The weather often plays an important role during episodes of poor air quality, allowing local surface pollution emissions to accumulate, but UK air quality is also affected by meteorological processes over much greater length scales. Observations and modelling can be used to study these influences, helping to improve air quality forecasts and understanding.
Nitrogen oxide (NO) and nitrogen dioxide (NO2), known together as nitrogen oxides (NOx), are produced from natural and anthropogenic sources, particularly by power generation, motor vehicles, fuel and solvent use, and other industrial activities.
Ozone is not emitted directly, but is formed in the atmosphere through photochemical reactions involving NOx, volatile organic compounds (VOCs) and ultraviolet radiation (sunlight).
Particulate matter (PM), or aerosols, are a suspension of tiny particles in air, each with a diameter of 0.001 – 100 µm. PM2.5 and PM10 are defined as having an aerodynamic diameter of less than 2.5 µm and 10 µm, respectively. The main species of PM are sulphates, nitrates, organics, dust, soot and sea salt.
Wind-driven atmospheric transport can allow these pollutants to move large distances away from its location of formation, which is essentially governed by the position of synoptic-scale (large-scale) pressure systems. Wind also strongly affects the vertical mixing of air. High pressure, anticyclonic
conditions are often associated with calm weather, which reduces pollution dispersion.
Understanding the effect of meteorology on pollution accumulation and dispersion/removal is important for improving public health, particularly through the improvement of national air quality forecasts. The AQUM is also used to inform policy decisions on emissions controls (i.e. by DEFRA), and for research into the wider effects of poor air quality.
The Met Office’s AQUM (Air Quality in the Unified Model) is operationally used to produce regional air quality forecasts, allowing air quality warnings to be delivered to the public. The model considers the emissions of pollutants, the transport and dispersion of pollutants by winds, the chemical reactions amongst reactive gases and aerosols, and the removal processes, such as wet and dry deposition.
The skill of AQUM forecasts is tested by comparison against surface observations. The Automatic Urban and Rural Network (AURN) provides measurements of pollutant concentrations on an hourly basis at approximately 200 sites across the UK. Observations from MODIS on NASA’s TERRA and AQUA satellites monitor the ambient aerosol optical depth (AOD) globally.
Data for the period 2006-2010 was used in this study and two poor air quality episodes were analysed: 18th-20th July 2006 and 26th-28th March 2007.
The influence of the UK synoptic weather regime on poor UK air quality episodes was indicated by the model to be significant.
High pressure was present over central Europe and the UK and synoptic set up caused there to be an easterly flow over the UK and allowed heatwave conditions to build across large parts of Europe and UK during this episode.
High NO2 levels remained close to sources due to its short summer lifetime, reaching 40 µgm-3 in the city hotspots. Being a summertime episode and with high levels of ozone precursors, ozone concentrations were also high (>130 µgm-3).
PM2.5 had concentrations of around 25 µgm-3 across the UK. PM2.5 was greatest over the north coast of Northern Ireland (> 50 µgm-3), over the Irish sea (~40 µgm-3), and over the English Channel (>50 µgm-3). PM2.5 was slightly higher than the average over the city hotspots noted with NO2, with PM2.5 concentrations of ~30 µgm-3. PM2.5 concentrations increased over the whole UK, by 15 µgm-3 on average. The greatest difference compared to the mean was on the north coast of Northern Ireland (~45 µgm-3 greater).
With high levels of NO present over the UK, the breakdown of ozone dominated over ozone formation, so ozone concentrations were lower than its average for this time of year. This was particularly the case to the west of UK city hotspots, due to the easterly winds causing streaks of ozone negative anomalies to form, collocated with NOx positive anomaly streaks.
There appears to be a sharp north-south gradient in ozone concentrations, cutting through central Northern Ireland, with higher concentrations to its west (>80 µgm-3) and lower concentrations to its east (<80 µgm-3).
The north-south sharp concentration gradient is also present in the PM2.5 output, but does not extend as far north as for ozone; instead cutting across southern Scotland and down the east coast of England. Unlike for ozone, there are lower PM2.5 concentrations to the west and north of the divide (~15 µgm-3) and higher PM2.5 concentrations to the east of the divide (~40 µgm-3). PM2.5 is greater than the mean, by about 35 µgm-3 to the east of the divide and by about 10 µgm-3 to the west of the divide.
The dividing boundary between higher and lower ozone in is likely due to the presence of a front separating two different air masses. Synoptic analysis charts show this north-south front moving eastwards towards the UK. The air mass behind (to the west of) the front progress over the North Atlantic, so has very low pollution levels. Hence, PM2.5 is low behind the front, and less NO2 means less is available to photochemically breakdown to form ozone. However, there is no sharp change in NO2 along this frontal line, but this may be due to NO2 much shorter lifetime.
