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Each year more than 29,000 premature deaths in the UK are linked to long term-exposure to ambient particulate matter (PM) with a diameter less than 2.5 μm (PM2.5). Many studies have focused on the long-term impacts of exposure to PM, but short-term increases in pollution can also exacerbate health effects, leading to deaths brought forward within exposed populations. This study investigates the impact of different atmospheric circulation patterns on UK PM2.5 concentrations and the relative contribution of local and transboundary pollutants to variations in PM2.5 concentrations. Daily mean PM2.5 observations from 42 UK background sites indicate that easterly, south-easterly and southerly wind directions and anticyclonic circulation patterns enhance background concentrations of PM2.5 at all UK sites by up to 12 μg m-3. Results from back trajectory analysis and the European Monitoring and Evaluation Programme for UK model (EMEP4UK) show this is due to the transboundary transport of pollutants from continental Europe. While back trajectories indicate under easterly, south-easterly and southerly flow 25–50% of the total accumulated primary PM2.5 emissions originate outside of the UK, with a very polluted footprint (0.25–0.35 μg m-2). Anticyclonic conditions, which occur frequently (21%), also lead to increases in PM2.5 concentrations (UK multi-annual mean 14.7 μg m-3). EMEP4UK results indicate this is likely due the build-up of local emissions due to slack winds. Under westerly and north-westerly flow 15–30% of the total accumulated primary PM2.5 emissions originate outside of the UK, and are much less polluted (0.1 μg m-2) with model results indicating transport of clean maritime air masses from the Atlantic. Results indicate that both wind-direction and stability under anticyclonic conditions are important in controlling ambient PM2.5 concentrations across the UK. There is also a strong dependence of high PM2.5 Daily Air Quality Index (DAQI) values on easterly, south-easterly and southerly wind-directions, with >70% of occurrences of observed 48–71+ μg m-3 concentrations occurring under these wind directions. While north-westerly and cyclonic conditions reduce PM2.5 concentrations at all sites by up to 8 μg m-3. PM2.5 DAQI values are also lowest under these conditions, with >80% of 0–11 μg m-3 concentrations and >50% of 12–23 μg m-3 concentrations observed during westerly, north-westerly and northerly wind directions. Indicating that these conditions are likely to be associated with a reduction in the potential health effects from exposure to ambient levels of PM2.5. Keywords: LWT, PM2.5, long-range transport, Air quality, Emissions, AURN

C2–C7 non-methane hydrocarbons were measured in clean maritime boundary layer air at latitudes between 53°N and 81°N. Measurements were made as part of the Arctic ice and environmental variability cruise aboard RRS James Clark Ross using a high sensitivity automated gas chromatography system. The data were collected during summer 1999, a period of continuous Arctic sunlight. Hydrocarbons of anthropogenic origin were observed to decrease in concentration with increasing latitude, a combination of dispersion and extensive atmospheric chemical processing. At high latitudes, low boundary layer conditions were common and species of exclusively anthropogenic origin reached highly stable although non-zero values (e.g. acetylene 27.8±2.4 pptV). A number of hydrocarbons believed to be of oceanic origin showed wide variability in these regions of atmospheric boundary layer stability (average ethene=37.2±20.9 pptV), highlighting inhomogeneity in the ocean to atmosphere flux rates. Whilst substantial increases in biological activity and productivity in the marginal ice zone have been previously reported, no evidence for increased biogenic hydrocarbon emissions were observed during this cruise. Removal mechanisms for atmospheric hydrocarbons were generally dominated by OH chemistry. Under clean maritime conditions the observed iso-/n pentane and butane ratios were in good agreement with published sea-fluxes. Using the relative ratios of iso-/n-butane and iso-butane/propane, there was evidence of significant chlorine atom chemistry at these high latitudes.

A detailed chemical mechanism has been used in conjunction with high-quality field measurements in order to assess the potential role of isoprene at a clean coastal Northern Hemisphere site. The importance of isoprene as a source of peroxy radicals, formaldehyde and ozone is discussed in some detail for a case study day (July 17 1996). Between 10:00 and 16:00 h GMT, isoprene accounts on average for 15% of the OH removal, yet is responsible for the production of about 30% of the HO2 radicals. Isoprene is also responsible for between 40–60% of the HCHO formation, and 20–40% of the 2 ppb h−1 conversion of NO to NO2 by peroxy radicals. We discuss the significant time lag between HCHO formation and destruction, a time lag that must be accounted for in order to accurately calculate the rate of HO2 production from isoprene degradation, and the implications of the isoprene oxidation for atmospheric chemistry.

The Cambridge Tropospheric Trajectory model of Chemistry and Transport (CiTTyCAT), a Lagrangian chemistry model, has burn evaluated using atmospheric chemical measurements collected during the East Atlantic Summer Experiment 1996 (EASE '96), This field campaign was part of the UK Natural Environment Research Council's (NERC) Atmospheric Chemistry Studies in the Oceanic Environment (ACSOE) programme, conducted at Mace Head, Republic of Ireland, during July and August 1996. The model includes a description of gas-phase tropospheric chemistry, and simple parameterisations for surface deposition, mixing from the free troposphere and emissions. The model generally compares well with the measurements and is used to study the production and loss of O-3 under a variety of conditions. The mean difference between the hourly O-3 concentrations calculated by the model and those measured is 0.6 ppbv with a standard deviation of 8.7 ppbv. Three specific air-flow regimes were identified during the campaign - westerly, anticyclonic (easterly) and south westerly. The westerly how is typical of background conditions for Mace Head. However, on some occasions there was evidence of long-range transport of pollutants from North America. In periods of anticyclonic flow, air parcels had collected emissions of NOx and VOCs immediately before arriving at Mace Head, leading to O-3 production. The level of calculated O-3 depends critically on the precise details of the trajectory, and hence on the emissions into the air parcel. In several periods of south westerly flow, low concentrations of O-3 were measured which were consistent with deposition and photochemical destruction inside the tropical marine boundary layer. (C) 2000 Elsevier Science Ltd. All rights reserved.

Observation of diurnal cycles in atmospheric concentrations of reactive alkenes are reported from measurements performed at a North Atlantic coastal site (Mace Head, Eire 53°19′34″N; 9°54′14″W). Species seen to exhibit distinct cycles included isoprene, ethene, propene, 1-butene, iso-butene and a substituted C6 alkene. Five hundred and thirty air mass classified measurements were performed over a 4 week period at approximately hourly frequency and demonstrate that during periods when air flow resulted from unpolluted oceanic regions a clear daily cycle in concentrations existed, peaking at around solar noon for all species. These observations support the proposed mechanism of production via photochemical degradation of organic carbon in sea water. The observed concentrations showed strong correlation (propene R2>0.75) with solar flux, with little relationship to other meteorological or chemical parameters. The species’ short atmospheric lifetimes indicate that the source of emission was from local coastal waters within close proximity of the sampling site. At solar noon concentrations of reactive alkenes from oceanic sources were responsible for up to 88% of non-methane hydrocarbon reaction with the hydroxyl radical at this coastal marine site.