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Workshop “Atmospheric monitoring and inverse modelling for verification of national and EU bottom-up GHG inventories” - reportWorkshop "Atmospheric monitoring and inverse modelling for verification of national and EU bottomupGHG inventories " - reportBaseline trends and top-down estimates of UK and NW EuropeanGHG emissionsAlistair ManningMet Office, Exeter, UKIntroductionHigh frequency observations on the remote west coast of Ireland have been used toestimate the Northern Hemisphere background trends of methane, nitrous oxide anda range of HFCs. Subtracting the background concentration from the observationyields a quantity that represents the impact of regional pollution on the atmosphericconcentration. By understanding the recent history of the air arriving at theobservation station and importantly how emissions dilute with time it is possible,through inverse modelling, to estimate the magnitude of emissions from differentgeographical regions. Emission estimates for the UK and other European regionsover a 12 year period (1995-2006) are presented for methane, nitrous oxide and arange of important HFCs (134a, 152a, 125, 365mfc).Northern Hemisphere baseline trendsThe Mace Head observing station is situated on the west coast of Ireland and is partof the global AGAGE (Advanced Global Atmospheric Gases Experiment) network ofsites. It is ideally situated to observe air from the Atlantic that has received no landemissions for thousands of kilometres. The majority of air therefore is a goodrepresentation of mid-latitude composition, referred to as baseline air, and can beused to assess annual and seasonal trends. The Mace Head station records acomprehensive set of greenhouse gases to state-of-the-art accuracy at high timeresolution.This study attempts to isolate those times from 1995 onwards that are representativeof mid-latitude baseline air by using a sophisticated atmospheric dispersion model,NAME (Numerical Atmospheric dispersion Modelling Environment) and statisticalpost-processing. NAME is a Lagrangian model developed following the Chernobylaccident to understand the movement of material in the atmosphere and has sincebeen used in a wide range of emergency and policy-based applications. In this workthe 3D meteorology has been obtained from the UK Met Office numerical weatherprediction model, the Unified Model, with a horizontal resolution of 55km falling to40km, a vertical resolution of 12 levels in 1995 increasing to 33 levels by 2006 andan analysis output every 3 hours.NAME has been run backwards in time for ten days for each 3 hour period from 1995until the end of 2006 releasing thousands of model particles at Mace Head. For each3 hour period therefore a map is produced estimating all of the surface (0-100m)contributions within ten days of travel arriving at Mace Head during that 3 hourinterval (figure 1). By selecting only those times when the history of the air massindicates negligible impact from the European land mass, or transport from tropical42
Workshop “Atmospheric monitoring and inverse modelling for verification of national and EU bottom-up GHG inventories” - reportWorkshop "Atmospheric monitoring and inverse modelling for verification of national and EU bottomupGHG inventories " - reportlatitudes or significant local contributions, it is possible to isolate those times classedas mid-latitude baseline. For a selection of greenhouse gases measured at MaceHead these baseline observations have been selected and then further refined byapplication of a statistical filter to remove outlying data points. Monthly averages ofthe resulting data are used to assess the annual and seasonal baseline trends ineach measured gas. The methodology has been applied to methane, nitrous oxide,and a range of HFCs covering the period 1995-2006 inclusive, figure 2.Figure 1: Example of air history map produced by NAME. The darker shades indicategreater surface contributions from that area to the air arriving at Mace Head during that 3hour period.Estimating European emissions using top-down inversion methodologyThe aim is to generate regional emission distribution estimates from ‘polluted’ (abovebaseline) observations. The emissions are defined as the geographical vector e thathas n elements indicating n geographical regions. The NAME model is used topredict the concentration time-series (t elements) at each observation point (MaceHead) from each potential source region. This information is captured in the transportand dilution matrix A which has t rows and n columns. The observation time-seriesrepresenting regional pollution is determined by subtracting the estimated timevaryingbaseline concentration from each observation, this generates the observationvector m that has t elements.For methane the observations from three stations have been used to better constrainthe inversion. The stations are Mace Head in Ireland, Deuselbach in westernGermany and Neuglobsow in eastern Germany. All three are Global AtmosphericWatch (GAW) stations. Concurrent data from all three stations only exist up until theend of 2004. In order to be able to use the data from all three stations it is necessarythat all the observations are inter-comparable so that all the data can be representedon the same calibration scale. In this case the data were all converted onto theJapanese scale. As both of the German sites are within the ‘polluted’ regional domainit is difficult to estimate the baseline concentrations at these stations. As all threestations have similar latitudes it has been assumed here that the baselineconcentration estimated at Mace Head would be applicable at both of these othersstations. Also because methane has natural biogenic sources that can exist close tothe observation sites and thus dominate the signal, all observations recorded duringperiods when ‘local’ effects would be significant, e.g. low wind speed and low43
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