Changes in visibility with PM2.5 composition and relative humidity at a background site in the Pearl River Delta region.

Abstract In fall–winter, 2007–2013, visibility and light scattering coefficients ( b sp ) were measured along with PM 2.5 mass concentrations and chemical compositions at a background site in the Pearl River Delta (PRD) region. The daily average visibility increased significantly ( p 2.5 mass concentration was below 75 μg/m 3 . By multiple linear regression on the chemical budget of particle scattering coefficient ( b sp ), we obtained site-specific mass scattering efficiency (MSE) values of 6.5 ± 0.2, 2.6 ± 0.3, 2.4 ± 0.7 and 7.3 ± 1.2 m 2 /g, respectively, for organic matter (OM), ammonium sulfate (AS), ammonium nitrate (AN) and sea salt (SS). The reconstructed light extinction coefficient ( b ext ) based on the Interagency Monitoring of Protected Visual Environments (IMPROVE) algorithm with our site-specific MSE revealed that OM, AS, AN, SS and light-absorbing carbon (LAC) on average contributed 45.9% ± 1.6%, 25.6% ± 1.2%, 12.0% ± 0.7%, 11.2% ± 0.9% and 5.4% ± 0.3% to light extinction, respectively. Averaged b ext displayed a significant reduction rate of 14.1/Mm·year ( p f (RH)) at rates of 2.5% and 0.16/year  − 1 ( p f (RH) partly offsets the positive effects of lowered AS in improving visibility, and aggravated the negative effects of increasing AN to impair visibility.