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Exposure to high levels of fine particulate matter with diameters <2.5 µm (PM 2.5 ) can lead to cardiovascular and respiratory diseases, and even death (Künzli et al., 2005;Pope et al., 2002). Concentrations of PM 2.5 in China have declined with the implementation of rigorous air pollution control measures since 2013, and the national population-weighted annual mean concentration has decreased from 61.8 μg/m 3 in 2013 to 42 μg/ m 3 in 2017 (Zhang et al., 2019). However, the significantly reduced PM 2.5 concentrations did not correspond to comparable health benefits due to the nonlinear response of mortality to ambient PM 2.5 concentrations (Apte et al., 2015;Zheng et al., 2017). Therefore, more strict controls for PM 2.5 are needed to better protect public health and improve air quality, and achieve the national standards and the levels recommended by the World Health Organization.Several studies have shown that organic aerosol (OA) is a critical component of global PM 2.5 , accounting for 20-90% of PM 2.5 , while secondary OA (SOA) accounts for more than half of all tropospheric OA by mass

Abstract. In order to quantify the aerosol impact on climate, a range of aerosol parameters is required. In this paper, two-year ground-based observations of aerosol optical properties are conducted at an urban site in Beijing of China. Aerosol absorption coefficient (σa) and scattering coefficient (σs), as well as single scattering albedo (ω) are analyzed for characterizing Beijing urban aerosol. Two-year averages (and standard deviations) for σa, σs and ω are 56±49 Mm−1, 288±281 Mm−1 and 0.80±0.09, respectively. Meanwhile, there is a distinct diurnal variation for σa, with its minimum occurs about 14:00 to 15:00 and maximum in the evening. While, σs peaks in late morning and the minimum occurs in the evening. σs in summer is higher than that in winter. ω in summer is higher than that in winter except before 07:00 a.m. and it peaks in the early afternoon. Both σa and σs show strong dependence on local wind in four seasons. When the wind blows from north with low speed (0–4 m/s), both σa and σs are high, while very low with wind speed higher than 4 m/s. When the wind blows from south with low speed (0–4 m/s), σa and σs are intermediate. ω also shows wind dependence to some extent though not as strong as σa and σs.