Secondary organic aerosol (SOA) accounts for a significant proportion of atmospheric particles, which has attracted wide attention because of its impact on visibility, climate change and human health. Although air quality in China has been significantly improved in recent years, atmospheric complex pollution is still serious. In order to better evaluate the influence of SOA on atmospheric complex pollution, obtain accurate parametric data and provide scientific basis for regional pollution prevention and control, comprehensive characterization of SOA is needed. In view of this, the research of environmental effects on SOA formation in recent years has been summarized, and that the synergetic effect of multi-environmental factors has an important influence on SOA formation has been demonstrated. In addition, the future prospect has also been suggested based on the present research.

Secondary organic aerosol (SOA) plays an important role in atmospheric visibility, radiation balance and climate change. Systematic study on the influence of different environmental factors on the optical properties of SOA is a frontier of atmospheric science. Field observation is an important way to understand the optical properties of SOA, and in recent years, the development of new observation instruments and models makes the understanding of optical properties of SOA more in-depth. On the other hand, by controlling specific variables to simulate the generation process of SOA under different conditions, laboratory research deepens the understanding of optical characteristics of SOA essentially. The review mainly focuses on the research of optical properties of SOA generated by gas-phase oxidation and multiphase process in recent years, and analyzes the influence of aging process on the SOA optical properties, as well as the comparison and connectionbetween field and laboratory results.

Aromatic compounds are one important kind of precursors of secondary organic aerosol (SOA). The atmospheric oxidation of aromatic compounds can produce light-absorbing brown carbon (BrC). Therefore, understanding the optical properties (e.g., light scattering, light absorption, and single scattering albedo) of SOA formed from aromatic compounds has great implications on the quantification of their impact on air quality, visibility, and climate change. In this review, we summarise the optical properties of SOA formed from three typical kinds of aromatic compounds in laboratory studies: single-ring aromatic hydrocarbons, oxygenated aromatic compounds, and polycyclic aromatic hydrocarbons. The similarities and differences of light scattering and absorption parameters from literature are analyzed and compared, the impacts of various environmental factors on SOA optical properties are summarised, and the key research directions in the future are proposed.

Brown carbon (BrC) is an important class of light absorbing carbonaceous aerosols in the atmosphere, which has attracted much attention because of its significant impact on regional and even global radiative forcing and climate. However, the lack of comprehensive understanding of the optical properties of BrC in the atmosphere is one of the key factors leading to the uncertainties of its radiative forcing evaluation. Researches on light-absorbing properties of BrC at home and abroad in recent years have been reviewed. It is shown that BrC sources, chromophore composition, atmospheric process, pH conditions and coexisting metal ions are potential influencing factors of the light absorption properties ofBrC. The current understanding of the influencing factors and mechanism of BrC light absorption properties are systematically summarized, and suggestions and prospects for the future research on BrC light absorption properties and associated influencing factors are also put forward.

Dust aerosols in the atmosphere have a strong impact on the climate system by interacting with solar radiation and participating in cloud formation. In the troposphere, dust aerosols are usually heterogeneous mixtures of various components, and their impact on climate can be fully understood only based on a detailed understanding of the physical and chemical properties of individual dust particles. The current understanding of atmospheric dust aerosols based on optical methods in the past 20 years is reviewed. The research achievements of optical techniques such as electron microscope, lidar, and single-particle mass spectrometry are summarized, the observation and analysis techniques based on scanning electron microscope and energy spectrum, polarization characteristics, and asymmetric factors are briefly introduced, and their advantages and disadvantages are discussed. The morphology and mixing state of dust aerosol are summarized, and the main waysof their participation in atmospheric physical and chemical processes are presented as well. The optical properties, hygroscopic behavior, and nucleation ability of polluted dust, all of which have important implications for climate change and prediction, are also discussed. It is well known that various aerosol particles from different sources lead to complex mixtures of dust aerosols in the atmosphere, so how to combine accurate single-particle analysis with fast response online detection technology will be the focus of future atmospheric detection research. And it is believed that these findings will be further extended to the study of the environmental and climate impacts of aerosols, as well as human health risk assessment.

Hygroscopic aerosols can take up water in ambient air, and their particle size will change with the increase of relative humidity, leading to dramatic changes in optical properties (such as extinction,scattering and absorption coefficients, and single scattering albedo, etc.). The hygroscopic growth factors of aerosol optical properties (the ratio of optical parameters in wet state to dry state) are characteristic parameters representing the growth capacities of aerosols optical hygroscopicity, and also key parameters in assessing the effects of aerosols on atmospheric visibility and radiative forcing. Thus, accurate measurment of optical hygroscopic growth factors is of great significance for the evaluation of aerosol environment and climate effects. The optical hygroscopic growth measurement system consists of an optical measurement device and a humidity control system. The humidity adjustment system is used to change and control the relative humidity of samples, and the optical measurement device measures the corresponding changes of optical parameters in real time to achieve the on-line measurement of optical hygroscopic properties. In view of the significance of aerosol hygroscopicity research, the existing optical hygroscopic measurement methods and their applications are analyzed and compared comprehensively, and the research directions of measurement techniques and experimental works on aerosol hygroscopicity in the future are prospected.

