By Chris Webber
In the past year the issue of air pollution within the UK has been elevated, driven by the loss of life that it causes (in 2013 > 500,000 years of UK lives lost due to air pollution 1). Air pollution concentrations within the UK are a function of both pollutant emissions and meteorology. This study set out to determine how synoptic meteorology impacts UK particulate matter (PM) concentrations with an aerodynamic diameter ≤ 10 µg m-3 ([PM10]).
The influence of synoptic meteorology on air pollution concentrations is well studied, with anticyclonic conditions over a region often found to be associated with the greatest pollutant concentrations 2,3. Webber et al. (2017) evaluated the impact of synoptic meteorology on UK Midlands [PM10]. They identified Omega block events as the synoptic meteorological condition that is associated with the most frequent UK daily mean [PM10] threshold exceedance events (episodes). A UK [PM10] episode is defined as daily mean [PM10] 10 µg m-3 above a mean UK Midlands concentration.
This study uses the Met-Office HADGEM3-GA4 atmosphere-only climate model to gather information on the flow regimes influencing the UK throughout Omega block events. For this study, temperature and wind velocity are constrained using ERA-Interim reanalysis data, in a process termed nudging. This study uses four inert tracers, emitted throughout Europe (Figure 1), to identify flow regimes from the highest PM10 emission regions throughout Europe. To enable their transport across Europe, the tracers are designed to replicate the lifetime of sulphate aerosol.
Figure 1. This study’s four tracer emission regions throughout Europe.
This study has identified 28 Omega blocks within the winter months (DJF) between December 1999 and February 2008. The anomalous mean sea level pressure composite for the 28 Omega blocks is shown for the onset day in Figure 2 and bears resemblance to a classical Omega block pattern (Figure 3). The Omega block onset is defined as in Webber et al. (2017), where the western flank of an upper level anticyclone has been detected within the northeast Atlantic/ European region.
Figure 2. Mean Sea Level Pressure Anomaly for 28 Omega block events on the day of onset, relative to a DJF 1999-2008 dataset mean.
Figure 3. An idealised schematic of an Omega block pattern. The High and Low refer to mean sea level pressure anomalies, while the solid black line represents flow streamlines (Met Office, 2017).
Figure 4 shows this study’s key result, the UK Midlands daily mean concentration of each tracer throughout the evolution of an Omega block. The observed UK Midlands [PM10] and modelled [PM10], the latter generated from the modelled tracers using multiple linear regression, are also shown. Within Figure 4 the solid line is the mean concentration throughout the Omega block subtracted by 1.65 x the standard deviation of that concentration (for a 1-tailed statistical test this equates to a 95th percentile confidence interval). The horizontal dashed lines represent the dataset means (negating the Omega block events) for each quantity. If the solid black line is greater than the horizontal dashed line in any of the panels, this represents a significant increase (p<0.05) in the concentration above the dataset mean.
Figure 4. Observed PM10, modelled PM10 and tracer concentrations throughout the 9 days of an Omega block event subtracted by 1.65 x the standard deviation of that concentration (solid lines). Horizontal dashed line represents the DJF 1999-2008 dataset mean for each tracer or PM10 concentration.
Figure 4 shows that Omega block events result in significant increases in both observed and modelled [PM10] on day +1 relative to the onset of an Omega block event. This is the maxima that was recognised by Webber et al. (2017) to lead to an elevated probability in UK Midlands PM10 episodes.
The key message from Figure 4 is that the peak in UK [PM10] throughout Omega block events is driven by an increase in locally sourced pollution and advected European pollution. Omega blocks have previously been thought to result in elevated [PM10] through the accumulation of locally sourced pollution, however this study is one of the first to show that this is not the whole story. We see significant influence from European tracers, which coincide with modelled UK [PM10] peaks.
1 EEA, 2016. Exceedance of air quality limit values in urban areas (Indicator CSI 004), European Environment Agency.
2 Barmpadimos, I., Keller, J., Oderbolz, D., Hueglin, C., Prevot, A. S. H., 2012. One decade of parallel fine (PM2.5) and coarse (PM10-PM2.5) particulate matter measurements in Europe: trends and variability. Atmos. Chem. Phys. 12, 3189-3203.
3 McGregor, G. R., Bamzelis, D., 1995. Synoptic Typing and Its Application to the Investigation of Weather Air-Pollution Relationships, Birmingham, United-Kingdom. Theor. Appl. Climatol. 51, 223-236.
4 Altenhoff, A. M., Martius, O., Croci-Maspoli, M. I., Schweirz, C., Davies, H. C., 2008. Linkage of atmospheric blocks and synoptic-scale Rossby waves: a climatological analysis. Tellus A, 60, no. 5, 1053-1063.
4 Webber, C. P., Dacre, H. F., Collins, W. J., Masato, G., 2017. The dynamical impact of Rossby wave breaking upon UK PM10 concentration. Atmos. Chem. Phys. 17, 867-881.
5 Met-Office, 2017. Blocking Patterns. Available online at: http://www.metoffice.gov.uk/learning/learn-about-the-weather/how-weather-works/highs-and-lows/blocks [Accessed July 2017].