Column-integrated Aerosol Optical Properties Research Articles

Variations in aerosol optical characteristics from SKYNET measurements in Beijing

Atmospheric aerosols have significant impacts on climate and environment, so the study on aerosol optical properties is of great scientific significance for climate change and atmospheric environment. The variations in column-integrated aerosol optical properties over Beijing were investigated from March 2016 to December 2019 based on the SKYNET data products. Higher values of AOD and AE found in summer indicate that it is important to pay more attention to fine particle pollution in summer. By fitting the AOD data of SKYNET with AERONET at same waveband, we found that monthly variations in aerosol optical depth estimated from SKYNET observation were mostly consistent with the AERONET data. Aerosols were divided into several modes according to optical characteristics, which deepen some insight into aerosol optical and radiative properties and revealed that the weak absorption aerosol dominated and contributed much to the radiation transport in Beijing. Data derived from the polarization lidar CALIPSO were combined with aerosol optical depth (AOD) from SKYNET and these results were consistent, which further identified the scientific of data. The random forest algorithm was used to interpolate the missing AOD500nm observations. The original results and the interpolation results were fitted with MODIS respectively, which proved that the random forest algorithm can supplement the missing results well. This work will be of great significance in addressing the impacts of aerosols on climate and environment, and will help reduce the uncertainty of the impacts of aerosols on the Earth's radiation budget. All work will provide scientific support for the prevention and control of specific pollution.

Column-integrated Aerosol Optical Properties Determined Using Ground-based Sun Photometry Measurements in the Hangzhou Region

To investigate the optical properties of aerosols in the Hangzhou region (Hangzhou, Tonglu, Jiande, and Chun'an), the aerosol optical depth (AOD), Ångström exponent (AE), single scattering albedo (SSA), and aerosol size distribution (ASD) were measured using CIMEL sun-photometers in 2012. The results showed that the annual average values of AOD440nm in Hangzhou, Tonglu, Jiande, and Chun'an were 0.94±0.16, 0.84±0.17, 0.82±0.22, and 0.71±0.20, respectively. The values generally decreased from the northeast to the southwest, and represented one of highest AOD districts in the Yangtze River Delta, China. The annual average values of AE440-870nm were 1.24±0.12, 1.19±0.17, 1.06±0.04, and 1.04±0.10, respectively, indicating that particles with small average effective radii were predominant. The relatively lower AE values in March and April were generally attributed to the long-range transport of dust aerosols from Northwest China. Obvious diurnal variations in the AOD were found in Hangzhou, Tonglu, and Jiande, but not in Chun'an. An average fine-mode effective radius of~0.15 μm was observed in spring, autumn, and winter, while a value of~0.25 μm was observed in summer, in conjunction with aerosol hygroscopic growth. An average coarse-mode effective radius of~2.94 μm was observed in summer, autumn, and winter, which was higher than the value in spring. The annual average values of SSA440nm were 0.91±0.01, 0.92±0.03, 0.92±0.02, 0.93±0.02, respectively, indicating that the particles had relatively strong to moderate absorption. Characterization of the aerosol types showed the predominance of biomass burning and urban industrial type aerosols in Hangzhou, while mixed type aerosols were observed in Tonglu, Jiande, and Chun'an.

Column-integrated aerosol optical properties of coarse- and fine-mode particles over the Pearl River Delta region in China

The sun-photometer data from 2011 to 2013 at Panyu site (Panyu) and from 2007 to 2013 at Dongguan site (Dg) in the Pearl River Delta region, were used for the retrieving of the aerosol optical depth (AOD), single scattering albedo (SSA), Ångström exponent (AE) and volume size distribution of coarse- and fine-mode particles. The coarse-mode particles presented low AOD (ranging from 0.05±0.03 to 0.08±0.05) but a strong absorption property (SSA ranged from 0.70±0.03 to 0.90±0.02) for the wavelengths between 440 and 1020nm. However, these coarse particles accounted for <10% of the total particles. The AOD of fine particles (AODf) was over 3 times as large as that of coarse particles (AODc). The fine particles SSA (SSAf) generally decreased as a function of wavelength, and the relatively lower SSAf value in summer was likely to be due to the stronger solar radiation and higher temperature. More than 70% of the aerosols at Panyu site were dominated by fine-mode absorbing particles, whereas about 70% of the particles at Dg site were attributed to fine-mode scattering particles. The differences of the aerosol optical properties between the two sites are likely associated with local emissions of the light-absorbing carbonaceous aerosols and the scattering aerosols (e.g., sulfate and nitrate particles) caused by the gas-phase oxidation of gaseous precursors (e.g., SO2 and NO2). The size distribution exhibited bimodal structures in which the accumulation mode was predominant. The fine-mode volume showed positive dependence on AOD (500nm), and the growth of peak value of the fine-mode volume was higher than that of the coarse volume. Both the AOD and SSA increased with increasing relative humidity (RH), while the AE decreased with increasing RH. These correlations imply that the aerosol properties are greatly modified by condensation growth.