Although NO2 concentrations were well within the WHO 1-hour NO2 mean threshold value of 200 µgm-3 during 18th-20th July 2006, the elevated NO2 levels are likely to have had some impact human health. Being a summertime episode and with high levels of ozone precursors, ozone concentrations were very high, reaching 130 µgm-3 in the south of England. The WHO ozone guideline value of 100 µgm-3 (8-hour mean) was therefore greatly exceeded, hence having a significant impact on human lung function. The WHO PM2.5 threshold value of 25 µgm-3 (24-hour mean) was also exceeded during this event.
NO2 concentrations were greater than 30 µgm-3 in the city hotspots during 26th-28th March 2007 - below the WHO NO2 threshold value of 200 µgm-3 (1-hour mean). The WHO ozone threshold (100 µgm-3 8-hour mean) was also not exceeded during this episode, reaching 65 µgm-3 for large parts of the UK. However, PM2.5 rose above the WHO PM2.5 threshold value of 25 µgm-3 (24-hour mean), to over 30 µgm-3 in large parts of England and Wales. Thus PM, in particular, is likely to have had a serious impact on human health during this poor air quality episode.
During 18th-20th July 2006, low winds and recirculating air flow over the UK and northwest Europe allowed pollution to accumulate close to UK emission sources, particularly close to large cities.
During 26th-28th March 2007, a well-defined easterly synoptic regime existed across most of Europe. This allowed pollution to build as air flowed over the continent, especially over highly-industrialised areas such as the Benelux region, and caused the long-range transport of pollution towards the UK, including NOx and PM.
WHO safe limit thresholds of NOx, ozone and PM were greatly exceeded in these two case studies.
The conclusions of this study will aid the interpretations of air quality forecasts and the effects of different weather regimes on UK air quality; for example, helping UK authorities prepare for, and help mitigate, the health impacts of poor air quality.
This study has multiple branches for future work. Most importantly, it would be interesting to determine why the NO2 positive anomaly streaks and ozone negative anomaly streaks seen over the Irish sea (figure 2 & 3), westwards of the city hotspots of Bristol, Liverpool and Glasgow, are not present downstream of other big cities.
2. DEFRA. 2010. Valuing the Overall Impacts of Air Pollution. [Online]. [Accessed 02 February 2017]. Available from: archive.defra.gov.uk.
3. Jones, A.M., Harrison, R.M., Baker, J., 2010. The wind speed dependence of the concentrations of airborne particulate matter and NOx. Atmospheric Environment. 44, pp.1682–1690.
4. Pope, R.J., Butt, E.W., Chipperfield, M.P., Doherty, R.M., Fenech, S., Schmidt, A., Arnold, S.R. and Savage, N.H. 2016. The impact of synoptic weather on UK surface ozone and implications for premature mortality. Environmental Research Letters. 11(12), p.124004.
5. Savage, N.H., Agnew, P., Davis, L.S., Ordónez, C., Thorpe, R., Johnson, C.E., O’Connor, F.M. and Dalvi, M. 2013. Air quality modelling using the Met Office unified model (AQUM OS24-26): model description and initial evaluation. Geosci.Model Dev. 6, pp.353–72.
6. Prior, J., and Beswick, M. 2007. The record-breaking heat and sunshine of July 2006. Weather. 62, pp.174–182.
7. World Health Organization, 2006. Air quality guidelines: global update 2005: particulate matter, ozone, nitrogen dioxide, and sulfur dioxide. World Health Organization.
8. Camalier, L., Cox, W. and Dolwick, P. 2007. The effects of meteorology on ozone in urban areas and their use in assessing ozone trends. Atmos. Environ. 41, pp.7127–37.
Background
Air pollution has detrimental impacts on human health. Exposure to air pollution increases the risk of disease from a stroke, heart disease, lung cancer and respiratory diseases, including asthma. Poor air quality, particularly from nitrogen dioxide (NO2), ozone (O3) and particulate matter (PM), contributes to an approximate 40,000 premature deaths per year in the UK1, as well as costing UK society £16 billion annually2.Nitrogen oxide (NO) and nitrogen dioxide (NO2), known together as nitrogen oxides (NOx), are produced from natural and anthropogenic sources, particularly by power generation, motor vehicles, fuel and solvent use, and other industrial activities.
Ozone is not emitted directly, but is formed in the atmosphere through photochemical reactions involving NOx, volatile organic compounds (VOCs) and ultraviolet radiation (sunlight).
Particulate matter (PM), or aerosols, are a suspension of tiny particles in air, each with a diameter of 0.001 – 100 µm. PM2.5 and PM10 are defined as having an aerodynamic diameter of less than 2.5 µm and 10 µm, respectively. The main species of PM are sulphates, nitrates, organics, dust, soot and sea salt.
