Influence of El Nino-Southern Oscillation on the interannual variability of tropospheric ozone in the nothern midlatitudes

We use the GEOS-Chem model to interpret long-term measurements of tropospheric ${O}_{3}$ and CO and to investigate the factors that contribute to their interannual variation (IAV) during the period from 1987 to 2005 with a focus on the northern mid-latitudes. We find that the model reproduces relatively well the observed IAV of ${O}_{3}$ and CO in general, except the negative anomalies in ${O}_{3}$ from 1991 to 1993 as well as the following upward trend in 1993-1996 observed at several sites in the northern mid-latitudes which are not represented by the model. We suggest that this likely results from a poor representation of stratospheric chemistry and dynamics in the model. Using a variety of sensitivity and tagged ${O}_{3}$ simulations, we examine in details the period of 1998-1999, when a large anomaly in TOC is observed and simulated over Europe (maximum of +4.9 DU or +14% in February 1998). Three consecutive periods can be distinguished during this longlasting anomaly, during which different processes affected the ${O}_{3}$ burden over Europe. The first period (spring 1998) is more largely influenced by the preceding 1997 El Nino that affects 1) stratosphere-troposphere exchange and 2) Asian pollution export and transport towards Europe by a change in convective activity in East Asia and a strengthening of the subtropical jet stream. An enhanced pollution export from North America is also noticed for this period. The second period (summer-fall 1998) shows a mixed influence from boreal wildfires occurring in summer 1998 and from Asian pollution. The third period is more significantly influenced by enhanced wildfires in Southeast Asia. Throughout the period from 1997 to 2005, positive anomalies in tropospheric ${O}_{3}$ column and in surface ${O}_{3}$ are found over Europe the following spring of an El Nino year.

Interannual variability in tropospheric ozone and its precursors

Symeon Koumoutsaris

Tropospheric ozone (O3) is a harmful pollutant and an increasingly important green-house gas. Tropospheric O3 distributions are determined by complex and non-linear interactions between photochemistry and transport processes. The first objective of this thesis is to examine the factors that contribute to the interannual variation (IAV) of tropospheric O3 and related compounds, such as carbon monoxide (CO), for a period of 19 years (1987-2005), with a focus over the northern mid-latitudes. A series of long-term measurements, including ozonesondes, groud-based sites and satellites, are interpreted with a global chemical-transport model (GEOS-Chem) and a number of sensitivity simulations are performed to quantitatively assess the relative contribution of individual processes to the IAV of O3 and CO. We first assess the capability of the model to reproduce O3 and CO IAV. We find that the model is able to reproduce reasonably well many of the observed features of the O3 and CO IAV. The model however, fails to reproduce the observed variation in O3 in the northern mid-latitudes during the first half of the 1990s (negative anomaly in 1991-1993 followed by an upward trend in 1993-1996) which is likely due to an inadequate representation of stratospheric dynamics and stratospheric O3 concentrations in the model. This study has demonstrated that the principal drivers of O3 and CO IAV are meteorology and emissions. We examine in particular the 1998-1999 period. A large anomaly in O3 was observed at several sites over Europe and is fairly well reproduced by the model. Different processes are found to contribute to this long-lasting anomaly. The first period (spring 1998) was largely influenced by the strong preceding El Niño episode of 1997. El Niño Southern Oscillation (ENSO) affects the stratosphere-troposphere exchange (STE) and the Asian pollution export, through a change in convective activity and a strengthening of the subtropical jet. North American pollution export was also enhanced through an increase in temperature and in photochemical activity. During the second period (summer-fall 1998), the O3 budget over Europe was influenced by both boreal wildfires and Asian pollution. Finally, the O3 anomaly during the third period (spring 1999) was affected by enhanced biomass burning emissions in Southeast Asia. We further noticed that a significant correlation exists between the Southern Oscillation Index (SO1) in fall and the European tropospheric O3 column in the following spring (R= -0.65, N=19), throughout the period from 1987 to 2005. In other words, increasing tropospheric O3 column is seen over Europe the following spring of an El Niño year. In the second part of this thesis, we used inversion methods to derive monthly NOx, emissions from GOME and SCIAMACHY space-based retrievals of nitrogen dioxide (NO2) columns for a period of 10 years (1996-2005). We calculated emission trends over the highly industrialized regions and large cities around the globe. Decreasing emissions were found in the U.S. and Europe. However, negligible or even positive trends were seen in urban areas in northeastern U.S. (e.g. Boston) and southern Europe (e.g. Lisbon, Istanbul) which are most probably due to increasing motor vehicle NOx, emissions. Furthermore, a slowdown in the decreasing trend in NOx, combustion fuel emissions over North Europe has been noticed after 2000 and especially in 2003-2004. This is probably related to a slowdown in the decrease in road transport emissions. Increasing trends, on the other hand, have been found in the combustion fuel NOx, emissions over east China (8.3% year-1), due to the fast industrial and economical development of China. In addition, we found an upward trend in soil NOx, emissions which may be related to the increasing use of fertilizers in the northeastern part of the country. Finally, we examined the influence of using two different retrievals of NO2 columns on the resulting a posteriori inventories. The inversion was carried out twice for the year 2000, using two different retrievals, one from Dalhouse/SAO and one from BIRA/KNMI. The two resulting inventories were found to correspond well globally, but show important differences (more than 60%) at specific times and regions.

EPFL2008

Long-range transport of pollution to Europe

Marion Auvray

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.

EPFL2006

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