With the incredibly fast development of China, the air quality in lots of Chinese cities has raised extra awareness of officials, scientists as well as the civilians. On February 26^th 2014, The PM2.5(Particulate matters suspended in air with the diameter less than or equal to 2.5μm) of Beijing rose up to 500μg/m³ which could make significantly adverse health effects on human bodies. That value has exceeded the Chinese daily air quality standard (35μg/m³) by almost 20 times. Recent studies have also shown that PM has remarkable ability to reduce the visibility. Due to the complexity of PM in the air, some of the species like organic matter (OM), ammonium sulphate (AS) and ammonium nitrate (AN) which come from different sources will reflect or absorb light after emitted into the atmosphere to lead to the visibility degradation. This study focuses on the application of the empirical formula called IMPORVE equation (Interagency Monitoring of Protected Visual Environments) initially demonstrated by[Malm et al] in 1994, and the equation is shown as below:

bext = 3f(RH)[sulfates] +3f(RH)[nitrates] +4[organics] +10[elemental carbon] +1[fine soil]+0.6[coarse matter] +10

Based on the original equation, some revisions have also been achieved by separating the organics part into two portions: WSOC (water-soluble organic carbon) and WISOC (water-insoluble organic carbon), the procedures of which is guided by the study of [Lin et al] in 2010. f(RH) is the growth factor calculated from the relative humility (RH) for those hygroscopic species which will uptake liquid water in the ambient air condition. And the scattering or absorption coefficient which is location dependent in front of the each species may differ a lot in different sampling sites and even for same sites in the different seasons. The Multiple Linear Regression (MLR) which is one of the most useful methods in calculating the coefficient serves as the auxiliary tool in this research.

The samples we used were collected at HKUST Supersite from March 2013 to February 2014.From the results we could see that the light extinction efficiency increased dramatically from 9Mm^-1 with its PM2.5 concentrations aournd1μg/m³ in July 17^th to 139 Mm^-1 with the PM2.5 around 30μg/m³ in March 1^st. At this stage, our group is aiming to figure out the contributions from each species mixed in the ambient aerosol to the light degradation and that should be helpful in the future policy making as well as the source apportion in the visibility degradation. In addition, we will keep conducting the empirical relation studies to get more general visibility degradation pattern in Hong Kong. This will be useful before we extend the scope of our research into the Pearl River Delta region.