Sensitivity Studies of the Effects of Model Formulation on the Evaluation of Control Strategies for Photochemical Air Pollution Formation in the United Kingdom

In the OZIPP (ozone isopleth plotting package, developed by United States Environmental Protection Agency) a number of model specific assumptions with respect to chemical and physical processes are made. These assumptions are introduced into an alternative model developed at AERE Harwell, United Kingdom, in which a detailed chemistry and mixture of organic emissions is included. The impact on the AERE Harwell model results of the assumptions made in OZIPP of omitting ground removal of ozone (O₃) and peroxyacetylnitrate (PAN) and of employing an incomplete PAN chemistry and adopting a reaction rate coefficient of the key reaction NO + HO₂ → NO₂ + OH which is a factor 10 lower than the accepted value, are discussed. The composition of the organic emissions is an important model parameter, and it is shown how grouping of nonmethane hydrocarbon (NMHC) emissions into a small group of NMHC thought to be representative, often implies that O₃ and other pollutants are overestimated. The O₃ isopleth diagram for London constructed using the AERE Harwell model gives a somewhat different picture from that obtained with OZIPP. OZIPP in general predicts that NO_x control or combined hydrocarbon(HC) and NO_x control is efficient with respect to O₃ reduction whilst the AERE Harwell model predicts that HC control alone usually is more efficient than combined HC and NO_x control. Furthermore NO_x control alone may often increase the O₃ burden downwind in the AERE Harwell model.     

The seasonal cycles and photochemistry of C2–C5 alkanes at Mace Head

Continuous in-situ measurements of NMHCs at Mace Head, Ireland during two full annual cycles from January 2005 to January 2007 were used to investigate NMHC emission sources and transport including dilution and photochemical oxidation. The Mace Head research station is ideally located to sample a wide range of air masses including polluted European transport, clean North Atlantic and Arctic air masses and the ultra-clean, Southern Atlantic air masses. The variety in air mass sampling is used to investigate interaction of emissions, transport, dilution and photochemistry. Variability of long-lived hydrocarbon ratios is used to assess and estimate typical transport times from emission source to the Mace Head receptor. Seasonality in the ratios of isomeric alkane pairs (for butane and pentanes) are used to assess the effects of atmospheric transport and photochemical ageing. Finally, the natural logarithms of NMHC ratios are used to assess photochemical oxidation.     

Using a reduced Common Representative Intermediates (CRIv2-R5) mechanism to simulate tropospheric ozone in a 3-D Lagrangian chemistry transport model

A reduced chemical scheme (CRIv2-R5) which describes ozone formation from the tropospheric degradation of methane and 22 emitted non-methane hydrocarbons and oxygenated volatile organic compounds has been applied in a global-3D chemistry transport model (STOCHEM). The scheme, which contains 220 species in 609 reactions, has been used to simulate ozone and its precursors for the meteorological year of 1998 and the results have been compared with those from STOCHEM runs with its original chemistry. Compared with the original chemistry scheme, the degradation of a larger number of more reactive VOCs in the CRI scheme results in the formation (and their consequent transportation) of more NO_x active reservoirs thus leading to formation of more ozone away from land-based sources. Conversely, the more reactive VOCs also lead to greater removal of OH in continental areas and greater formation of OH in marine environments. STOCHEM run with the CRI scheme simulates more ozone (by up to 10 ppb), which results in better agreement with observed vertical ozone profiles. The CRI scheme transforms the globally and annually integrated ozone budget for the considered year in STOCHEM from a net loss of ?55 Tg yr^?1 to a net gain of +50 Tg yr^?1.     

Measurements of gaseous hydrogen peroxide in southern England during a photochemical episode

Simultaneous measurements of gaseous hydrogen peroxide and ozone made in southern England are reported. The hydrogen peroxide measurements are the first reported for the United Kingdom and show clear diurnal trends and correlate with ozone measurements. Measurements were made during a photochemical episode when a peak hydrogen peroxide concentration of 2.5 microg m(-3) was recorded with a simultaneous peak of 168 microg m(-3) in the ozone concentration. From observations on the rate of decay in the measured concentrations, an evening-time deposition velocity of 0.28 cm s(-1) was derived for hydrogen peroxide.     

The effects of selected NOx reduction scenarios on long-term nitrogen deposition and episodic ozone levels in Europe

For computational reasons, evaluations of NO(x) emission controls usually concentrate on either episodic or annual impacts on pollution or deposition levels. However, previously published model results indicate that the consequences of NO(x) controls can be quite different on these different time scales. In this paper we analyse the impact of a consistent set of NO(x) control scenarios on both the episodic and annual time-scales. Using similar models, we compute levels of episode peak O(3) and NO(2) and annual NO(y)-N and total N deposition at three locations in Europe due to six emission scenarios derived from OECD estimates. An NO(x) control scenario which reduces European emissions by 63%, only results in total annual N deposition reductions of 19, 36 and 26% at the three locations examined because of the influence of ammonia-nitrogen deposition. The same scenario results in either increases or decreases in episode peak O(3) due to the influence of hydrocarbons. Emission reduction strategies should take into account not only NO(x) emissions, but emissions of other pollutants, such as hydrocarbons and ammonia.     

