Optical properties of PM2.5 and the impacts of chemical compositions in the coastal city Xiamen in China.

Continuous in situ measurements of optical properties of fine aerosols (PM2.5) were conducted in the urbanized coastal city Xiamen in Southeast China from November 2013 to January 2014. PM2.5 samples were also collected and chemical compositions including organic carbon (OC), elemental carbon (EC) and water-soluble inorganic ions were determined to investigate the impacts of chemical compositions on aerosol optical properties. Average values of scattering coefficient (bscat), absorption coefficient (babs), extinction coefficient (bext) and single scattering albedo (SSA) were 164.0Mm(-1), 22.4Mm(-1), 187.0Mm(-1) and 0.88, respectively. bscat, babs and bext showed obvious bi-modal diurnal variations with high values in the morning and at night while low value in the early afternoon, whereas SSA exhibited an opposite diurnal variation. Average bscat and babs were largest in the wind direction of southwest and were larger with slower wind. babs was mainly affected by EC, while bscat was affected by ammonium, sulfate, nitrate and OC. The IMPROVE formula was applied to estimate bext based on the chemical species. Results shows that ammonium sulfate was the largest contributor, accounting for 36.4% of bext, followed by organic matter (30.6%), ammonium nitrate (20.1%), EC (9.0%) and sea salt (3.9%). The deterioration in visibility was mainly led by increases in secondary aerosols including sulfate and nitrate. Backward trajectories analysis showed that during the sampling period Xiamen was significantly affected by the air masses originating from the Northern and Northeastern areas. Air masses from the Northern associated with relative higher bext and less relative contribution from ammonium sulfate and more relative contribution from ammonium nitrate, organic matter and sea salt.