The team used high-precision measurement equipment based at Royal Holloway College in Egham, but also found similar declines in even the most traffic-populated regions of London, including Marylebone Road, show. The authors argue that the clean-up of gasoline cars was effective, but recognize the challenge facing diesel vehicles.

Linear trends of CO at the measurement site. Lowry et al. Click to enlarge.

CO is representative of many pollutants, and London of many northern European cities. While decadal variations in frequency of meteorological events, especially long-lived anticyclones such as those of summer 2003, may result in anomalous yearly averages, the sustained decline in CO suggests a significant and progressive underlying improvement in air quality in London. This observed decline is consistent with emission inventories and satellite observations, and comparable to the decline in other European cities but much higher than rates of decline observed at European continental background sites15. The cause of the London decline is almost certainly the strict controls on vehicle emissions introduced by the UK government, first in the 1991 Road Vehicles Regulations, and then the 1997 National Air Quality Strategy. … The UK legislation accompanied parallel moves across Western Europe in response to a European Union Directive, and the general improvement in London’s easterly air quality further confirms the betterment of air in the northern European source areas contributing to this flow.

The comparison with Hong Kong is instructive. Strong pollution control was introduced in the United Kingdom in 1997, and in Hong Kong in 1999. There the resemblance ends: the sub-continental regions differ in CO emissions history. Over the past 15 years the Egham area has reached near-oceanic background air quality, while urban Hong Kong has seen only limited decline in ambient winter CO. London benefits greatly from the wide emission reductions over NW Europe since 1990. Even in prolonged episodes of easterly air in winter, ambient CO mixing ratios at Egham now do not reach the excessive levels characteristic of the mid-late 1990s. However if it had not been for strenuous local emission control in the Hong Kong Special Administrative Region (SAR), the annual average CO mole fractions in Hong Kong could by now have been very much higher. Further improvements in Hong Kong during the sustained continental airflow of the winter NE Monsoon will depend on continued efforts to improve emission controls and thus air quality in mainland China.

… To conclude: effective improvement in ambient CO is indeed possible within a relatively short timeframe (a decade or less), but it needs both strong local action and co-ordinated regional policy.

—Lowry et al.

Resources

• D. Lowry, M. E. Lanoisellé, R. E. Fisher, M. Martin, C. M. R. Fowler, J. L. France, I. Y. Hernández-Paniagua, P. C. Novelli, S. Sriskantharajah, P. O’Brien, N. D. Rata, C. W. Holmes, Z. L. Fleming, K. C. Clemitshaw, G. Zazzeri, M. Pommier, C. A. McLinden & E. G. Nisbet (2016) “Marked long-term decline in ambient CO mixing ratio in SE England, 1997–2014: evidence of policy success in improving air quality” Scientific Reports 6, Article number: 25661 doi: 10.1038/srep25661