Contrast in column-integrated aerosol optical properties during heating and non-heating seasons at Urumqi — Its causes and implications

Aerosol optical properties were retrieved from two years' worth of Sunphotometer measurements at Urumqi, an urban station in western China. Distinct seasonal variations of aerosol optical properties were revealed. During the heating season, mean aerosol optical depth at 550nm (τa), Angstrom exponent calculated from aerosol optical depth at wavelength of 440 and 870nm (α) as well as PM2.5 concentration were 0.58±0.33, 1.11±0.34 and 79.5±69.6μgm−3, respectively, which contrasted their counterparts during the non-heating season of 0.32±0.22, 0.79±0.26, and 35.0±20.1μgm−3. Seasonal variations of τa and PM2.5 at Urumqi contrasted with corresponding values in eastern China. Enhancement of τa was associated with fine mode radius (rf) exceeding 0.15μm. Relative humidity frequently exceeded 80% during the heating season, which probably resulted in rf>0.15 as a result of the hygroscopic growth under the humid environment. rf was larger than value assigned to the fine-mode dominated aerosol models used in the dark-target algorithm of the Moderate Resolution Spectroradiometer (MODIS). Annual mean single scattering albedo at 550nm (ω) was 0.87 that was close to the value assigned to the absorption aerosol model in the MODIS algorithm. ω increased as τa increased, probably as a result of the growth of aerosol size. ω of dust aerosols at Urumqi was slightly lower than that in dust source and downwind regions. Substantial differences in aerosol optical and physical properties and their seasonal variation between western and eastern China require maintaining long-term ground based remote sensing aerosols, which would be expected to play an important role in studying aerosol's effects on weather, climate and atmospheric environment.

Characterization of aerosol events based on the column integrated optical aerosol properties and polarimetric measurements

Aerosol optical properties are very useful tools for analyzing their radiative effects, which are directly or indirectly related to the global radiation budget. Investigation of column-integrated aerosol optical properties is a worldwide and well-accepted method. The introduction of new methodologies, like those of operation with polarimetric measurements, represent a new challenge to interpret the measurement data and give more detailed information about the aerosol events and their characteristics. Aerosol optical properties during the period June – August 2015 in AERONET Strzyzow station in Poland were analyzed. The aerosol properties like aerosol optical depth, Ångström exponent, fine mode fraction, fine mode contribution on AOD, asymmetry parameter, single scattering angle are analyzed synergistically with the polarimetric measurements of the degree of polarization in different solar zenith and zenith viewing angles at several wavelengths. The overall results show that aerosol events in Strzyzow were characterized mostly by fine mode aerosols. Backward-trajectories suggest that the majority of air masses come from the west. The principal component of the aerosol load was urban/industrial contamination, especially from the inner part of the continent. Additionally, the maximal values of the degree of linear polarization were found to be dependent on the solar zenith and zenith viewing angles and aerosol optical properties like aerosol optical depth and Ångström exponent. These dependencies were further analyzed in a specific case with very high mean values of AOD500 (0.59) and AE440–870 (1.91). The diurnal variations of aerosol optical properties investigated during this special case, suggest that biomass burning products are the main cause of that aerosol load over the stations.

Column-integrated aerosol optical properties and direct radiative forcing over the urban-industrial megacity Nanjing in the Yangtze River Delta, China.

