Light-absorbing aerosol properties retrieved from the sunphotometer observation over the Yangtze River Delta, China

Coherent high-spectral-resolution lidar for the measurement of the atmospheric Mie-Rayleigh-Brillouin backscatter spectrum

In this study, a 1550 nm coherent high-spectral-resolution lidar (CHSRL) is developed to measure the optical properties of aerosols and atmospheric wind profiles in the atmospheric boundary layer. To determine the optical properties, a coherent frequency discriminator based on the fast Fourier transform is designed in the CHSRL to separate the Mie and the Rayleigh-Brillouin backscatter spectra to fulfill the needs of high-spectral measurements. The atmospheric wind velocity is retrieved using the simultaneously measured Doppler shift. This non-optical frequency discriminator is a feasible and low-cost solution compared to a narrow-bandwidth optical filter, such as a Fabry-Perot interferometer or an atomic filter. However, shot, amplifier spontaneous emission, and detector noise interfere with the Rayleigh-Brillouin spectrum. Therefore, a spectrum correction algorithm is proposed to recover the interfered Rayleigh-Brillouin spectrum, and the measurement results of the spectral line agree well with those modeled with Tenti S6 at different central frequencies. Finally, field observations for comparison are conducted with the co-located CHSRL, Raman lidar, and coherent Doppler wind lidar. The comparison results indicate that the correlation coefficient of the aerosol backscatter coefficient is 0.84. The correlation coefficient and standard deviation of wind velocity are 0.98 and 0.13 m · s^-1, respectively.

Aerosol optical properties and the mixing state of black carbon at a background mountainous site in Eastern China

In-situ measurements of aerosol optical properties were conducted at Mt. Huang from September 23 to October 28, 2012. Low averages of 82.2, 10.9, and 14.1 Mm^-1 for scattering coefficient (σ_{sp, neph, 550}), hemispheric backscattering coefficient (σ_{hbsp, neph, 550}), and absorption coefficient (σ_ap, 550), respectively, were obtained. Atmospheric aging process resulted in the increase of σ_ap, 550 but the decrease of the single scattering albedo (ω₅₅₀) at constant aerosol concentration. However, the proportion of non-light-absorbing components (non-BCs) was getting higher during the aging process, resulting in the increase of aerosol diameter, which also contributed to relatively higher σ_{sp, neph, 550} and ω₅₅₀. Diurnal cycles of σ_{sp, neph, 550} and σ_ap, 550 with high values in the morning and low values in the afternoon were observed closely related to the development of the planetary boundary layer and the mountain-valley breeze. BC mixing state, represented by the volume fraction of externally mixed BC to total BC (r), was retrieved by using the modified Mie model. The results showed r reduced from about 70% to 50% when the externally mixed non-BCs were considered. The periodical change and different diurnal patterns of r were due to the atmospheric aging and different air sources under different synoptic systems. Local biomass burning emissions were also one of the influencing factors on r. Aerosol radiative forcing for different mixing state were evaluated by a "two-layer-single-wavelength" model, showing the cooling effect of aerosols weakened with BC mixing state changing from external to core-shell mixture.

Effects of aerosols on the surface ozone generation via a study of the interaction of ozone and its precursors during the summer in Shanghai, China

Previous studies have shown that heavy aerosol pollution can inhibit the surface ozone generation. More recent studies, however, have revealed that aerosol loading is positively correlated with ozone concentrations in large cities, such as Shanghai, particularly during the summer. Whether the correlation between aerosol pollution and ozone concentrations is positive or negative, it is an issue that needs to be considered by atmospheric scientists. Although the presence of ozone precursors, such as nitrogen oxide (NOx) and volatile organic compounds (VOCs), affect ozone concentrations, the roles of aerosols in the formation of ozone have rarely been investigated. Therefore, an analysis of the effect of aerosols on photochemical ozone generation via a study of the interaction of ozone and its precursors is important. In our research, we found that both aerosol and ozone concentrations were higher in Shanghai under polluted conditions than they were under clean conditions during the summer, but the ozone formation was controlled by VOCs, not by aerosol loading. The decrease in the AOD (SSA) increased (decreased) the surface UV radiation and promoted (inhibited) photochemical ozone production. We also found that the lower the concentration of photochemically active VOCs, the weaker the effect of the AOD on the ozone concentrations. The other results were shown as follows: (1) Aerosol pollution decreased the amount of UV radiation reaching the Earth's surface, but the surface UV radiation increased with increasing aerosol particle scattering; (2) Aerosol pollution inhibited the photolysis of nitrogen dioxide (NO₂), while the scattering property of aerosols facilitated this phenomenon; (3) When both the concentration of ozone precursors and the SSA were constant, the ozone concentration decreased, but the attenuation rate increased significantly with an increase in AOD.

Contrasting Aerosol Optical Characteristics and Source Regions During Summer and Winter Pollution Episodes in Nanjing, China

Two episodes with heavy air pollution in Nanjing, China, one in the summer and another one in the winter of 2017, were selected to study aerosol properties using sun photometer and ground-based measurements, together with source region analysis. The aerosol properties, the meteorological conditions, and the source regions during these two episodes were very different. The episodes were selected based on the air quality index (AQI), which reached a maximum value of 193 during the summer episode (26 May–3 June) and 304 during the winter episode (21–31 December). The particulate matter (PM) concentrations during the winter episode reached maximum values for PM2.5/10 of 254 g m−3 and 345 g m−3, much higher than those during the summer (73 and 185 g m−3). In contrast, the value of aerosol optical depth (AOD) at 500 nm was higher during the summer episode (2.52 0.19) than during that in the winter (1.38 0.18). A high AOD value does not necessarily correspond to a high PM concentration but is also affected by factors, such as wind, Planetary Boundary Layer Height (PBLH), and relative humidity. The mean value of the Ångström Exponent (AE) varied from 0.91–1.42, suggesting that the aerosol is a mixture of invaded dust and black carbon. The absorption was stronger during the summer than during the winter, with a minimum value of the single scattering albedo (SSA) at 440 nm of 0.86 on 28 May. Low values of asymmetry factor (ASY) (0.65 at 440 nm and 0.58 at 1020 nm) suggest a large number of anthropogenic aerosols, which are absorbing fine-mode particles. The Imaginary part of the Refractive Index (IRI) was higher during the summer than during the winter, indicating there was absorbing aerosol during the summer. These differences in aerosol properties during the summer and winter episodes are discussed in terms of meteorological conditions and transport. The extreme values of PM and AOD were reached during both episodes in conditions with stable atmospheric stratification and low surface wind speed, which are conducive for the accumulation of pollutants. Potential source contribution function (PSCF) and concentration weighted trajectory (CWT) analysis show that fine mode absorbing aerosols dominate during the summer season, mainly due to emissions of local and near-by sources. In the winter, part of the air masses was arriving from arid/semi-arid regions (Shaanxi, Ningxia, Gansu, and Inner Mongolia provinces) covering long distances and transporting coarse particles to the study area, which increased the scattering characteristics of aerosols.