C. E. Johnson

Tropospheric Ozone in a Global-Scale Three-Dimensional Lagrangian Model and Its Response to NOX Emission Controls

Abstract A three-dimensional Lagrangian tropospheric chemistry modelis used toinvestigate the impact of human activities on the tropospheric distributionofozone and hydroxyl radicals. The model describes the behaviour of 50 speciesincluding methane, carbon monoxide, oxides of nitrogen, sulphur dioxide andnineorganic compounds emitted from human activities and a range of other sources.Thechemical mechanism involves about 100 chemical reactions of which 16 arephotochemical reactions whose diurnal dependence is treated in full. The modelutilises a five minute chemistry time step and a three hour advection timestepfor the 50,000 air parcels. Meteorological data for the winds, temperatures,clouds and so on are taken from the UK Meteorological Office global model for1994 onwards. The impacts of a 50% reduction in European NOXemissions onglobal ozone concentrations are assessed. Surface ozoneconcentrations decrease in summertime and rise in wintertime, but to differentextents.

Possible role of wetlands, permafrost, and methane hydrates in the methane cycle under future climate change: A review

We have reviewed the available scientific literature on how natural sources and the atmospheric fate of methane may be affected by future climate change. We discuss how processes governing methane wetland emissions, permafrost thawing, and destabilization of marine hydrates may affect the climate system. It is likely that methane wetland emissions will increase over the next century. Uncertainties arise from the temperature dependence of emissions and changes in the geographical distribution of wetland areas. Another major concern is the possible degradation or thaw of terrestrial permafrost due to climate change. The amount of carbon stored in permafrost, the rate at which it will thaw, and the ratio of methane to carbon dioxide emissions upon decomposition form the main uncertainties. Large amounts of methane are also stored in marine hydrates, and they could be responsible for large emissions in the future. The time scales for destabilization of marine hydrates are not well understood and are likely to be very long for hydrates found in deep sediments but much shorter for hydrates below shallow waters, such as in the Arctic Ocean. Uncertainties are dominated by the sizes and locations of the methane hydrate inventories, the time scales associated with heat penetration in the ocean and sediments, and the fate of methane released in the seawater. Overall, uncertainties are large, and it is difficult to be conclusive about the time scales and magnitudes of methane feedbacks, but significant increases in methane emissions are likely, and catastrophic emissions cannot be ruled out. We also identify gaps in our scientific knowledge and make recommendations for future research and development in the context of Earth system modeling.

The European regional ozone distribution and its links with the global scale for the years 1992 and 2015

Abstract Because of global-scale increases in trace gas emissions, ozone concentrations in northern hemisphere may increase over the next decade, driving up ozone concentrations within Europe. Over this same period, policy actions are anticipated which will reduce the internal European regional-scale ozone production capacity. The overall success of these regional policies will be determined by the resultant of these global- and regional-scale influences. A global three-dimensional Lagrangian chemistry model STOCHEM has been used to look at the relative magnitudes of these two influences on the European regional ozone distribution under some illustrative emission scenarios up to the year 2015. Substantial reductions in European NOx emissions should bring a significant improvement in ozone air quality, but they may not be enough to keep future peak ozone levels below internationally accepted environmental criteria without action on the global scale to control emissions of tropospheric ozone precursors: methane, carbon monoxide, NOx and VOCs.

Evolution of tropospheric ozone radiative forcing

We present the rst estimate of the evolution of tropospheric ozone (O3(T)) radiative forcing since 1860 and into the future. The UKMO 3-D chemistry-transport model (STOCHEM) was used to simulate the tropospheric composition in 1860, 1950, 1970, 1990 and 2100, by chang- ing trace gas emissions. The future scenario used a doubled CO2 climate. STOCHEM includes extensive non-methane hydrocarbon (NMHC) chemistry, and produces a reasonable simulation of present-day O3(T). Radiative forcings caused by the modelled changes in O3(T) since 1860 were calculated using the UKMO radiation code, and included clouds and stratospheric temperature adjustment. Calculated changes in the global annual mean forcing since 1860 were 0.13, 0.22, 0.29 and 0.48 W m 2 for the four years. Up to 1990 this forcing scales linearly with the change in total NOx emis- sions since 1860; this linearity breaks down in 2100. The 1990 forcing is at the lower end of the range from previous modelling studies (0.28 - 0.51 W m 2 ), but is still signif- icant, enhancing the well-mixed greenhouse gas forcing by over 10 %.

Simulation of Global Hydrogen Levels Using a Lagrangian Three-Dimensional Model

Many previous assessments of the global hydrogen budget have used assumed global averages of temperatures and levels of key reactants to calculate the magnitudes of the various sinks. Dry deposition is by far the largest hydrogen sink but has not been considered in detail in previous estimates of the hydrogen budget. Simulations of hydrogen using a global three-dimensional Lagrangian chemistry-transport model and two different dry deposition schemes were compared with surface measurements. An improved dry deposition scheme which included the effects of soil moisture gave better agreement between the modelled hydrogen levels and surface measurements. The seasonal variation in the hydrogen levels was also simulated much more accurately with the new dry deposition scheme. The model results at high southern latitudes were insensitive to the relative partitioning of the sources between fossil fuel combustion and biomass burning. The results indicate a global mean hydrogen dry deposition velocity of 5.3×10−4 m s−1 which is lower than the previously used 7×10−4 m s−1.

