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The hygroscopicity of organic aerosol (OA) is important for investigation of its climatic and environmental impacts. However, the hygroscopicity parameter κOA remains poorly characterized, especially in the relatively polluted environment on the North China Plain (NCP). Here we conducted simultaneous wintertime measurements of bulk aerosol chemical compositions of PM2.5 and PM1 and bulk aerosol hygroscopicity of PM10 and PM1 on the NCP using a capture-vaporizer time-of-flight aerosol chemical speciation monitor (CV-ToF-ACSM) and a humidified nephelometer system which measures the aerosol light-scattering enhancement factor f(RH). A method for calculating κOA based on f(RH) and bulk aerosol chemical-composition measurements was developed. We found that κOA varied in a wide range with significant diurnal variations. The derived κOA ranged from almost 0.0 to 0.25, with an average (±1σ) of 0.08 (±0.06) for the entire study. The derived κOA was highly correlated with f44 (fraction of m∕z 44 in OA measured by CV-ToF-ACSM), an indicator of the oxidation degree of OA (R=0.79), and the relationship can be parameterized as κOA=1.04×f44-0.02 (κOA=0.3×O:C-0.02, based on the relationship between the f44 and O∕C ratio for CV-ToF-ACSM). On average, κOA reached the minimum (0.02) in the morning near 07:30 local time (LT) and then increased rapidly, reaching the peak value of 0.16 near 14:30 LT. The diurnal variations in κOA were highly and positively correlated with those of mass fractions of oxygenated OA (R=0.95), indicating that photochemical processing played a dominant role in the increase in κOA in winter on the NCP. Results in this study demonstrate the potential wide applications of a humidified nephelometer system together with aerosol composition measurements for investigating the hygroscopicity of OA in various environments and highlight that the parameterization of κOA as a function of OA aging processes needs to be considered in chemical transport models for better evaluating the impacts of OA on cloud formation, atmospheric chemistry, and radiative forcing.

The Yangtze River Delta (YRD) is one of the most densely populated regions in China with severe air quality issues that have not been fully understood. Thus, in this study, based on 1-year (2013) continuous measurement at a National Reference Climatological Station (NRCS, 30.22° N, 120.17° E; 41.7 m a.s.l.) in the center of Hangzhou in the YRD, we investigated the seasonal characteristics, interspecies relationships, and the local emissions and the regional potential source contributions of trace gases (including O3, NOx, NOy, SO2, and CO) and particulate matter (PM2.5 and PM10). Results revealed that severe two-tier air pollution (photochemical and haze pollution) occurred in this region, with frequent exceedances in O3 (38 days) and PM2.5 (62 days). O3 and PM2.5 both exhibited distinct seasonal variations with reversed patterns: O3 reaching a maximum in warm seasons (May and July) but PM2.5 reaching a maximum in cold seasons (November to January). The overall results from interspecies correlation indicated a strong local photochemistry favoring the O3 production under a volatile organic compound (VOC)-limited regime, whereas it moved towards an optimum O3 production zone during warm seasons, accompanied by the formation of secondary fine particulates under high O3. The emission maps of PM2.5, CO, NOx, and SO2 demonstrated that local emissions were significant for these species on a seasonal scale. The contributions from the regional transport among inland cities (Zhejiang, Jiangsu, Anhui, and Jiangxi Province) on a seasonal scale were further confirmed to be crucial to air pollution at the NRCS site by using backward trajectory simulations. Air masses transported from the offshore areas of the Yellow Sea, East Sea, and South Sea were also found to be highly relevant to the elevated O3 at the NRCS site through the analysis of potential source contribution function (PSCF). Case studies of photochemical pollution (O3) and haze (PM2.5) episodes both suggested the combined importance of local atmospheric photochemistry and synoptic conditions during the accumulation (related with anticyclones) and dilution process (related with cyclones). Apart from supplementing a general picture of the air pollution state in the city of Hangzhou in the YRD region, this study specifically elucidates the role of local emission and regional transport, and it interprets the physical and photochemical processes during haze and photochemical pollution episodes. Moreover, this work suggests that cross-regional control measures are crucial to improve air quality in the YRD region, and it further emphasizes the importance of local thermally induced circulation for air quality.