Black carbon aerosol is an important product from incomplete combustion of fossil fuels and biomass, which can not only affect air quality and human health, but also alter the radiation balance by absorbing solar radiation, thus influencing regional and global climate. As the most populous and extensive region of the world′s seven continents, Asia accounts for more than half of global emissions of black carbon. This work systematically summarizes the main measurement techniques and instruments available for atmospheric black carbon as well as the major findings in Asia since the 1990s. The results show that the spatial distribution of black carbon concentrations in Asia is consistent with that of the population, and the boundary of the distribution of black carbon concentrations in China roughly overlaps with the Hu Huanyong line, which supports the fact that black carbon is mainly governed by anthropogenic activities. This work further showsthe sources and influencing factors of black carbon in Asia, as well as its optical properties. Finally, the shortcomings of current studies and future research needs regarding black carbon are discussed.

Off-line membrane filtration, solvent extraction and continuous spectral analysis were mainly used to monitor and analyze the atmospheric particulate matter in Xi’an city, China, for a continuous year from December 25, 2016 to December 26, 2017. Atmospheric PM2.5 samples were collected by quartz fiber filter firstly, and then water-soluble organic carbon (WSOC) and methanol-soluble organic carbon (MSOC) were ultrasonically extracted by ultra-pure water and methanol respectivelly. Finally, UV-vis absorption spectrum was used to obtain the light absorption characteristics of the samples. By analyzing the absorption contribution of water soluble brown carbon (BrC) and methanol soluble BrC in winter and summer in Xi′an at 365 nm, it was found that the absorption efficiency of methanol extraction was higher than that of water extraction in winter and summer, and the annual mean of methanol-soluble organic carbon mass absorption efficiency (MAE(MSOC)) ((1.60±0.67) m2·g－1) is 1.17 times of that of water-soluble organic carbon mass absorption efficiency (MAE(WSOC)) ((0.90±0.47) m2·g－1), indicating that there is a large amount of organics with stronger light absorption in MSOC. In winter, MAE(WSOC) and MAE(MSOC) are (2.05±0.86) m2·g－1 and (1.53±0.36) m2·g－1 respectivelly, while MAE(WSOC) and MAE(MSOC) in summer are (1.06±0.24)) m2·g－1 and (0.51±0.17) m2·g－1 respectivelly, which shows that MAE in winter is higher than that in summer. And the calculated E250/E265 value of WSOC in winter (5.25) is lower than that in summer (5.58), which may be caused by winter coal-fired heating emissions. The correlation analysis between WSOC and the six meteorological elements was carried out, and it is shown that WSOC is correlated with PM2.5 (R2 = 0.6417) and PM10 (R2 = 0.4035), while there is no obvious correlation between WSOC and O3 (R2 = 0.0682), indicating that the source of the secondary photochemical reaction accounts for a small proportion.

Aerosol samples were collected at urban and rural sites in Chongqing, China, from December 2015 to July 2017, and then the spatial and seasonal variations of light absorption properties and implications of brown carbon (BrC) in aerosols in this area were systematically investigated, and the radiative forcing of BrC was evaluated. The results show that the average light absorption coefficient b405,BrC, contribution to aerosol light absorption, and mass absorption efficiency of BrC at 405 nm in winter were (13.0±9.0) Mm－1, (24.5±6.1)% and (0.9±0.2) m2·g－1 respectively, about 11.3, 2.9 and 3.4 times of those in summer. The average b405,BrC at urban site (YB) in summer and winter were 2.8 and 1.8 times of those at rural site (JY) respectively. However, the absorption ngstrm exponent (E =1.2±0.1) and the contribution to aerosol light absorption of BrC at 405 nm (16.7±5.9)% at YB weresignificantly lower than those at JY ((1.6±0.2) and (32.3±6.3)%, respectively). The relationships between b405,BrC and the indicators of pollution sources indicated that the light absorption properties of BrC were mainly affected by the formation of secondary organic aerosols (SOA) in summer, while mainly affected by biomass burning, coal burning and the formation of SOA in winter. Compared with those in summer (p> 0.1), the significant correlation between b405,BrC and NH4+ concentrations (p< 0.001), as well as the significant correlations between E and the mass fractions of NO3－ and NH4+, were observed in winter. The shape of fit line between E values and φ in winter was similar to that of combustion chamber results of biomass burning, indicating that BrC light absorption was mainly affected by biomass burning and the aged BrC produced from the reactions of organic compounds with NO3－ and NH4+ during winter. The fractional contribution of radiation absorbed by BrC was (29.1±5.8)% in 405～980 nm and (60.8±13.7)% in 405～445 nm in winter, about 2.9 and 3.2 times those in summer, indicating that the radiation absorbed by BrC was the main contributor affecting the radiative balance during winter, especially at short wavelengths.

