The surface aerosol optical properties in the urban area of Nanjing, west Yangtze River Delta, China

Abstract

Abstract. Observational studies of aerosol optical properties are useful for reducing uncertainties in estimations of aerosol radiative forcing and forecasting visibility. In this study, the observed near-surface aerosol optical properties in urban Nanjing are analysed from March 2014 to February 2016. Results show that near-surface urban aerosols in Nanjing are mainly from local emissions and the surrounding regions. They have lower loadings but are more scattering than aerosols in most cities in China. The annual mean aerosol extinction coefficient (EC), single-scattering albedo (SSA) and asymmetry parameter (ASP) at 550 nm are 381.96 Mm −1 , 0.9 and 0.57, respectively. The aerosol absorption coefficient (AAC) is about 1 order of magnitude smaller than its scattering coefficient (SC). However, the absorbing aerosol has a larger Ångström exponent (AAE) value, 1.58 at 470/660 nm, about 0.2 larger than the scattering aerosols (SAE). All the aerosol optical properties follow a near-unimodal pattern, and their values are mostly concentrated around their averages, accounting for more than 60 % of the total samplings. Additionally, they have substantial seasonality and diurnal variations. High levels of SC and AAC all appear in winter due to higher aerosol and trace gas emissions. AAE (ASP) is the smallest (largest) in summer, possibly because of high relative humidity (RH) which also causes considerably larger SC and smaller SAE, although intensive gas-toparticle transformation could produce a large number of finer scattering aerosols in this season. Seasonality of EC is different from the columnar aerosol optical depth. Larger AACs appear during the rush hours of the day while SC and backscattering coefficient (Bsp) only peak in the early morning. Aerosols are fresher in the daytime than at night-time, leading to their larger Ångström exponent and smaller ASP. Different temporal variations between AAC and SC cause the aerosols to be more absorbing (smaller SSA) in autumn, winter and around rush hours. ASP has a good quasi-log-normal growth trend with increasing SC when RH is below 60 %. The correlation between AAC and SC at the site is close but a little smaller than that in suburban Nanjing in spring. Atmospheric visibility decreases exponentially with increasing EC or SC, more sharply in spring and summer, and it could be further deteriorated with increasing SSA and ASP.

Aerosol Optical Properties at SORPES in Nanjing, East China

“…The overall SAE average ± standard deviation for the wavelength pair 450 / 635 nm was 1.30 ± 0.34 (Table 1) which is 25 very close to the respective value of 1.32 ± 0.41 in the center of Nanjing (Zhuang et al, 2017). The overall average backscatter fraction b = 0.12 ± 0.02 at  = 525 nm (Table 1) is also very similar to that at several sites around the world (e.g., Delene and Ogren, 2002;Andrews et al, 2011;Virkkula et al, 2011).…”

“…The Hong Kong measurements were conducted at the Hok Tsui monitoring station on the southeast tip of Hong Kong Island facing the South China Sea. Even though it is not far from the city, its aerosols 15 are dominated by sea-salt particles which explains the high SSA (Wang et al, 2017).…”

“…In Nanjing center, the annual averages were very close to those at SORPES, even though σap was slightly higher and σsp lower in the center of the city, respectively, which can be explained by the proximity to fresh traffic emissions. Another difference is that Zhuang et al (2017) used the Cref = 3.56 at  = 520 nm for calculating σap from aethalometer data whereas the value Cref = 4. 26 30 was used for all wavelengths in the present study.…”

“…Yangtze River Delta (YRD) region (Kim et al, 2004;Andreae et al, 2008;Yan et al, 2008;Cheng et al, 2008;Yang et al, 2009;Ma et al, 2011;Zhuang et al, 2014Zhuang et al, , 2016Wang et al, 2017). These three regions are highly industrialized with an extremely high population density; due to the complex anthropogenic emissions and consequent chemical reactions these three regions not only have some of the highest aerosol loadings worldwide but also some of the greatest uncertainties of radiative forcing of aerosols.…”

“…In the results, therefore, the mass concentration calculated from the number size distributions will be denoted as PM1. The 25 reasoning for the use of density of 1.7 g cm -3 was presented by Wang et al (2017).…”