Search

Rapid economic and industrial growth in China has led to serious air pollution with high concentrations of suspended fine particulate matter (PM2.5), in particular during winter. On a regional scale, meteorological conditions play a major role in modulating the accumulation, transport, removal and transformation of air pollutants. These meteorological conditions are affected by large-scale circulation patterns over China, dominated by the East Asian winter monsoon (EAWM). However, previous studies of how the large-scale winter circulation modulates air quality in China primarily focused on the North China Plain. The study of regional differences in the dominant large-scale circulation patterns needed to project future climate-driven PM2.5 concentration changes is far from complete. On longer inter-annual timescales, the EAWM is in turn influenced by El Niño–Southern Oscillation (ENSO) and the ensuing Pacific-East Asia teleconnection pattern. Better understanding of the ENSO-EAWM relationship and changes in this relationship under global warming is needed. Firstly, the influence of large-scale circulation on daily PM2.5 variability through its direct effect on key regional meteorological variables over three major populated regions of China (Beijing–Tianjin–Hebei, BTH; the Yangtze River Delta, YRD; the Pearl River Delta, PRD) is examined, based on a new high-resolution air quality reanalysis dataset for China for five winters from December 2013 to February 2018. In BTH, a shallow East Asian trough curbs northerly cold and dry air from the Siberian High, enhancing PM2.5 pollution levels. Weak near-surface southerly winds in eastern and southern China, associated with a weakened Siberian High, suppress horizontal dispersion, contributing to air pollution accumulation over YRD. In PRD, weak southerly winds and precipitation deficits over southern China are conducive to high PM2.5 concentrations. To account for these dominant large-scale circulation–PM2.5 relationships, we propose three new circulation-based indices: a 500 hPa geopotential height-based index for BTH, a sea level pressure-based index for YRD and an 850 hPa meridional wind-based index for PRD. These three indices can effectively distinguish clean days from heavily polluted days in these regions, assuming PM2.5 variability is solely due to meteorology. Subsequently, the influence of the winter large-scale circulation on daily PM2.5 concentrations and on the sensitivity of PM2.5 to emissions over major populated regions of China with a focus on YRD is investigated, using the United Kingdom Earth System Model, UKESM1. Weak flow of near-surface cold, dry air from the north and weak inflow of maritime air are conducive to air pollution over YRD for 1999–2019. These provide favourable conditions for the accumulation of local pollution but limit the transport of air pollutants into YRD from the north for 2014– 2019. Based on the dominant large-scale circulation, we construct a new index using the north-south pressure gradient to project PM2.5 concentrations over the region. We show that this index can effectively distinguish different levels of pollution over YRD and explain changes in PM2.5 sensitivity to emissions from local and northern regions. We then project future changes in PM2.5 concentrations using this index under the weak climate and air pollutant mitigation scenario (SSP3- 7.0). We find an increase in PM2.5 concentrations over YRD due to climate-driven circulation changes that is expected to partially offset the effect of emission control measures in the near-term future. Finally, changes in the relationship between ENSO and the EAWM at various global warming levels during the 21st century are examined based on experiments from the Max Planck Institute Grand Ensemble (MPI-GE) that represent the upper boundary of the range of emissions scenario (RCP 8.5). The externally forced component of this relationship (i.e. forced by greenhouse gases and anthropogenic aerosol emissions) strengthens under moderate warming (+1.5 ◦ C), and then weakens for +3 ◦ C warming. These changes are characterised by variations in strength and location of the core of El Niño-related warming and associated deep convection anomalies over the equatorial Pacific leading to circulation anomalies across the Asian-Pacific region. Under global warming, the ENSO–EAWM relationship is strongly related to the background mean state of both the EAWM and ENSO, through changesin the EAWM strength and a shift of the ENSO pattern. Anthropogenic aerosols also play a key role in influencing the ENSO–EAWM relationship under moderate warming (up to 1.5 ◦C). These results demonstrate the importance of understanding the occurrence of days with elevated PM2.5 concentrations and explaining changes in the sensitivity of PM2.5 to emissions from local and surrounding regions from a large-scale perspective. These findings could help project the occurrence of heavily polluted PM2.5 days during wintertime and assess future emission control strategies for PM2.5 air quality improvement under climate change. Furthermore, the ENSO–EAWM relationship is shown to have a substantial inter-decadal variation under global warming. This may further improve the accuracy of future predictions of air quality in China.