To explore the aerosol chemical extinction during complex air pollution, a field campaign was carried out to investigate the composition and extinction characteristics of PM2.5 in spring when O3 and PM2.5 combined pollution occurred in Chengdu, China, and the relationship between chemical composition and extinction characteristics was studied using the chemical extinction algorithm proposed by IMPROVE in the United States. The results showed that the average concentration of PM2.5 and scattering coefficient bsp in the spring of 2018 in Chengdu were (50.3±22.4) μg·m－3 and (237.5±140.2) Mm－1, respectively, and both of them showed the similar diurnal variation trend with “single peak and single valley”. The extinction coefficient of spring PM2.5 in Chengdu was (268.4±153.7) Mm－1, and the chemical components that contributed the most to it were NH4NO3 (26.0%) and organic matter (OM) (24.4%). In the case of PM2.5 and O3 combined pollution, the mass of secondary pollution components (SNA, the sum of SO42－, NO3－, NH4+) and secondary organic carbon (SOC) increased significantly, which increased by 1.0 and 1.3 times respectively compared with the clean day, and OM became the largest extinction contributor (32.2%), followed by NH4NO3 and (NH4)2SO4, contributing 22.8% and 20.5% respectively. Therefore, it is shown that further reducing the emission of precursor gases such as SO2, NOx, NH3 and VOCs may be efficient to improve the air quality and visibility in Chengdu district.

Based on the POLDER-3 multi-angle polarimetric observation data from 2005 to 2013, the global aerosol comprehensive products are obtained through the newly developed aerosol component method which enables simultaneous inversion of aerosol optical properties and component information, and then the AERONET (Aerosol Robotic Network) global site observation data are used to validate and comprehensively evaluate the retrieval of aerosol optical property products, and the applicability and superiority of the component retrieval approach are also discussed. The overall validation results show that the quality of spectral AOD derived by the aerosol component approach from POLDER measurements is comparable to AERONET measurements. In addition, the polarimetric observations based on aerosol component approach can provide more information on aerosol properties, such as spectral absorption AOD (AAOD) and ngstrm exponent (AE) for different band combinations (440/670 nm, 670/870 nm, 870/1020 nm, 440/1020 nm), and all these aerosol optical property products have fairly small biases, demonstrating that the algorithm is able to achieve a better fit to the observation data, which provides a basis for further improvement of the algorithm.

Sea salt aerosol (SSA) is an important link between ocean and atmosphere. Surface-active organics constitute a major fraction of the organic film of SSA. It is believed that the changes in chemical composition and surface organization of the organic film have important influences on physical, chemical and optical properties of SSA, and then affect many atmospheric processes. A Langmuir trough was used in combination with infrared reflection absorptionspectroscopy (IRRAS) to detect the molecular arrangement of the surface film from macroscopic viewpoints as well as microscopic consideration. Focusing on the mixed monolayers of stearic acid/oleic acid (OA) and stearic acid/elaidic acid (EA) at the air-water interface, the intermolecular interactions between long-chain fatty acids at the SSA interface were investigated, and the effects of saturation degree and double bond configuration of organics on the surface properties of SSA were clarified. It is found that compared with EA with trans-double bond, the obstructive effect on the molecular ordering was found to be significantly remarkable for OA where the cis-double bond exists in the middle of molecular chain. Although the trans-double bond in the alkyl chain of EA also produces the obstruction, the degree is not as much as that caused by the cis-one.

Xylene is one of the most important anthropogenic volatile organic compounds (VOCs) for the secondary organic aerosol (SOA) formation. As the effect of NH3 on the yield and reaction mechanism of SOA formed by photooxidation of xylene is not clear, SOA formation from atmospheric photooxidation of xylene with different NH3 concentrations was studied in a laboratory smog chamber, focusing on the mass concentration, optical properties and chemical composition of SOA. The results show that NH3 can obviously promote the formation of SOA at low concentration. And according to aerosol mass spectra results, the promotion of carbonyl compounds into the particle-phase and the formation of nitrogen-containing organic in the presence of NH3 are the main reasons for the increase of SOA mass concentration. In addition, it is found that NH3 can increase the mass absorption coefficient (MAC) of o-xylene SOA, but has no significant effect on p-xylene SOA. The analysis shows that the formation of aldehydes through photooxidation of o-xylene is more efficient than that of p-xylene, and then Maillard reaction between NH3 and aldehydes leads to the increase of light absorption of SOA.