Aerosol optical properties were measured and analyzed through the ground-based remote sensing Aerosol Robotic Network (AERONET) over an urban-industrial site, Nanjing (32.21° N, 118.72° E, and 62m above sea level), in the Yangtze River Delta, China, during September 2007-August 2008. The annual averaged values of aerosol optical depth (AOD500) and the Ångström exponent (AE440-870) were measured to be 0.94 ± 0.52 and 1.10 ± 0.21, respectively. The seasonal averaged values of AOD500 (AE440-870) were noticed to be high in summer (autumn) and low in autumn (spring). The characterization of aerosol types showed the dominance of mixed type followed by the biomass burning and urban-industrial type of aerosol at Nanjing. Subsequently, the curvature (a 2) obtained from the second-order polynomial fit and the second derivative of AE (α') were also analyzed to understand the dominant aerosol type. The single scattering albedo at 440nm (SSA440) varied from 0.88 to 0.93 with relatively lower (higher) values during the summer (spring), suggesting an increase in black carbon and mineral dust (desert dust) aerosols of absorbing (scattering) nature. The averaged monthly and seasonal evolutions of shortwave (0.3-4.0μm) direct aerosol radiative forcing (DARF) values were computed from the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model both at the top of atmosphere (TOA) and bottom of atmosphere (SUR) during the study period. Further, the aerosol forcing efficiency (AFE) and the corresponding atmospheric heating rates (AHR) were also estimated from the forcing within the atmosphere (ATM). The derived DARF values, therefore, produced a warming effect within the atmosphere due to strong absorption of solar radiation.

Column-integrated aerosol optical properties and direct radiative forcing based on sun photometer measurements at a semi-arid rural site in Northeast China

Ground and satellite remote sensing measurements have revealed heavy aerosol loading in China; however, aerosol optical properties and direct radiative forcing in Northeast China – important in climate modeling and remote sensing – have not been widely studied. We studied four years of continuous sun photometer measurements at Tongyu, a typical semi-arid rural site in Northeast China, to better understand column-integrated aerosol optical properties and direct radiative forcing. The annual average aerosol optical depth (AOD) at 500nm was 0.20±0.26; the Ångström exponent (AE) between 440 and 870nm was 1.37±0.64; and the single scattering albedo (SSA) at 440nm was 0.91±0.05. The AOD at this rural site was a quarter of that observed in the polluted North China Plain and Yangtze River Delta regions. Anthropogenic fine-mode particles were the dominant contributor to AOD. The AOD and AE showed generally opposite seasonal variation patterns. Relatively higher AOD values in summer (0.26±0.27) and spring (0.24±0.30) were likely related to long-range transportation of anthropogenic aerosols from southern industrial regions in summer, and the increased contribution of dust events in spring. The minimum AOD (0.16±0.22) was concurrent with the maximum AE (1.75±0.76), observed in winter. On average, the absorption AOD (AAOD) at 440nm and its absorption Ångström exponent (AAE) between 440 and 870nm were 0.06±0.03 and 1.04±0.43, respectively. The mean AAE was considerably higher than 1 in autumn and winter, indicating that brown carbon from biomass burning contributed greatly to aerosol absorption. The AAE was lower than 1 in summer and spring, related to the coating of black carbon particles. Large negative aerosol direct radiative forcing was estimated at the bottom of the atmosphere, with relatively lower values estimated at the top of the atmosphere; the means were −26.28 and −9.42Wm−2, respectively. This resulted in a strong cooling effect on the surface, but warming in the atmosphere, potentially impacting the regional climate.

Column-Integrated Aerosol Optical Properties during Summer and Autumn of 2012 in Xi’an, China

Column-integrated aerosol optical properties were derived systematically from measurements made in Xi’an, which is located in Guanzhong Plain of central China with a ground-based CIMEL sun photometer from May to November 2012. Aerosol optical depths (AODs), Ångström exponents, water vapor contents, and aerosol optical and micro-physical properties, including aerosol volume size distribution, complex refractive indices and single scattering albedo (SSA), were determined. Daily variations in AODs at 440 nm (τ440) generally followed those of the 24-hr PM2.5 mass concentrations, but there were differences in the relationships in summer and autumn. August showed the highest monthly τ440 (1.13) while the largest monthly Ångström exponent (α440–870 = 1.30) and water vapor content (Cw = 4.28) both occurred in July. Monthly averages of the aerosol size distributions showed the dominance of coarse mode aerosols, except in July and August, when the contribution of the accumulation and coarse modes were fairly comparable. Monthly changes in the complex refractive index (including both real and imaginary parts) and SSA were also studied, including their wavelength dependences; these analyses implied changes in the abundances of the aerosol types. Finally, an episode involving urban and dust aerosols was analyzed using sun photometer aerosol retrievals; MODIS images captured by Aqua satellite and average wind vectors from the NCEP operational global analyses were also considered in the case study.

