Marked long-term decline in ambient CO mixing ratio in SE England, 1997-2014: evidence of policy success in improving air quality

Abstract

Atmospheric CO at Egham in SE England has shown a marked and progressive decline since 1997, following adoption of strict controls on emissions. The Egham site is uniquely positioned to allow both assessment and comparison of 'clean Atlantic background' air and CO-enriched air downwind from the London conurbation. The decline is strongest (approximately 50 ppb per year) in the 1997-2003 period but continues post 2003. A 'local CO increment' can be identified as the residual after subtraction of contemporary background Atlantic CO mixing ratios from measured values at Egham. This increment, which is primarily from regional sources (during anticyclonic or northerly winds) or from the European continent (with easterly air mass origins), has significant seasonality, but overall has declined steadily since 1997. On many days of the year CO measured at Egham is now not far above Atlantic background levels measured at Mace Head (Ireland). The results are consistent with MOPITT satellite observations and 'bottom-up' inventory results. Comparison with urban and regional background CO mixing ratios in Hong Kong demonstrates the importance of regional, as opposed to local reduction of CO emission. The Egham record implies that controls on emissions subsequent to legislation have been extremely successful in the UK.

Figure 1. Background and meteorology for the Egham (EGH) site.

(a) Royal Holloway Univ. London (RHUL – green) is located WSW of central London. Atlantic air typically crosses the UK south coast and then descends over large tracts of agricultural or wooded land and scattered towns to reach Egham. Image created in ArcGIS ArcMap 10.2 using basemap source: Esri, HERE, DeLorme, MapmyIndia. © OpenStreetMap contributors, and the GIS user community. (b) Wind directions for air arriving at Egham (EGH), 2000–2014, by percentage, season and wind speed. Note the dominance of winds from the western sectors. (c) HYSPLIT 4 frequency plots of 10 day back trajectories from Egham for 2011 to give an indication of annual variability of the air mass origins. Each panel represents 3 months of data, with a new trajectory plotted every 6 hours. Quarter 1 (Jan–Mar), Q2 (Apr–Jun), Q3 (Jul–Sep), Q4 (Oct–Dec). Red star is EGH. Lime green indicates >10% of trajectories and pale red >1% of trajectories. Air is mainly from Canada, Europe and the Arctic, with very little air from high CO regions in east Asia and the USA. Note for 2011 the greater incidence of easterly trajectories in Q4 autumn and the dominance of south-westerlies in Q1 winter. For ease of interpretation, some Arctic background sites (squares on the maps) have been added: Zeppelin (Ny-Ålesund) is yellow, Alert is dark blue. HYSPLIT models are produced by ARL (Air Resources Laboratory). HYSPLIT.trajectory maps are produced using archive data and can be freely redistributed (https://www.ready.noaa.gov/HYSPLIT_agreement.php).

Figure 2. CO trends measured at the EGH site.

(a) The overall London (Egham) CO Record from 1997–2014. Circles show average monthly mixing ratio (in ppb). Analysed by Trace Analytical RGD-2 instrument (black circles) prior to 2009, and Peak Performer 1 analyser thereafter (orange circles). (b) Daily averaged mixing ratios during Jan 1997-Dec 2014 of CO recorded at Royal Holloway following the format described by Hernandez-Panagua et al.. (c) Linear trends of CO at the EGH site during 1997–2014 calculated with the Sen’s estimate for annual averages and secular trend. The secular trend was obtained by filtering monthly averages with the STL technique. The best fit to the data is an Exponential curve to the annual CO average (black dashed curve) using an offset exponential function in the form: where A = 159.26, B = 430.25 and C = 5.0887, and x₀ is the initial year of measurements, 1997. (d) Comparison of filtered Egham background CO (black solid circles) vs. Mace Head monthly averages (red open squares), 1997–2014.

Figure 3. NAME particle dispersion modelling of air masses reaching the EGH site.

Scenarios run using version 6.1 of the Met Office NAME particle dispersion model (http://www.metoffice.gov.uk/research/modelling-systems/dispersion-model) to allow back trajectory analysis of certain events during the measurement period. © British Crown copyright 2016, Met Office. Plots show the surface influence (0 to 100 m) in the preceding 5 days. All plots are for 0300 UTC because the time for lowest influence of local area combustion sources is between 0200 and 0400 and mixing ratios shown below are averages of this 2-hour period. Two events are highlighted from July 2001 (a,b) and November 2011 (c,d). (a) Low CO (104 ppb) representing Atlantic background on 18 July 2001, (b) higher CO of 224 ppb from the addition of UK and near continental emissions on the previous day 17 July 2001, (c) Atlantic air with added emissions from France and SE England reaches 228 ppb on 19 November 2011, (d) 3 days later on 22 November 2011 the 5 days of air movement crosses Europe from the Mediterranean Sea and reaches 366 ppb CO. During a still-air period of this event at 00:00 UTC on 21/11/2011 the CO reached a high of 1257 ppb due to emissions of local CO sources under the inversion.

Figure 4. Assessment of CO reductions at EGH compared to background and by wind direction.

(a) Monthly averaged residual CO increment at Egham for all data, after subtracting the Mace Head background. (b) Comparison of monthly averaged CO in easterly winds (>0.1 m/s) arriving at Egham (Black open squares) compared to the Mace Head monthly-averaged Atlantic background values, (Red solid circles). (c) Directional analysis of the ‘Egham residual’; the CO increment over Mace Head background mixing ratios. Black curve: All directions. Blue curve: Northerly winds arriving from the northern UK and North Sea; Red curve: Easterly winds only: air from London and NW continental Europe; Green curve: SW winds from Atlantic background sector.

Figure 5. CO trend comparisons between background, peri-urban and city roadside sites.

(a) Comparison between CO annual averages from Marylebone Rd., Egham (this work), and Mace Head. (b) Exponentially fitted year-on-year change from annual averaged CO at Egham and Marylebone Rd.

Figure 6. Satellite analysis of CO reductions over London.

MOPITT total column mean for CO, zonally integrated (±10 km) for each 2 km after repositioning of all satellite observations about the city centre in order to co-align all wind vectors (rotation of all pixels) in an upwind-downwind direction at the time of observation over London, as a function of the distance from the city centre, for 2000–2003 (blue), 2004–2007 (red) and 2008–2011 (green). Wind data has been averaged to encompass the boundary layer from the surface up to 800 hPa. The error bars correspond to a standard deviation. Note sustained reduction in CO, sharper between 2000–2003 and 2004–2007 than between 2004–2007 and 2008–2011. Data were obtained from the NASA Langley Research Center Atmospheric Science Data Center (ftp://l5eil01.larc.nasa.gov/MOPITT/).

Figure 7. CO Emissions as reported in national and global emissions inventories.

(a) UK National CO emissions inventory (totals), 1970–2013, summarised using data from the UK National Atmospheric Emissions Inventory. UK emissions decreased by 76% between 1990 and 2010 compared to 62% for the EC27. (b) Comparison of CO emissions estimates 1990–2010: US, Western Europe, India and China using the ACCMP/MACCity estimates.

Figure 8. Hong Kong: annual averages of CO mixing ratios at Tap Mun (NE Hong Kong background) and Causeway Bay (east-central urban).

Data from Hong Kong Environmental Protection Department. Note that year-on-year meteorological changes can have large impact (for South China Sea Monsoon Index see http://web.lasg.ac.cn/staff/ljp/data-monsoon/SCSSMI.htm).