Aerosol optical properties at SORPES in Nanjing, east China

Aerosol optical properties (AOPs) and supporting parameters – particle number size distributions, PM_2.5 mass concentrations, and the concentrations of trace gases (NO_<i>x</i> and NO_<i>y</i>) – were measured at SORPES, a regional background station in Nanjing, China from June 2013 to May 2015. The aerosol was highly scattering: the average scattering coefficient was <i>σ</i>_sp = 403 ± 314 Mm⁻¹, the absorption coefficient <i>σ</i>_ap = 26 ± 19 Mm⁻¹, and the single-scattering albedo SSA = 0.93 ± 0.03 for green light. The SSA in Nanjing appears to be slightly higher than published values from several other sites in China and elsewhere. The average Ångström exponent of absorption (AAE) for the wavelength range 370–950 nm was 1.04 and the AAE range was 0.7–1.4. These AAE values can be explained with different amounts of non-absorbing coating on pure black carbon (BC) cores and different core sizes rather than contribution by brown carbon. The AOPs had typical seasonal cycles with high <i>σ</i>_sp and <i>σ</i>_ap in winter and low ones in summer: the averages were <i>σ</i>_sp = 544 ± 422 and <i>σ</i>_ap = 36 ± 24 Mm⁻¹ in winter and <i>σ</i>_sp = 342 ± 281 and <i>σ</i>_ap = 20 ± 13 Mm⁻¹ in summer. The intensive AOPs had no clear seasonal cycles, the variations in them were rather related to the evolution of pollution episodes. The diurnal cycles of the intensive AOPs were clear and in agreement with the cycle of the particle number size distribution. The diurnal cycle of SSA was similar to that of the air photochemical age, suggesting that the darkest aerosol originated from fresh traffic emissions. A Lagrangian retroplume analysis showed that the potential source areas of high <i>σ</i>_sp and <i>σ</i>_ap are mainly in eastern China. Synoptic weather phenomena dominated the cycle of AOPs on a temporal scale of 3–7 days. During pollution episodes, modeled boundary layer height decreased, whereas PM_2.5 concentrations and <i>σ</i>_sp and <i>σ</i>_ap typically increased gradually and remained high during several days but decreased faster, sometimes by even more than an order of magnitude within some hours. During the growth phase of the pollution episodes the intensive AOPs evolved clearly. The mass scattering efficiency MSE of PM_2.5 grew during the extended pollution episodes from ∼ 4 to ∼ 6 m² g⁻¹ and the mass fraction of BC_e decreased from ∼ 10 to ∼ 3 % during the growth phase of the episodes. Particle growth resulted in the backscatter fraction decreasing from more than 0.16 to less than 0.10, SSA growing from less than 0.9 to more than 0.95, and radiative forcing efficiency (RFE) changing from less than −26 W m⁻² to more than −24 W m⁻², which means that the magnitude of RFE decreased. The RFE probability distribution at SORPES was clearly narrower than at a clean background site which is in agreement with a published RFE climatology