[1] Haze clouds often form over the North China Plain (NCP) of eastern China, where large amounts of aerosol particles and their precursors are emitted. To obtain general insights into regional pollution, a large-scale, long-term study was conducted using A-Train satellite observations, ground measurements, and meteorological data. Contrary to previous analyses, most of the haze clouds appeared to form abruptly (within 2-3 h). Case studies show that natural sources contribute significantly to the formation of regional haze. Dust plumes can mix with local pollutants, causing smog clouds to form abruptly, while moist airflows can cause widespread haze-fog pollution. The combined observations revealed highly inhomogeneous haze clouds, in terms of both vertical and horizontal distribution, leading to clear discrepancies between site measurements near the surface and satellite observations at the top of the atmosphere. Surprisingly, prevailing dust plumes, which are closely connected with the haze clouds, were observed in winter. Airborne dust and water vapor transported from outside the region are the main drivers of regional haze over the NCP. Accumulation of local pollutants also leads to common occurrences of urban smog; however, the occurrence of most haze clouds shows no obvious correlation with local pollution. Local-and regional-scale haze pollution are common over the NCP, but they have differing formation mechanisms, and contrasting chemical and physical properties. The present findings improve our understanding of heavy pollution over eastern China and its links to climate.

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The sensitivities of Weather Research and Forecasting model coupled with chemistry (WRF‐Chem) to the Air Force Weather Agency (AFWA) and Shao2011 (S11) dust emission schemes, and to various land surface properties generated from United States Geological Survey (USGS) and Beijing Normal University (BNU) soil data over East Asia for spring 2012, are examined in this study. The results show that the dust emissions generated with the S11 scheme are 2–5 times that generated with the AFWA emission scheme, with emissions ranging from 0.2–1 Tg/d over East Asia in the relevant period. The AFWA emission scheme omits almost the entire Gobi desert and produces low dust emissions, whereas large amounts of dust emission in this region are produced with the S11 emission scheme, regardless of whether USGS or BNU soil data are used. The surface particulate matter 10 (PM10) concentrations are reasonably well reproduced by the model with different configurations. However, the S11 emission scheme has better performance in simulating the surface PM10 concentrations than the AFWA scheme, especially near the Gobi desert. Comparisons with satellite‐based observations suggest that WRF‐Chem gives better performance with S11 emission scheme in reproducing the horizontal and vertical distribution of aerosol optical properties. The discrepancy between the performances of the AFWA and S11 emission schemes is mainly due to the underestimation of the dust emission over the Gobi desert by the AFWA scheme, which scales the dust emission directly based on the erodibility factor, indicating that the erodibility factor over the Gobi desert is highly underestimated and highlighting an urgent need to improve the erodibility data set.

Atmospheric particulate matter (PM) remains poorly understood due to the lack of comprehensive measurements at high time resolution for tracking its dynamic features and the lack of long-term observation for tracking its seasonal variability. Here, we present highly timeresolved and seasonal compositions and characteristics of non-refractory components in PM with a diameter less than 1 µm (NR-PM 1 ) at a suburban site in Hong Kong. The measurements were made with an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) at the Hong Kong University of Science and Technology (HKUST) Air Quality Research Supersite for 4 months, with one in each season of the year. The average NR-PM 1 concentration of ∼ 15 µg m −3 is higher than those AMS measurements made in South Korea and Japan, but lower than those in North China, the Yangtze River Delta and the nearby Pearl River Delta. The seasonal dependence of the total NR-PM 1 monthly averaged concentrations was small, but that of the fractions of the species in NR-PM 1 was significant. Site characteristic plays an important role in the relative fractions of species in NR-PM 1 and our results are generally consistent with measurements at other non-urban sites in this regard. Detailed analyses were conducted on the AMS data in the aspects of (1) species concentrations, (2) size distributions, (3) degree of oxygenation of organics, and (4) Published by Copernicus Publications on behalf of the European Geosciences Union. 38 Y. J. Li et al.: Seasonal characteristics of fine particulate matter current study provide a better understanding of the role of air mass origin in the seasonal characteristics of the PM composition and the relative importance of local vs. transported organic aerosols in this region.

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The Himalayas were recently identified as a global hot spot for deep stratosphere-to-troposphere transport (STT) in spring. Although the STT in this region may play a vital role in tropospheric chemistry, the hydrological cycle and aquatic ecosystems in Asia, there is no direct measurement of a chemical stratospheric tracer to verify and evaluate its possible impacts. Here we use cosmogenic 35 S as a tracer for air masses originating in the stratosphere and transported downward. We measure concentrations of 35 S in fresh surface snow and river runoff samples collected from Mount Everest in April 2013 to be more than 10 times higher than previously reported by any surface measurement, in support of the Himalayas as a gateway of springtime STT. In light of this result, measurements of 35 SO 2 and 35 SO 4 2À at Nam Co in spring 2011 are reanalyzed to investigate the magnitudes of stratospheric air masses from the Himalayas to the tropospheric sulfur cycle and surface O 3 level over the Tibetan Plateau. A simple one-box model reveals that the oxidative lifetime of SO 2 is reduced in aged STT plumes. Triple oxygen isotopic measurements of sulfate samples suggest that enhanced O 3 levels may shift the oxidation pathway of SO 2 in the troposphere, which may be constrained by further intensive sampling and measurements. Comparison with surface O 3 measurements and traditional meteorological tracing methods shows that 35 S is a potentially unique and sensitive tracer to quantify the contribution of stratospheric air to surface O 3 levels in fresh or aged STT plumes.