This study investigated the air pollution characteristics of synoptic-scale circulation in the Beijing megacity, and provided quantitative evaluation of the impacts of circulation patterns on air quality during the 2008 Beijing Summer Olympics. Nine weather circulation types (CTs) were objectively identified over the North China region during 2000–2009, using obliquely rotated T-mode principal component analysis (PCA). The resulting CTs were examined in relation to the local meteorology, regional transport pathways, and air quality parameters, respectively. The FLEXPART-WRF model was used to calculate 48-h backward plume trajectories for each CT. Each CT was characterized with distinct local meteorology and air mass origin. CT 1 (high pressure to the west with a strong pressure gradient) was characterized by a northwestern air mass origin, with the smallest local and southeasterly air mass sources, and CT 6 (high pressure to the northwest) had air mass sources mostly from the north and east. On the contrary, CTs 5, 8, and 9 (weak pressure field, high pressure to the east, and low pressure to the northwest, respectively) were characterized by southern and southeastern trajectories, which indicated a greater influence of high pollutant emission sources. In turn, poor air quality in Beijing (high loadings of PM₁₀, BC, SO₂, NO₂, NO_x, O₃, AOD, and low visibility) was associated with these CTs. Good air quality in Beijing was associated with CTs 1 and 6. The average visibilities (with ±1&sigma;) in Beijing for CTs 1 and 6 during 2000–2009 were 18.5 ± 8.3 km and 14.3 ± 8.5 km, respectively. In contrast, low visibility values of 6.0 ± 3.5 km, 6.6 ± 3.7 km, and 6.7 ± 3.6 km were found in CTs 5, 8, and 9, respectively. The mean concentrations of PM₁₀ for CTs 1, 6, 5, 8, and 9 during 2005–2009 were 90.3 ± 76.3 μg m⁻³, 111.7 ± 89.6 μg m⁻³, 173.4 ± 105.8 μg m⁻³, 158.4 ± 90.0 μg m⁻³, and 151.2 ± 93.1 μg m⁻³, respectively. <br></br> Analysis of the relationship between circulation pattern and air quality during the emission control period suggests that CTs are the primary drivers of day-to-day variations in pollutant concentrations over Beijing and its vicinity. During the Olympics period, the frequency of CT 6 was twice that of the mean in August from 2000 to 2009. This CT had northerly transport pathways and favorable meteorological conditions (e.g. frequent precipitation) for clean air during the Olympics. Assuming that relationships between CTs and air quality parameters in the same season are fixed in different years, the relative contributions of synoptic circulation to decreases in PM₁₀, BC, SO₂, NO₂, NO_x, CO, and horizontal light extinction during the Olympics were ...

Abstract. In order to quantify the aerosol impact on climate, a range of aerosol parameters are required. In this paper, twoyear of ground-based observations of aerosol optical properties from an urban site in Beijing of China are assessed. The aerosol absorption coefficient (σ a ), scattering coefficient (σ s ), as well as single scattering albedo (ω) are analyzed to aid in characterizing Beijing's urban aerosol. Two-year averages (and standard deviations) for σ a at 532 nm, σ s at 525 nm and ω at 525 nm are 56±49 Mm −1 , 288±281 Mm −1 and 0.80±0.09, respectively. Meanwhile, there is a distinct diurnal variation for σ a , with its minimum occurring at approximately 14:00 to 15:00 and maximum at midnight. σ s peaks in the late morning and the minimum occurs in the evening. σ s in summer is higher than that in winter. ω is also higher in summer than that in winter, except before 07:00 a.m., and peaks in the early afternoon. Both σ a and σ s show strong dependence on local wind in all four seasons. When the wind blows from the north with low speed (0-2 m/s), the values of σ a are high, and in contrast, very low with wind speeds higher than 4 m/s. When the wind blows from south with low speed (0-4 m/s), σ a is intermediate. The patterns of the wind dependence of σ a indicates that σ a is mainly dominated by local emissions. σ s displays a similar dependence on wind speed and direction to σ a , except in summer. In summer, the σ s value is highest when wind is from southeast with speed of 0-6 m/s. This indicates that the particle pollution resulting from regional transport is only significant in the summer season. ω also shows wind dependence to some extent though not as strong as σ a or σ s . Overall, the wind dependence results provide valuable information about the locations of Correspondence to: C. C. Li (ccli@pku.edu.cn) aerosol pollution sources and suggest that the air pollution in summer is a regional problem but in other seasons it is mainly affected by local urban emissions.

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A unified microscopic principle is proposed to clarify resistive switching behaviors of transition metal oxide based resistive random access memories (RRAM) for the first time. In this unified microscopic principle, both unipolar and bipolar switching characteristics of RRAM are correlated with the distribution of localized oxygen vacancies in the oxide switching layer, which is governed by the generation and recombination with dissociative oxygen ions. Based on the proposed microscopic principle, an atomistic simulation method is developed to evaluate critical memory performance, and successfully conduct the device optimization. The experimental data are well in line with the developed simulation method.