An intensive study of aerosol optical properties in Beijing urban area

Abstract. In order to quantify the aerosol impact on climate, a range of aerosol parameters are required. In this paper, two-year 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 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.