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 (bsp) were measured along with PM2.5 mass concentrations and chemical compositions at a background site in the Pearl River Delta (PRD) region. The daily average visibility increased significantly (p<0.01) at a rate of 1.1km/year, yet its median stabilized at ~13km. No haze days occurred when the 24-hr mean PM2.5 mass concentration was below 75μg/m3. By multiple linear regression on the chemical budget of particle scattering coefficient (bsp), 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±1.2m2/g, respectively, for organic matter (OM), ammonium sulfate (AS), ammonium nitrate (AN) and sea salt (SS). The reconstructed light extinction coefficient (bext) 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 bext displayed a significant reduction rate of 14.1/Mm·year (p<0.05); this rate would be 82% higher if it were not counteracted by increasing relative humidity (RH) and hygroscopic growth factor (f(RH)) at rates of 2.5% and 0.16/year−1 (p<0.01), respectively, during the fall–winter, 2007–2013. This growth of RH and f(RH) partly offsets the positive effects of lowered AS in improving visibility, and aggravated the negative effects of increasing AN to impair visibility.