Wind-driven atmospheric transport can allow these pollutants to move large distances away from its location of formation, which is essentially governed by the position of synoptic-scale (large-scale) pressure systems. Wind also strongly affects the vertical mixing of air. High pressure, anticyclonic
conditions are often associated with calm weather, which reduces pollution dispersion.
Methods
The interactions between meteorology and air quality is relatively well understood theoretically, but few studies have used the UK's air quality measurements network and air quality forecast models to monitor the effects of meteorology on UK air quality.Understanding the effect of meteorology on pollution accumulation and dispersion/removal is important for improving public health, particularly through the improvement of national air quality forecasts. The AQUM is also used to inform policy decisions on emissions controls (i.e. by DEFRA), and for research into the wider effects of poor air quality.
The Met Office’s AQUM (Air Quality in the Unified Model) is operationally used to produce regional air quality forecasts, allowing air quality warnings to be delivered to the public. The model considers the emissions of pollutants, the transport and dispersion of pollutants by winds, the chemical reactions amongst reactive gases and aerosols, and the removal processes, such as wet and dry deposition.
The skill of AQUM forecasts is tested by comparison against surface observations. The Automatic Urban and Rural Network (AURN) provides measurements of pollutant concentrations on an hourly basis at approximately 200 sites across the UK. Observations from MODIS on NASA’s TERRA and AQUA satellites monitor the ambient aerosol optical depth (AOD) globally.
Data for the period 2006-2010 was used in this study and two poor air quality episodes were analysed: 18th-20th July 2006 and 26th-28th March 2007.
Results
Comparison of observations and modelling showed the ability of the Met Office’s regional air quality model (AQUM) to reproduce surface pollution concentrations, making it a practical tool for investigating the influence of synoptic meteorology on UK air quality. Some bias does exist however, with an NO2 negative bias of around 30 µgm-3 in cities (neutral away from cities) and an ozone positive bias of around 10 µgm-3 (figure 1).
Figure 1. Difference between gridded midday AQUM output and gridded midday AURN data (AQUM – AURN), during July 2006 for (a) NO2 and (b) ozone, and during March 2007 for (c) NO2 and (d) ozone.
Poor air quality episode: 18th-20th July 2006
During 18th-20th July 2006, low winds and recirculating air flow over the UK and northwest Europe allowed pollution to accumulate close to UK emission sources6, particularly close to the city hotspots in southern Scotland, central England, Bristol and London (figure 2).High pressure was present over central Europe and the UK and synoptic set up caused there to be an easterly flow over the UK and allowed heatwave conditions to build across large parts of Europe and UK during this episode.
Figure 2. Gridded midday AQUM output for the 18th-20th July 2006 episode, showing the absolute values for (a) NO2 and (b) ozone, and the difference from the 5-year mean for (c) NO2 and (d) ozone, with surface wind vectors overlaid.
Poor air quality episode: 26th-28th March 2007
During 26th-28th March 2007, a well-defined easterly synoptic regime existed across most of Europe. The air mass arriving in the UK, from the ground to 3km up (above the boundary layer), passed long distances over Europe. This allowed pollution to build as air flowed over the continent, especially over highly-industrialised areas such as the Benelux region, and caused the long-range transport of pollution towards the UK (figure 3), including NOx and PM.
Figure 3. Gridded midday AQUM output for the 26th-28th March 2007 episode, showing the absolute values for (a) NO2 and (b) ozone, and the difference from the 5-year mean for (c) NO2 and (d) ozone, with surface wind vectors overlaid.
There appears to be a sharp north-south gradient in ozone concentrations, cutting through central Northern Ireland, with higher concentrations to its west (>80 µgm-3) and lower concentrations to its east (<80 µgm-3).
The north-south sharp concentration gradient is also present in the PM2.5 output, but does not extend as far north as for ozone; instead cutting across southern Scotland and down the east coast of England. Unlike for ozone, there are lower PM2.5 concentrations to the west and north of the divide (~15 µgm-3) and higher PM2.5 concentrations to the east of the divide (~40 µgm-3). PM2.5 is greater than the mean, by about 35 µgm-3 to the east of the divide and by about 10 µgm-3 to the west of the divide.
The dividing boundary between higher and lower ozone in is likely due to the presence of a front separating two different air masses. Synoptic analysis charts show this north-south front moving eastwards towards the UK. The air mass behind (to the west of) the front progress over the North Atlantic, so has very low pollution levels. Hence, PM2.5 is low behind the front, and less NO2 means less is available to photochemically breakdown to form ozone. However, there is no sharp change in NO2 along this frontal line, but this may be due to NO2 much shorter lifetime.
