| Summary: | To clarify the aerosol optical properties under different pollution levels and their impacting factors, hourly organic carbon (OC), elemental carbon (EC), and water-soluble ion (WSI) concentrations in PM<sub>2.5</sub> were observed by using monitoring for aerosols and gases (MARGA) and a semicontinuous OC/EC analyzer (Model RT-4) in Wuhan from 9 to 26 January 2018. The aerosol extinction coefficient (<i>b<sub>ext</sub></i>) was reconstructed using the original Interagency Monitoring of Protected Visual Environment (IMPROVE) formula with a modification to include sea salt aerosols. A good correlation was obtained between the reconstructed <i>b<sub>ext</sub></i> and measured <i>b<sub>ext</sub></i> converted from visibility. <i>b<sub>ext</sub></i> presented a unimodal distribution on polluted days (PM<sub>2.5</sub> mass concentrations > 75 μg⋅m<sup>−3</sup>), peaking at 19:00. <i>b<sub>ext</sub></i> on clean days (PM<sub>2.5</sub> mass concentrations < 75 μg⋅m<sup>−3</sup>) did not change much during the day, while on polluted days, it increased rapidly starting at 12:00 due to the decrease of wind speed and increase of relative humidity (RH). PM<sub>2.5</sub> mass concentrations, the aerosol scattering coefficient (<i>b<sub>scat</sub></i>), and the aerosol extinction coefficient increased with pollution levels. The value of <i>b<sub>ext</sub></i> was 854.72 Mm<sup>−1</sup> on bad days, which was 4.86, 3.1, 2.29, and 1.28 times of that obtained on excellent, good, acceptable, and poor days, respectively. When RH < 95%, <i>b<sub>ext</sub></i> exhibited an increasing trend with RH under all pollution levels, and the higher the pollution level, the bigger the growth rate was. However, when RH > 95%, <i>b<sub>ext</sub></i> on acceptable, poor and bad days decreased, while <i>b<sub>ext</sub></i> on excellent and good days still increased. The overall <i>b<sub>ext</sub></i> in Wuhan in January was mainly contributed by NH<sub>4</sub>NO<sub>3</sub> (25.2%) and organic matter (20.1%). The contributions of NH<sub>4</sub>NO<sub>3</sub> and (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> to <i>b<sub>ext</sub></i> increased significantly with pollution levels. On bad days, NH<sub>4</sub>NO<sub>3</sub> and (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> contributed the most to <i>b<sub>ext</sub></i>, accounting for 38.2% and 27.0%, respectively.
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