The European network of regulatory air quality monitoring stations is now dense enough to explore long term terms.We combine the Airbase data base for the years 2000 to 2012 and AQ e-reporting after 2013. Trends are computed for SO2, NO2, O3, PM10 and PM2.5, including various metrics and indicators. Quantitative information if provided for the evolution of all these compounds and, when relevant, put in perspective with the trend in air pollutant emissions. We present an assessment of observed air quality trends in Europe aimed at making the most of the regulatory monitoring network to document and explain the effectiveness of air pollution mitigation policies. The focus is on the 2000-2017 time period and surface SO2, NO2, ozone, PM10 and PM2.5, for which we can rely on more than 10,000 stations. Data related to 3,500 stations complied with the requirements, in terms of completeness and representativeness for long-term trend assessments. Such long-term records are only available for countries of the European Union, with one exception for Norway. Substantial improvements are found for all air pollutants. We assess in detail the absolute and relative trends for a wide range of air pollutant indicators and also discuss the spatial variability of the trends as well as changes in monthly, weekly and hourly variability. These changes are put in perspective with emission reductions in Europe in order to point out the pollutants where a potential mismatch may occur between expected and observed improvements in air pollutant concentration. The relative change in SO2 concentrations lies in the 70 to 85% range. This reduction is lower but still in line with the reported emission decrease in Europe (-89%). There is however a slight mismatch between emissions and concentrations in the aftermath of the 2008 economic crisis. Such a mismatch after 2008 also appears for NO2. But on the contrary to SO2, this mismatch has a more substantial impact on the overall trend as reduction in concentrations is 30% which is lower than expected given the 53% reduction in emission over the same time period. The magnitude of ozone peaks (as the fourth highest annual daily maximum of 8hr running mean) decreases by 10% and the number of days exceeding the long-term objective of daily maximum hourly ozone above 120µg/m3 is reduced by 30 to 50%. Annual ozone mean however increases, especially at urban sites. The increase is less pronounced at rural sites, suggesting that it is mainly related to lower NOx titration effect rather than hemispheric changes. Annual mean ozone increase also contributes to higher health exposure, with median SOMO35 and SOMO10 increasing by 1.3% and 13.4%, respectively at urban stations. This needs however to be considered with respect to the NO2 reduction in order to understand the net impact on health, for instance by looking at Ox (sum of NO2 and O3) which decreases. Particulate matter annual mean concentrations decrease by 25 to 45%, depending on station typology. The reductions are similar for PM10 and PM2.5 when comparing collocated measurements. The highest peaks of particulate matter exhibit less relative reduction than the average, showing that episodes of high PM would deserve more focus. PM concentrations decrease faster than primary PM emissions (-30% for primary PM10 and -18% for primary PM2.5), thanks to the additional impact of the reduction of precursors of secondary PM, such as SOx, NOx and NH3. The air quality index over Europe was computed for the whole time period. It gradually improves over the 2000-2017 time period. But most of the improvement concern days in the “moderate” air quality category, whereas the number of days classified as “poor” for air quality remain quite constant. This observation raises specific concern for future improvement of high air pollution episodes., OCP/EEA/ACC/18/001-ETC/ATNI

Air pollution has been recognized as a threat to human health since the time of Hippocrates, ca 400 BC. Successive written accounts of air pollution occur in different countries through the following two millennia until measurements, from the eighteenth century onwards, show the growing scale of poor air quality in urban centres and close to industry, and the chemical characteristics of the gases and particulate matter. The industrial revolution accelerated both the magnitude of emissions of the primary pollutants and the geographical spread of contributing countries as highly polluted cities became the defining issue, culminating with the great smog of London in 1952. Europe and North America dominated emissions and suffered the majority of adverse effects until the latter decades of the twentieth century, by which time the transboundary issues of acid rain, forest decline and ground-level ozone became the main environmental and political air quality issues. As controls on emissions of sulfur and nitrogen oxides (SO 2 and NO x ) began to take effect in Europe and North America, emissions in East and South Asia grew strongly and dominated global emissions by the early years of the twenty-first century. The effects of air quality on human health had also returned to the top of the priorities by 2000 as new epidemiological evidence emerged. By this time, extensive networks of surface measurements and satellite remote sensing provided global measurements of both primary and secondary pollutants. Global emissions of SO 2 and NO x peaked, respectively, in ca 1990 and 2018 and have since declined to 2020 as a result of widespread emission controls. By contrast, with a lack of actions to abate ammonia, global emissions have continued to grow. This article is part of a discussion meeting issue ‘Air quality, past present and future’.