G. W. Campbell

New high resolution maps of estimated background ambient NOx and NO2 concentrations in the U.K.

Abstract Maps of estimated annual mean background surface concentrations of NOx and NO2 in the U.K. have been calculated on 1 km x 1 km grid for 1994. These maps have been calculated using a multi-stage approach which includes the interpolation of rural measurements and the empirical modelling of the local source contribution to ambient concentrations. The empirical regression models were calibrated with reference to automatic monitoring data and then validated by comparison with diffusion tube measurements of NO2 at several hundred sites. Estimates of the population potentially exposed to annual mean concentrations of pollutants above certain thresholds were calculated from these maps. These threshold annual mean concentrations can be approximately equated with proposed and existing air quality standards for NO2, benzene and 1,3 butadiene. These estimates indicate that about 8% of the population live in regions where the annual mean NO2 concentration is above the European Union Directive Guide Value for a median concentration of 26 ppb; about 23% live in areas at risk of exceeding the U.K. Government proposed 99.9th percentile target for NO2 of 104.6 ppb and about 30% live in areas with running annual mean benzene concentrations exceeding 1 ppb.

Regional mass budgets of oxidized and reduced nitrogen and their relative contribution to the nitrogen inputs of sensitive ecosystems

Wet deposition of nitrogen is reasonably well monitored throughout Europe, whereas the dry deposition inputs are provided largely by models. Recent long-term measurements of NO2 and NH3 fluxes to semi-natural vegetation have shown that rates of NH3 deposition exceed those of NO2, typically by an order of magnitude. Incorporating the results of these dry deposition measurements in regional deposition budgets shows that the inputs of reduced nitrogen contribute the dominant fraction of the total nitrogen inputs in most regions of the UK. The results are illustrated by comparing the atmospheric mass-budget for oxidized nitrogen over the UK. Of the annual UK emissions of NOx, amounting to 780 kt N (Salway et al., 1997), only 5% is dry deposited to terrestrial surfaces within the country while 15% is wet deposited, whereas for the reduced nitrogen, 42% of emissions (of the 260 kt N, Salway et al., 1997) are dry deposited and 46% are wet deposited. Even more striking are the relative contributions of oxidized and reduced nitrogen to semi-natural vegetation, which is a particularly efficient sink for NH3. The species composition of semi-natural vegetation is also regarded as very sensitive to nitrogen inputs. The distribution of nitrogen deposition among different land uses shows that the average input to forest in the UK is 33 kg N ha−1 annually of which 78% is reduced nitrogen. The other land uses receive about 15 kg N ha−1 of nitrogen of which between 55% and 65% is NHx. Critical loads for nutrient nitrogen are exceeded primarily in forested and moorland areas as a consequence of NH3 dry deposition and wet NH4+ deposition. For forests the area in exceedance of 20 kg N ha−1 year−1 critical load represents 70% of the forest area (1.4 × 106 ha−1) while for moorland the area in exceedance is 13% of the moorland area and occupies 1.04 × 106 ha−1).

Trends in sulphate and nitrate wet deposition over the United Kingdom: 1986–1999

Since 1986 annual wet deposition of non-sea-salt sulphur has decreased over all regions of the United Kingdom, with the largest decrease in eastern areas. Nationally wet deposited sulphur has decreased by about one-third compared with an approximately two-thirds decrease in sulphur dioxide emissions. Over the same period daily measurements of sulphur dioxide and particulate sulphate concentrations at a small number of sites showed a statistically significant decrease at all but the most remote locations. The comparative magnitude of the changes in UK emissions and wet deposition suggests some change in the partitioning between wet and dry sulphur deposition. Trajectory analyses of precipitation events show that background sulphate deposition has not changed significantly in northwestern coastal regions, where it accounts for approximately half of the total. Wet nitrate deposition has decreased at a slower rate and increased slightly in some remote western areas. Sector analyses suggest that this increase is linked with increased deposition during easterly airflow while particulate sulphate concentrations in these airmasses have decreased. Nitrate now accounts for at least one-third of the acidifying effect of the combined wet deposition of non-sea-salt sulphate and nitrate over all parts of the United Kingdom.

Trends in wet and dry deposition of sulphur in the United Kingdom

UK data on sulphur deposition trends between the 1960s and 1990s are presented. Long term data sets of sulphur dioxide (SO2) concentrations at two sites have been analysed and dry deposition determined using a resistance model. Wet deposition has been calculated from non-marine sulphate concentration and rainfall fields for 1978–80 and 1989–93. These maps have been interpolated and corrected for seeder feeder enhancement. The wet deposition of sulphur declined by about 43 % between 1979 and 1993 whereas emissions of sulphur declined by about 32 %. An indication of the trends in sulphur dry deposition is provided by data from Eskdalemuir, a site in southern Scotland where wet deposition and SO2 concentration have been measured since 1979. Dry deposition at Eskdalemuir has decreased by 70 % and wet deposition by 48 %. Hence, while wet deposition has responded approximately linearly with the decline in UK emissions of sulphur dioxide, dry deposition has declined at twice the rate of decline in UK emissions.

Wet deposition of non sea-salt sulphate in the United Kingdom: The influence of natural sources

Because of its position to the west of Europe, much of the wet sulphur deposition in the west of the UK is background in the sense that it is not attributable to pollutants emitted within Europe less than four days previously. There are both natural and anthropogenic sources of this sulphur. An important natural source, especially during the summer, is dimethylsulphide (DMS) produced by marine phytoplankton. To identify the contribution of marine biogenic sulphur we have measured stable sulphur isotope ratios in precipitation. We show that biogenic sulphur is significant in summer but contributes little in winter and that around 5–10% of the annual background wet sulphur deposition is due to biogenic sources. During July and December 1993, airflow across the UK was predominantly westerly. The measured biogenic component of precipitation sulphate accounted for around 30 % of background sulphate in July but was negligible in December. Investigation of five day back-trajectories for the period indicated little opportunity for re-circulation of European emissions, suggesting that other (non-DMS) natural sources and non-European anthropogenic emissions were responsible for most of the background sulphur.

Measurements of precipitation composition at UK EMEP sites 1987 - 1992 and comparison with the HARM Model

Precipitation composition has been measured daily at five UK EMEP sites since 1987. Sulphur dioxide and sulphate aerosol concentrations are also measured daily at the sites. Back trajectories and wind sectors calculated by the Norwegian Meteorological Institute have been used to characterise the variation in wet deposition in terms of air mass source. Contributions to wet deposition from various source regions have been estimated for Eskdalemuir. Observations from the EMEP sites have been compared with output from the Hull Acid Rain Model (HARM). HARM is a Lagrangian model using simplified meteorology but straight-line trajectories. Results are compared on a site-by-site and sector-by-sector basis and the model reproduces the general features of pollutant concentration and wet deposition indicated by the measurements. The possible effects of future reductions in emissions of SO2 and NOx on precipitation concentrations by wind sector are described.