AR4 did not report trends in long-term surface-based in situ measurements of particulate matter, its components or its properties. This section summarizes reported trends of PM10, PM2.5 (particulate matter with aerodynamic diameters <10 and <2.5 μm, respectively), sulphate and equivalent black carbon/elemental carbon, from regionally representative measurement networks. An overview of current networks and definitions pertinent to aerosol measurements is given in Supplementary Material 2.SM.2.3. Studies reporting trends representative of regional changes are presented in Table 2.2. Long-term data are almost entirely from North America and Europe, whereas a few individual studies on aerosol trends in India and China are reported in Supplementary Material 2.SM.2.3. Figure 2.10 gives an overview of observed PM10, PM2.5, and sulphate trends in North America and Europe for 1990–2009 and 2000–2009.
In Europe, strong downward trends are observed for PM10, PM2.5 and sulphate from the rural stations in the EMEP (European Monitoring and Evaluation Programme) network. For 2000–2009, PM2.5 shows an average reduction of 3.9% yr–1 for the six stations with significant trends, while trends are not significant at seven other stations. Over 2000–2009, PM10 at 12 (out of 24) sites shows significant downward trend of on average 2.6% yr–1. Similarly sulphate strongly decreased at 3.1% yr–1 from 1990 to 2009 with 26 of 30 sites having significant reductions. The largest decrease occurred before 2000, while for 2000–2009, the trends were weaker and less robust. This is consistent with reported emission reductions of 65% from 1990 to 2000 and 28% from 2001 to 2009 (Yttri et al., 2011; Torseth et al., 2012). Model analysis (Pozzoli et al., 2011) attributed the trends in large part to emission changes.
In the USA, the largest reductions in PM and sulphate are observed in the 2000s, rather than the 1990s as in Europe. IMPROVE (U.S. Interagency Monitoring of Protected Visual Environments Network) PM2.5 measurements (Hand et al., 2011) show significant downward trends averaging 4.0% yr–1 for 2000–2009 at sites with significant trends, and 2.1% yr–1 at all sites, and PM10 decreases of 3.1% yr–1 for 2000–2009. Declines of PM2.5 and SO4 2– in Canada are very similar (Hidy and Pennell, 2010), with annual mean PM2.5 at urban measurement sites decreasing by 3.6% yr–1 during 1985–2006 (Canada, 2012).
In the eastern and southwestern USA, IMPROVE data show strong sulphate declines, which range from 2 to 6% yr–1, with an average of 2.3% yr–1 for the sites with significant negative trends for 1990–2009. However, four IMPROVE sites show strong SO4 2– increases from 2000 to 2009, amounting to 11.9% yr–1, at Hawaii (1225 m above sea level), and 4 to 7% yr–1 at three sites in southwest Alaska.
A recent study on long-term trends in aerosol optical properties from 24 globally distributed background sites (Collaud Coen et al., 2013) reported statistically significant trends at 16 locations, but the sign and magnitude of the trends varied largely with the aerosol property considered and geographical region (Table 2.3). Among the sites, this study reported strong increases in absorption and scattering coefficients in the free troposphere at Mauna Loa, Hawaii (3400 m above sea level), which is a regional feature also evident in the satellite-based AOD trends (illustrated in Figure 2.9). Possible explanations for these changes include the influence of increasing Asian emissions and changes in clouds and removal processes. More and longer Asian time series, coupled with transport analyses, are needed to corroborate these findings. Aerosol number concentrations (Asmi et al., 2013) are declining significantly at most sites in Europe, North America, the Pacific and the Caribbean, but increasing at South Pole based on a study of 17 globally distributed remote sites.
Total carbon (= light absorbing carbon + organic carbon) measurements indicate highly significant downward trends between 2.5 and 7.5% yr–1 along the east and west coasts of the USA, and smaller and less significant trends in other regions of the USA from 1989 to 2008 (Hand et al., 2011; Murphy et al., 2011).
In Europe, Torseth et al. (2012) suggest a slight reduction in elemental carbon concentrations at two stations from 2001 to 2009, subject to large interannual variability. Collaud Coen et al. (2013) reported consistent negative trends in the aerosol absorption coefficient at stations in the continental USA, Arctic and Antarctica, but mostly insignificant trends in Europe over the last decade.
In the Arctic, changes in aerosol impact the atmosphere’s radiative balance as well as snow and ice albedo. Similar to Europe and the USA, Hirdman et al. (2010) reported downward trends in equivalent black carbon and SO4 2– for two out of total three Arctic stations and attributed them to emission changes.
In summary, declining AOD in Europe and North America is corroborated by very likely downward trends in ground-based in situ particulate matter measurements since the mid-1980s. Robust evidence from around 200 regional background sites with in situ ground based aerosol measurements indicate downward trends in the last two decades of PM2.5 in parts of Europe (2 to 6% yr–1) and the USA (1 to 2.5% yr–1),