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 (σa), scattering coefficient (σs), as well as single scattering albedo (ω) are analyzed to aid in characterizing Beijing's urban aerosol. Two-year averages (and standard deviations) for σa at 532 nm, σs 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 σa, with its minimum occurring at approximately 14:00 to 15:00 and maximum at midnight. σs peaks in the late morning and the minimum occurs in the evening. σs 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 σa and σs 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 σa 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), σa is intermediate. The patterns of the wind dependence of σa indicates that σa is mainly dominated by local emissions. σs displays a similar dependence on wind speed and direction to σa, except in summer. In summer, the σs 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 σa or σs. 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.