WHO guideline threshold pollution values
The World Health Organization (WHO) air quality guidelines provide safe limits for the key air pollutants that pose health risk, based on expert evaluation of current scientific evidence. The bias in the AQUM must be considered when comparing the pollution concentrations to the WHO limits. The WHO guideline values are for mean concentrations over a number of hours, depending on the lifetime of the pollutant; this must be taken into account when comparing the WHO thresholds to the three-day episode midday means used in this study.Although NO2 concentrations were well within the WHO 1-hour NO2 mean threshold value of 200 µgm-3 during 18th-20th July 2006, the elevated NO2 levels are likely to have had some impact human health. Being a summertime episode and with high levels of ozone precursors, ozone concentrations were very high, reaching 130 µgm-3 in the south of England. The WHO ozone guideline value of 100 µgm-3 (8-hour mean) was therefore greatly exceeded, hence having a significant impact on human lung function. The WHO PM2.5 threshold value of 25 µgm-3 (24-hour mean) was also exceeded during this event.
NO2 concentrations were greater than 30 µgm-3 in the city hotspots during 26th-28th March 2007 - below the WHO NO2 threshold value of 200 µgm-3 (1-hour mean). The WHO ozone threshold (100 µgm-3 8-hour mean) was also not exceeded during this episode, reaching 65 µgm-3 for large parts of the UK. However, PM2.5 rose above the WHO PM2.5 threshold value of 25 µgm-3 (24-hour mean), to over 30 µgm-3 in large parts of England and Wales. Thus PM, in particular, is likely to have had a serious impact on human health during this poor air quality episode.
Conclusions
This study concludes that the processes of pollution accumulation and long-range transport have a strong influence on UK air quality under favourable synoptic meteorology. It highlights that background air quality is not just a local scale issue and is affected by European pollution sources under easterly flow regimes.During 18th-20th July 2006, low winds and recirculating air flow over the UK and northwest Europe allowed pollution to accumulate close to UK emission sources, particularly close to large cities.
During 26th-28th March 2007, a well-defined easterly synoptic regime existed across most of Europe. This allowed pollution to build as air flowed over the continent, especially over highly-industrialised areas such as the Benelux region, and caused the long-range transport of pollution towards the UK, including NOx and PM.
WHO safe limit thresholds of NOx, ozone and PM were greatly exceeded in these two case studies.
The conclusions of this study will aid the interpretations of air quality forecasts and the effects of different weather regimes on UK air quality; for example, helping UK authorities prepare for, and help mitigate, the health impacts of poor air quality.
Limitations and future work
Analysis was limited by systematic bias of the AQUM model, causing an overestimate of the absolute values of ozone and an underestimate of the absolute values of NO2. This affects the assessment of the individual impacts of the pollutants. The multiple factors affecting air quality results in it being extremely difficult to separate effects of synoptic meteorological processes on air quality from other factors, including the effect of local emissions and local meteorology, vertical atmospheric mixing and pollution diurnal cycles.This study has multiple branches for future work. Most importantly, it would be interesting to determine why the NO2 positive anomaly streaks and ozone negative anomaly streaks seen over the Irish sea (figure 2 & 3), westwards of the city hotspots of Bristol, Liverpool and Glasgow, are not present downstream of other big cities.
Key references
1. Holgate, S.T. 2017. ‘Every breath we take: the lifelong impact of air pollution’ – a call for action. Clinical Medicine. 17(1), pp.8–12.2. DEFRA. 2010. Valuing the Overall Impacts of Air Pollution. [Online]. [Accessed 02 February 2017]. Available from: archive.defra.gov.uk.
3. Jones, A.M., Harrison, R.M., Baker, J., 2010. The wind speed dependence of the concentrations of airborne particulate matter and NOx. Atmospheric Environment. 44, pp.1682–1690.
4. Pope, R.J., Butt, E.W., Chipperfield, M.P., Doherty, R.M., Fenech, S., Schmidt, A., Arnold, S.R. and Savage, N.H. 2016. The impact of synoptic weather on UK surface ozone and implications for premature mortality. Environmental Research Letters. 11(12), p.124004.
5. Savage, N.H., Agnew, P., Davis, L.S., Ordónez, C., Thorpe, R., Johnson, C.E., O’Connor, F.M. and Dalvi, M. 2013. Air quality modelling using the Met Office unified model (AQUM OS24-26): model description and initial evaluation. Geosci.Model Dev. 6, pp.353–72.
6. Prior, J., and Beswick, M. 2007. The record-breaking heat and sunshine of July 2006. Weather. 62, pp.174–182.
7. World Health Organization, 2006. Air quality guidelines: global update 2005: particulate matter, ozone, nitrogen dioxide, and sulfur dioxide. World Health Organization.
8. Camalier, L., Cox, W. and Dolwick, P. 2007. The effects of meteorology on ozone in urban areas and their use in assessing ozone trends. Atmos. Environ. 41, pp.7127–37.
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