Aerosol Optical Properties at SORPES in Nanjing, East China

Aerosol optical properties (AOPs) and supporting parameters – particle number size distributions, mass concentrations and trace gases (NOx and NOy) – 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 σsp = 410 ± 320 Mm−1, the absorption coefficient σap = 26 ± 19 Mm−1 and the single-scattering albedo SSA = 0.93 ± 0.03 for the 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 0.7–1.4. These AAE values can be explained with different amounts of non-absorbing coating on pure BC cores and different core sizes so the data does not suggest any significant contribution to absorption by brown carbon. The AOPs had typical seasonal cycles with high σsp and σap in winter and lower in summer: the averages were σsp = 545 ± 425 Mm−1 and σap = 36 ± 24 Mm−1 in winter and σsp = 364 ± 294 Mm−1 and σap = 20 ± 13 Mm−1 in summer. The intensive AOPs had no clear seasonal cycles, the variations of them were rather related to the evolution of pollution episodes. The diurnal cycles of the intensive AOPs were clear and in 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 sources of high σsp and σap are mainly in eastern China. Synoptic weather dominated the cycle of AOPs in a temporal scale of 2–7 days. During pollution episodes, modeled PBLH decreased, whereas PM2.5 concentrations, σsp and σap 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 of PM2.5 grew during the extended pollution episodes from ~4 m2 g−1 to ~6 m2 g−1 and the mass fraction of BCe decreased from ~10 % to ~2 % during the growth phase of the episodes. Particle growth resulted in b 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 growing from less than −26 W m−2 τ−1 to more than −24 W m−2 τ−1. In other words, the darker aerosol – the aerosol that had a higher BC mass fraction – had a more negative radiative forcing efficiency, i.e., they have the property of cooling the atmosphere more efficiently per unit optical depth than the aerosol with the higher SSA and a lower BC mass fraction. This counterintuitive result is due to the size of the particles: the upscatter fraction of small particles is higher than that of the big ones which more than compensates the darkness of them. The RFE probability distribution at SORPES was clearly more narrow than at a clean background site which is in agreement with a published RFE climatology.