John R. Stedman

The origin and day-of-week dependence of photochemical ozone episodes in the UK

Analyses are presented which focus specifically on photochemical ozone events when hourly mean levels of ozone at UK monitoring sites have reached or exceeded the information threshold of 90 ppbv. The geographical origins of such episodes are determined from an analysis of the trajectories associated with elevated ozone levels at 14 rural sites and six urban sites in the UK over the period 1992–1999. The analysis shows that the highest levels of ozone occur under summertime anticyclonic conditions, when the back trajectories have ‘looped’ over mainland Europe and arrive in the UK from a broadly easterly or southeasterly direction. Monitoring data from the same sites over the period 1989–1999 clearly demonstrate that ozone episodes are more prevalent at the end of the week, with the greatest numbers of hours ⩾90 ppbv occurring on Fridays. Relative to Fridays, the number of hours exceedence on the other days are: Sundays 51%, Mondays 50%, Tuesdays 54%, Wednesdays 39%, Thursdays 70% and Saturdays 82%. The observed day-of-week dependence is believed to result from the temporal dependence in the emissions of the precursor VOC and NOx (which are greater on weekdays) and the multi-day timescale required for chemical processing and transport to lead to elevated ozone levels under photochemical episode conditions. To test this hypothesis, a photochemical trajectory model, incorporating temporal variations in precursor emissions, has been used to investigate the formation of ozone along 96 h trajectories arriving at eight southern UK rural sites, for the meteorological conditions of an ozone episode on 31 July 1999. As a direct result of imposing a day-of-week variation in emissions, the model is able to recreate some of the features of the observed general pattern of 90 ppbv exceedences with day of week. In particular, Friday is calculated to be the day with the highest peak ozone mixing ratio at 7 of the 8 sites. The calculated peak mixing ratios on each day at each site are used to infer the number of hours exceedence of the 90 ppbv threshold, using a relationship derived from archived monitoring data. The inferred hours exceedence of the 90 ppbv threshold at all sites combined is calculated to be a factor of two greater on Friday than it is on Monday or Tuesday.

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.

Receptor modelling of PM10 concentrations at a United Kingdom national network monitoring site in central London

Abstract A receptor model for predicting future PM10 concentrations has been developed within the framework of the UK Airborne Particles Expert Group and applied during the recently completed review of the UK National Air Quality Strategy. The model uses a combination of measured PM10, oxides of nitrogen and particulate sulphate concentrations to provide daily estimates of the contributions to total particle concentrations from primary combustion, secondary and other (generally coarse) particle sources. Projections of past and future concentrations of PM10 are estimated by applying appropriate reductions to the current concentrations of the three components based on an understanding of the likely impact of current policies on future levels. Projections have been derived from 1996, 1997 and 1998 monitoring data and compared with UK national air quality objectives and European Union limit values. One of the key uncertainties within the receptor modelling method is the assignment of the residual PM10, remaining after the assignment of primary combustion and secondary particle contributions, to the ‘other’ particle fraction. An examination of the difference between measured PM10 and PM2.5 concentrations confirms our assignment of the bulk of this residual to coarse particles. Projections based on 1996 monitoring data are the highest and those based on 1998 monitoring data are the lowest. Whilst there is considerable difference between these projections they are consistent with measured concentrations for previous years. All three projections suggest that with current agreed policies the EU annual mean limit value will be achieved. The 24-h mean limit value is projected to be achievable when projections are derived from 1997 and 1998 data, but not from 1996 data. All three projections suggest that with current agreed policies the central London site will not achieve the provisional 1997 UK National Air Quality Strategy objective.

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.