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ABSTRACT The effects of El Niño/Southern Oscillation ( ENSO) under negative Arctic Oscillation ( AO) phase on the Asian dust activity are investigated for springs of the period 1961-2002. The spring dust index ( DI) describing the monthly frequencies of three types of dust events (e.g. dust storm, blowing dust, and floating dust) exhibits a significant increase in the years of negative AO phase (hereafter AO−) and El Niño, compared with that in the years of AO− and La Niña. Averaged over all observation stations, the spring DI (49.7) during the El Niño/ AO− years is higher by 11.4% or 29.8% than that (38.3) during the La Niña/ AO− years. We suggest possible physical mechanism that the anomalous large-scale environments associated with AO− and El Niño are more effective to provide favourable conditions to enhance Asian dust activity. During the El Niño/ AO− years, meridional gradients of pressure and temperature over the dust source regions are significantly enhanced by decreasing the geopotential height and warming air temperature that originated from the north and south of source regions, respectively, under the influence of AO− and El Niño. These also intensify the zonal wind shear and atmospheric baroclinicity, thereby producing enhanced cyclogenesis and dust occurrences over the major source regions. At the same time, dust transport paths with the stronger westerly winds are developed by the combined constraints of anomalous cyclone over the Siberia and the Mongolia and anomalous anticyclone over the western North Pacific, and thus strengthen dust transport to the downwind regions. [ABSTRACT FROM AUTHOR]

Exposure to outdoor fine particulate matter (PM 2.5 ) is a leading risk factor for mortality. We develop global estimates of annual PM 2.5 concentrations and trends for 1998-2018 using advances in satellite observations, chemical transport modeling, and ground-based monitoring. Aerosol optical depths (AODs) from advanced satellite products including finer resolution, increased global coverage, and improved long-term stability are combined and related to surface PM 2.5 concentrations using geophysical relationships between surface PM 2.5 and AOD simulated by the GEOS-Chem chemical transport model with updated algorithms. The resultant annual mean geophysical PM 2.5 estimates are highly consistent with globally distributed ground monitors ( R 2 = 0.81; slope = 0.90). Geographically weighted regression is applied to the geophysical PM 2.5 estimates to predict and account for the residual bias with PM 2.5 monitors, yielding even higher cross validated agreement ( R 2 = 0.90-0.92; slope = 0.90-0.97) with ground monitors and improved agreement compared to all earlier global estimates. The consistent long-term satellite AOD and simulation enable trend assessment over a 21 year period, identifying significant trends for eastern North America (-0.28 ± 0.03 μg/m 3 /yr), Europe (-0.15 ± 0.03 μg/m 3 /yr), India (1.13 ± 0.15 μg/m 3 /yr), and globally (0.04 ± 0.02 μg/m 3 /yr) for China over 2011-2018 are remarkable, with implications for the health of billions of people.3 /yr) for India over 2005-2013 and the negative trend (-3.37 ± 0.38 μg/m 3 /yr) for China over 2011-2018 are remarkable, with implications for the health of billions of people.