of the Azores anticyclone. Model sensitivity simulations indicate that the summer mean North American

{O}_{3}

contribution ranges from 3 to 5 ppb (7–11%) in the planetary boundary layer and 10 to 13 ppb (18–23%) in the middle and upper troposphere. During particular episodes, North American sources can result in

{O}_{3}

enhancements up to 25–28 ppb in the layer between 800–600 hPa and 10–12 ppb in the boundary layer. The impact of the zonal transport events on

{O}_{3}

distribution over Europe is more clearly seen below 700 hPa as they tend to transport pollution at lower levels while the events associated with transient cyclones are more likely to have an impact on the middle and upper troposphere (i.e. above 600 hPa). The air mass origins found in the GEOS-CHEM model are clearly confirmed by back trajectory analyses. During most of the 9 events, a strong contribution in North American

{O}_{3}

is in general associated with only little European

{O}_{3}

and viceversa (in particular at the Jungfraujoch). A substantial North American contribution (e.g., 30% or higher) to

{O}_{3}

over Europe does not always result in pronounced

{O}_{3}

enhancements in the observations during our period of study.

2006

Chemistry and climate interactions

Luca Pozzoli

Atmospheric trace gases and aerosols and climate interact in many ways. A quantitative assessment of the influence of trace gases and aerosols on climate can only be achieved if the interactions and feedbacks among these three major components are accounted for. The goal of this thesis was to develop, evaluate and apply the ECHAM5-HAMMOZ chemistry-aerosol-climate model. The model includes fully interactive simulations of the NOx-Ox-hydrocarbons chemistry and of the aerosol chemistry and microphysics, the two simulations being embedded into the well established ECHAM5 climate model. In particular, on-line calculation of the photolysis rates (that accounts for aerosols and clouds) and heterogeneous reactions of trace gases on aerosols are accounted for in the model. In addition, the aerosol simulation provides a prognostic representation of the mixing state of the aerosol components, a feature that is particularly important to examine the effect of the SO2 uptake and subsequent sulfate formation on aerosols. A thorough model evaluation of the model was performed using observations from the Transport and Chemical Evolution over the Pacific (TRACE-P) aircraft campaign. We also used ground based measurements from the European network EMEP (SO2 and sulfate), from the north American network IMPROVE (sulfate, black carbon and organic carbon), and from AERONET (aerosol optical depth). The coupled model is able to reproduce fairly well many of the observations over the TRACE-P region, even though some improvements are still needed. For example, we find a mean absolute bias of 20 ppbv (30%) and 40 ppbv (13%) for the simulated O3 and CO, respectively. Sulfate concentrations are represented fairly well, for all the regions considered, while SO2 is overestimated by a factor of 2 in general. Black carbon concentrations are underestimated over the TRACE-P region (mean absolute bias of 80%), most probably because of too low emissions, but well reproduced over north America. The aerosol optical depths compare well with observations at many sites in general, both in terms of annual means and seasonal variations. We show the results from a series of sensitivity simulations which goals are to assess the impact of heterogeneous reactions, photolysis reactions and sulfur chemistry on the regional and global trace gas distribution, and aerosol distribution, composition and optical properties. We found that heterogeneous reactions result in a reduction of 7% in the global O3 burden, and by up to 15% in surface O3 over regions rich in mineral dust. OH burden decreases by 10%, NO and NO2 by 20% and 29%, respectively while CO burden increases by 7%. Our numbers fall in general within the range of previous studies. We find that the effect of aerosols through the modifications of photolysis rates do not affect significantly the trace gas distributions and global burdens, while previous studies suggested larger effect. Heterogeneous reactions reduce the global mean SO2 surface concentration by 14%, while the global burden remains unchanged. This reflects the effect of competing processes. In particular, the depletion in SO2 by direct uptake on sea salt and dust is counterbalanced by the decrease in SO2 oxidation that is associated with the decrease in OH. The balance between the two competing effects differs depending on the regions (SO2 increases by more than +10% over north Africa and Indian Ocean and decreases by up to -20% at high latitudes). These processes, in turn, affect the sulfate formation. We find in general an increase (by up to 4%) in sulfate burden over the oceans because of sea salt uptake of SO2, and a decrease by up to 6% over Sahara and India, where SO2 increases. Our global burden of SO2 and sulfate amount to 0.77 and 0.87 (Tg(S)), respectively, which fall in the range of previous estimates. We find that SO2 uptake on sea salt contributes to the production of 3.69 Tg(S) yr-1 of sulfate (5% of total sulfate production) in good agreement with recent observation-based estimates. Uptake of SO2 on dust only contributes to the production of 0.55 Tg(S) yr-1 (< 1%) of sulfate, but it is an important process as it is responsible for the coating of mineral dust particles. A total of 300 Tg yr-1 of dust are transferred from the insoluble to the soluble mixed modes (56% of total emissions) because of this later process. The total burden of black carbon does not change significantly because of the heterogeneous reactions, but it is redistributed between insoluble/soluble modes. The changes in the mixing state of aerosols produce significant regional variations in aerosol optical depth and absorption. The aerosol optical depth and aerosol absorption increase by 10 to 30% and by 15%, respectively, during winter in the main polluted regions of the north hemisphere because of enhancing absorption efficiency of black carbon mixed with sulfate. On the contrary aerosol absorption decreases by up to 20% over the Atlantic ocean because of enhancing black carbon removal by wet scavenging. ECHAM5-HAMMOZ is a fully coupled model which includes many of the trace gas-aerosol and chemistry-climate interactions, and is thus a tool that allows an integrated assessment of the impact of the major short-life species that contribute to climate change and air quality.

EPFL2007