Role of convection in determining the budget of odd hydrogen in the upper troposphere

This paper presents a model study of the changes in upper tropospheric HOx ( = OH + HO2) due to upward convective transport of surface pollutants. The model used is a three-dimensional global Lagrangian tropospheric chemistry transport model of 70 chemical species and 150 reactions including nonmethane hydrocarbon chemistry. It is driven by meteorological data from the U.K. Meterological Office with a 6 hour time resolution. We find that the effect of convection is to increase upper tropospheric (300–200 hPa) HOx globally by over 50%. The effect is greatest over the tropical continents where convection and VOC emissions from vegetation are colocated. The convection of isoprene, and hydroperoxides has the greatest effect. Convecting formaldehyde and acetone has a lesser effect. The contribution from isoprene depends more on the convection of its degradation products than the convection of isoprene itself. The upper tropospheric HOx budget is shown to be very sensitive to the model implementation of convective wet deposition.

Relative radiative forcing consequences of global emissions of hydrocarbons, carbon monoxide and NOx from human activities estimated with a zonally-averaged two-dimensional model

A global two-dimensional (altitude-latitude) chemistry transport model is used to follow the changes in the tropospheric distribution of the two major radiatively active trace gases, methane and ozone, following step changes to the sustained emissions of the short-lived trace gases methane, carbon monoxide and non-methane hydrocarbons. The radiative impacts were dependent on the latitude chosen for the applied change in emissions. Step change global warming potentials (GWPs) were derived for a range of short-lived trace gases to describe their time-integrated radiative forcing impacts for unit emissions relative to that of carbon dioxide. The GWPs show that the tropospheric chemistry of the hydrocarbons can produce significant indirect radiative impacts through changing the tropospheric distributions of hydroxyl radicals, methane and ozone. For aircraft, the indirect radiative forcing impact of the NOx emissions appears to be greater than that from their carbon dioxide emissions. Quantitative results from this two-dimensional model study must, however, be viewed against the known inadequacies of zonally-averaged models and their poor representation of many important tropospheric processes.

The tropospheric sulphur cycle and the role of volcanic SO2

Abstract A global three-dimensional chemistry-transport model has been applied to study the tropospheric sulphur cycle, and in particular the volcanic component. The model is in general agreement with previous studies of the global S budget. We find that volcanic emissions constitute 10% of the present-day global SO2 source to the atmosphere, but form 26% of the SO2 burden, and 14% of the sulphate aerosol burden. Two previous modelling studies suggested that the volcanic fraction of sulphate was 18% and 35%, from sources representing 7% and 14%, respectively, of the global total SO2 emission. The results are dependent upon various assumptions about volcanic emissions (magnitude, geographical location, altitude), the global distribution of oxidants, and the physical processes of dry and wet deposition. Because of this dependence upon poorly constrained parameters, it is unclear which modelling study is closest to the truth.

A comparison of two schemes for the convective transport of chemical species in a Lagrangian global chemistry model

We have developed a detailed parametrization scheme to represent the effects of subgrid-scale convective transport in a three-dimensional chemistry-transport model (CTM). The CTM utilizes the meteorological fields generated by a general-circulation model (GCM) to redistribute over 70 chemical species. The convective transport is implemented using the convective mass fluxes, entrainment rates and detrainment rates from the GCM. We compare the modelled distributions of 222Rn with observations. This shows that the vertical profile of this species is affected by the choice of convective-transpo rt parametrization. The new parametrization is found to improve significantly the simulation of 222Rn over the summertime continents.

The impact of European legislative and technology measures to reduce air pollutants on air quality, human health and climate

European air quality legislation has reduced emissions of air pollutants across Europe since the 1970s, affecting air quality, human health and regional climate. We used a coupled composition-climate model to simulate the impacts of European air quality legislation and technology measures implemented between 1970 and 2010. We contrast simulations using two emission scenarios; one with actual emissions in 2010 and the other with emissions that would have occurred in 2010 in the absence of technological improvements and end-of-pipe treatment measures in the energy, industrial and road transport sectors. European emissions of sulphur dioxide, black carbon (BC) and organic carbon in 2010 are 53%, 59% and 32% lower respectively compared to emissions that would have occurred in 2010 in the absence of legislative and technology measures. These emission reductions decreased simulated European annual mean concentrations of fine particulate matter (PM2.5) by 35%, sulphate by 44%, BC by 56% and particulate organic matter by 23%. The reduction in PM2.5 concentrations is calculated to have prevented 80 000 (37 000–116 000, at 95% confidence intervals) premature deaths annually across the European Union, resulting in a perceived financial benefit to society of US