Aerosol Optical Properties at Four Sites in Thailand

This paper presents column integrated aerosol optical properties including aerosol optical depth (AOD), Angstrom wavelength exponent (α), single scattering albedo (SSA), and size distribution from ground-based measurements at four sites in Thailand: Chiang Mai (18.。N, 98.。 E), Ubon Ratchathani (15.25。N, 104.87。 E), Nakhon Pathom (13.82。N, 100.04。E), and Songkhla (7.2。N, 100.60。E). Results show a marked seasonal trend in AOD at 500 nm for the first three stations, with the monthly average maxima of 0.92, 0.78 and 0.61 for Chiang Mai, Ubon Ratchathani and Nakhon Pathom, respectively. These maxima occur in the dry season (November-April). Minimum values for these stations were recorded during the wet season (May-October). A similar pattern is exhibited in the α for the three stations, with maxima in the dry season and minima in the wet season. The lowest SSA values occur at Chiang Mai, which means this station has the highest absorption, with the highest SSA values occurring at Songkhla which corresponds to the lowest absorption. The southern station at Songkhla differs from the other three as it has less local pollution sources and is subjected to the influence of the tropical maritime environment. AOD at Songkhla maintains a low and more constant value year round with the maximum monthly average AOD of 0.27 and the minimum of 0.16. Diurnal changes in AOD at the four stations are discussed and related to various external variables.

A multi-year analysis of clear-sky aerosol optical properties and direct radiative forcing at Gosan, Korea (2001–2008)

A seven-year analysis of clear-sky column-integrated aerosol optical properties and direct radiative forcing at Gosan in Korea is presented in this paper; the Gosan site is heavily impacted by aerosols emitted from the Asian continent. MODIS and AERONET Sun/sky radiometer measurements showed significant seasonal and geographical variability in aerosol optical depth (AOD), with high AODs over the east coastal industrialized regions of China in spring and summer. At the Gosan site, the monthly mean AOD at 675 nm ranged between 0.12 and 0.36. High AODs were observed from April to June (> 0.33), while low-to-moderate monthly mean AODs were apparent during the remaining months. The single scattering albedo (SSA) exhibited relatively low values from February to May (< 0.93), indicating the presence of light-absorbing aerosols, whereas high values of SSA (> 0.93) were observed during the remaining months. The annual average clear-sky direct forcing (forcing efficiency) at the surface was − 27.55 ± 9.21 W m − 2 (− 91.85 ± 11.12 W m − 2 τ − 1 ) and at the top of the atmosphere (TOA) was − 15.79 ± 4.44 W m − 2 (− 53.76 ± 6.70 W m − 2 τ − 1 ), thereby leading to an atmospheric absorption of 11.76 ± 5.82 W m − 2 . From March to June, the surface aerosol radiative forcing (forcing efficiency) ranged between − 27.29 (– 85.33) and − 34.76 W m − 2 (− 97.19 W m − 2 τ − 1 ), whereas at TOA it ranged between − 16.84 (− 51.82) and − 19.10 W m − 2 (− 56.05 W m − 2 τ − 1 ), and the atmospheric forcing ranged between 10.45 and 16.41 W m − 2 . The atmospheric forcing (i.e., atmospheric absorption) caused an increase in atmospheric heating (about 1.5 to 3.0 K day − 1 ). The strongest radiative heating was observed from April to May (> 2.5 K day − 1 ).

Column-integrated aerosol optical properties in the free troposphere: case study-Sierra Nevada, Spain

From spectral solar irradiance extinction measurements in the visible range carried out at Sierra Nevada (2950 m a.s.l.) during July 1998, the characteristics of the aerosol column in the free troposphere have been measured. The results show that in stable atmospheric conditions, the column-integrated aerosol takes an optical depth (AOD) at 500 nm of 0.02 /spl plusmn/ 0.02, and an average radius in the accumulation mode of 0.15 /spl mu/m, with a likely Atlantic origin. In unstable situations, the AOD at 500 nm can increase by more than an order of magnitude, and the aerosols' origin is uncertain, since air masses can arrive from different sources at different altitudes.