European abatement of surface ozone in a global perspective

EU's programme Clean Air for Europe (CAFE) is presently revising the policy on air quality which will lead to the adoption of a thematic strategy on air pollution under the Sixth Environmental Action Programme by mid-2005. For the abatement of surface ozone it is becoming evident that processes outside European control will be crucial for meeting long-term aims and air quality guidelines in Europe in the future. Measurements and modelling results indicate that there is a strong link between climate change and surface ozone. A warmer and dryer European climate is very likely to lead to increased ozone concentrations. Furthermore, increased anthropogenic emissions in developing economies in Asia are likely to raise the hemispheric background level of ozone. A significant increase in the background concentration of ozone has been observed at several sites in Northern Europe although the underlying causes are not settled. The photochemical formation of tropospheric ozone from increased concentrations of methane and CO may also lead to a higher ozone level on a global scale. Gradually, these effects may outweigh the effect of the reduced European ozone precursor emissions. This calls for a global or hemispheric perspective in the revision of the European air quality policy for ozone.     

The contribution from shipping emissions to air quality and acid deposition in Europe

A global three-dimensional Lagrangian chemistry-transport model STOCHEM is used to describe the European regional acid deposition and ozone air quality impacts along the Atlantic Ocean seaboard of Europe, from the SO2, NOx, VOCs and CO emissions from international shipping under conditions appropriate to the year 2000. Model-derived total sulfur deposition from international shipping reaches over 200 mg S m(-2) yr(-1) over the southwestern approaches to the British Isles and Brittany. The contribution from international shipping to surface ozone concentrations during the summertime, peaks at about 6 ppb over Ireland, Brittany and Portugal. Shipping emissions act as an external influence on acid deposition and ozone air quality within Europe and may require control actions in the future if strict deposition and air quality targets are to be met.     

Sensitivity of modelled sulphate and nitrate aerosol to cloud,pH and ammonia emissions

A Lagrangian dispersion model has been used to predict daily sulphate aerosol in 2006 at five UK rural measurement sites and hourly nitrate aerosol in April 2003 at Harwell (UK). The sensitivity of aqueous phase sulphate production to the meteorological input has been investigated. Large differences were found between cloud fraction and cloud liquid water output from the regional and mesoscale Met Office Unified Model. The impact on the sulphate aerosol was relatively small, with the mesoscale meteorology giving better results.Sulphate aerosol production in the aqueous phase was found to be very sensitive to modelled cloud pH. As the cloud becomes acidic sulphate production is greatly limited, conversely if the cloud is basic large amounts of sulphate aerosol are produced. A fixed model pH of 5.8 was found to produce better results than allowing the model to calculate pH which resulted in large over-predictions of measured sulphate aerosol in some episodes.Nitrate aerosol was not sensitive to cloud variables or pH, but showed a slight increase with 30% more ammonia emissions, and a slight decrease with 30% less ammonia.Sulphate production in model runs with fixed pH was not sensitive to changing ammonia emissions, however the sulphate production with modelled pH was very sensitive to plus or minus 30% ammonia. This work suggests that good modelling of ammonia is essential to correct estimation of aqueous phase sulphate aerosol if cloud pH is modelled. It is concluded that modelling to estimate the effect of reduced ammonia emission scenarios on future ambient aerosol levels should also take into account the neutralising effect of ammonia in cloud and hence the effect on aqueous phase production of sulphate.     

Development and application of a Lagrangian model to determine the origins of ozone episodes in the UK

This paper describes the further development and application of the Edinburgh–Lancaster Model for Ozone (ELMO). We replace straight-line back-trajectories with trajectories and associated meteorology supplied by the US National Oceanic and Atmospheric Administration Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) service to allow more realistic modelling of specific UK ozone episodes. We call this ELMO-2. Model performance is rigorously tested against observed ozone concentrations for two episodes recorded across 14 rural UK monitoring stations during the spring and summer of 1995. For both episodes, the afternoon concentrations (usually coinciding with the daily maxima) are captured well by the model and the diurnal ozone cycle is reproduced, although the amplitude in concentrations is generally smaller than the observed. The summer episode is investigated further through indicator species analysis and source attribution, and found to be mainly VOC-limited. European emissions account for the majority of ozone production. We demonstrate how improved modelling leads to better understanding of regional and local ozone production across the UK under episodic conditions.