Inter-annual variability and long-term trends in tropospheric ozone are both environmental and climate concerns. Ozone measured at Mt Waliguan Observatory (WLG, 3816 m a.s.l.) on the Tibetan Plateau over the period of 1994–2013 has increased significantly by 0.2–0.3 ppbv yr−1 during spring and autumn but shows a much smaller trend in winter and no significant trend in summer. Here we explore the factors driving the observed ozone changes at WLG using backward trajectory analysis, chemistry–climate model hindcast simulations (GFDL AM3), a trajectory-mapped ozonesonde data set, and several climate indices. A stratospheric ozone tracer implemented in GFDL AM3 indicates that stratosphere-to-troposphere transport (STT) can explain ∼ 60 % of the simulated springtime ozone increase at WLG, consistent with an increase in the NW air-mass frequency inferred from the trajectory analysis. Enhanced STT associated with the strengthening of the mid-latitude jet stream contributes to the observed high ozone anomalies at WLG during the springs of 1999 and 2012. During autumn, observations at WLG are more heavily influenced by polluted air masses originating from South East Asia than in the other seasons. Rising Asian anthropogenic emissions of ozone precursors are the key driver of increasing autumnal ozone observed at WLG, as supported by the GFDL AM3 model with time-varying emissions, which captures the observed ozone increase (0.26 ± 0.11 ppbv yr−1). AM3 simulates a greater ozone increase of 0.38 ± 0.11 ppbv yr−1 at WLG in autumn under conditions with strong transport from South East Asia and shows no significant ozone trend in autumn when anthropogenic emissions are held constant in time. During summer, WLG is mostly influenced by easterly air masses, but these trajectories do not extend to the polluted regions of eastern China and have decreased significantly over the last 2 decades, which likely explains why summertime ozone measured at WLG shows no significant trend despite ozone increases in eastern China. Analysis of the Trajectory-mapped Ozonesonde data set for the Stratosphere and Troposphere (TOST) and trajectory residence time reveals increases in direct ozone transport from the eastern sector during autumn, which adds to the autumnal ozone increase. We further examine the links of ozone variability at WLG to the quasi-biennial oscillation (QBO), the East Asian summer monsoon (EASM), and the sunspot cycle. Our results suggest that the 2–3-, 3–7-, and 11-year periodicities are linked to the QBO, EASM index, and sunspot cycle, respectively. A multivariate regression analysis is performed to quantify the relative contributions of various factors to surface ozone concentrations at WLG. Through an observational and modelling analysis, this study demonstrates the complex relationships between surface ozone at remote locations and its dynamical and chemical influencing factors.

Water can be a major component of an aerosol particle. Also water serves as a medium for aqueous-phase reactions in aerosols. In this study, a novel method is presented to calculate the aerosol liquid water content at high relative humidity based on measurements of aerosol hygroscopic growth factor, particle number size distribution and relative humidity in the Haze in China (HaChi) summer field campaign (July–August 2009) in the North China Plain. The aerosol liquid water content calculated using this method agreed well with that calculated using a thermodynamic equilibrium model (ISORROPIA II) at high relative humidity (>60 %) with a correlation coefficient of 0.9658. At low relative humidity (

This work analyses optical properties of the dry tropospheric aerosol measured at the regional Global Atmosphere Watch (GAW) observation site Melpitz in East Germany. For a continuous observation period between 2007 and 2010, we provide representative values of the dry-state scattering coefficient, hemispheric backscattering coefficient, absorption coefficient, single scattering albedo, and scattering Ångström exponent. Besides the direct measurement, the aerosol scattering coefficient was alternatively computed from experimental particle number size distributions using a Mie model. Within pre-defined limits, a closure could be achieved with the direct measurement. The achievement of closure implies that such calculations can be used as a high-level quality control measure for data sets involving multiple instrumentation. All dry-state optical properties show pronounced annual and diurnal variations, which are attributed to the corresponding variations in the regional emission fluxes, the intensity of secondary particle formation, and the mixing layer height. Air mass classification shows that atmospheric stability is a major factor influencing the dry aerosol properties at the GAW station. In the cold season, temperature inversions limit the volume available for atmospheric mixing, so that the dry-state aerosol optical properties near the ground proved quite sensitive to the geographical origin of the air mass. In the warm season, when the atmosphere is usually well-mixed during daytime, considerably less variability was observed for the optical properties between different air masses. This work provides, on the basis of quality-checked in situ measurements, a first step towards a climatological assessment of direct aerosol radiative forcing in the region under study.