An Investigation of Aerosol Scattering and Absorption Properties in Wuhan, Central China

New insights into black carbon light absorption enhancement: A comprehensive analysis of two differential behaviors

High uncertainty in optical properties of black carbon (BC) involving heterogeneous chemistry has recently attracted increasing attention in the field of atmospheric climatology. To fill the gap in BC optical knowledge so as to estimate more accurate climate effects and serve the response to global warming, it is beneficial to conduct site-level studies on BC light absorption enhancement (Eabs) characteristics. Real-time surface gas and particulate pollutant observations during the summer and winter over Wuhan were utilized for the analysis of Eabs simulated by minimum R squared (MRS), considering two distinct atmospheric conditions (2015 and 2017). In general, differences in aerosol emissions led to Eabs differential behaviors. The summer average of Eabs (1.92 ± 0.55) in 2015 was higher than the winter average (1.27 ± 0.42), while the average (1.11 ± 0.20) in 2017 summer was lower than that (1.67 ± 0.69) in winter. Eabs and RBC (representing the mass ratio of non-refractory constituents to elemental carbon) constraints suggest that Eabs increased with the increase in RBC under the ambient condition enriched by secondary inorganic aerosol (SIA), with a maximum growth rate of 70.6% in 2015 summer. However, Eabs demonstrated a negative trend against RBC in 2017 winter due to the more complicated mixing state. The result arose from the opposite impact of hygroscopic SIA and absorbing OC/irregular distributed coatings on amplifying the light absorbency of BC. Furthermore, sensitivity analysis revealed a robust positive correlation (R > 0.9) between aerosol chemical compositions (including sulfate, nitrate, ammonium and secondary organic carbon), which could be significantly perturbed by only a small fraction of absorbing materials or restructuring BC through gaps filling. The above findings not only deepen the understanding of BC, but also provide useful information for the scientific decision-making in government to mitigate particulate pollution and obtain more precise BC radiative forcing.

Columnar Aerosol Optical Property Characterization and Aerosol Typing Based on Ground-Based Observations in a Rural Site in the Central Yangtze River Delta Region

Accurate and updated aerosol optical properties (AOPs) are of vital importance to climatology and environment-related studies for assessing the radiative impact of natural and anthropogenic aerosols. We comprehensively studied the columnar AOP observations between January 2019 and July 2020 from a ground-based remote sensing instrument located at a rural site operated by Central China Comprehensive Experimental Sites in the center of the Yangtze River Delta (YRD) region. In order to further study the aerosol type, two threshold-based aerosol classification methods were used to investigate the potential categories of aerosol particles under different aerosol loadings. Based on AOP observation and classification results, the potential relationships between the above-mentioned results and meteorological factors (i.e., humidity) and long-range transportation processes were analyzed. According to the results, obvious variation in aerosol optical depth (AOD) during the daytime, as well as throughout the year, was revealed. Investigation into AOD, single-scattering albedo (SSA), and absorption aerosol optical depth (AAOD) revealed the dominance of fine-mode aerosols with low absorptivity. According to the results of the two aerosol classification methods, the dominant aerosol types were continental (accounting for 43.9%, method A) and non-absorbing aerosols (62.5%, method B). Longer term columnar AOP observations using remote sensing alongside other techniques in the rural areas in East China are still needed for accurate parameterization in the future.

Three-years monitoring of PM2.5 and scattering coefficients in Shanghai, China

The absorption and scattering of aerosols are critical factors that influence in global climate and visibility degradation. From January 2013 to December 2015, aerosol scattering coefficients, PM_2.5, and meteorological parameters were continuously measured at a monitoring site in Shanghai, China. The annual means of scattering coefficients were 312.3, 232.1, and 261.9 Mm^-1 for the years 2013, 2014, and 2015, respectively. The corresponding values for PM_2.5 were 61.6, 51.6, and 52.9 μg/m³. Compared with the average scattering coefficient of the year 2013, those of 2014 and 2015decreased by 26% and 16%, respectively. Furthermore, the annual average PM_2.5 decreased by 16% and 14% in 2014 and 2015, respectively. Although this study concluded that PM_2.5 was generally correlated with scattering coefficients during the entire measurement period, the decrease in the former was much less than the latter. On this basis, ultrafine particles may decrease significantly because they cause aerosol scattering. This finding should be investigated further in the future. The inter-annual meteorological changes affected PM_2.5 and scattering coefficient inter-annual variations. In the northwest and southwest direction, the seasonal and diurnal variations of aerosol scattering coefficients showed larger values when the wind speeds were about 3-5 m/s. The serious pollution in the northwest direction were mainly due to long-distance transport of pollutants during winter, whereas those in the southwest direction were attributed to local emission. The westerly wind frequency is the crucial factor influencing local pollution transport significantly. Backward trajectory analysis indicated that the air pollution in Shanghai in 2013-2015 is attributed to long-distance transport and primarily affected by the air mass from northwest direction. Observations on long-term aerosol optical properties on the basis of in-situ measurements can help thoroughly understand the radiative forcing characteristics of aerosol.

Aerosol Optical Properties and Contribution to Differentiate Haze and Haze-Free Weather in Wuhan City

Haze is an atmospheric phenomenon in which different types of particulates obscure the sky, and hence affect almost all human activities. Over a couple of recent decades, China has witnessed increasingly worse air quality as well as atmospheric haziness in its cities. There are various haze contributing factors including the rapid industrialization, excessive biomass burning, and an increase in the number of vehicles. This study proposes a methodology based on the aerosols scattering and absorption properties, to predict the likelihood of an episode of hazy days. This case study employs the aerosol optical properties data from integrated nephelometer and aethalometer sensors from December 2009 to September 2014 over Wuhan. The role and contribution of each aerosol optical parameter (e.g., aerosol scattering and absorption coefficients, single scattering albedo, scattering, and absorption Ångström exponents, backscatter ratio, and asymmetry factor) in distinguishing haze and haze-free conditions has been quantitatively determined based on a machine learning approach. Each aerosol optical parameter was classified independently by the support vector machine (SVM) algorithm, and the aerosol scattering (85.37%) and absorption (74.53%) coefficients were found to be primary potential indicators. Through the Kolmogorov-Smirnov test and traditional statistical analysis, the aerosol scattering and absorption coefficients were then verified as important indicators in distinguishing haze and haze-free days. Finally, through a probability density diagram and frequency histogram, we propose a simple quantitative standard to distinguish between haze and haze-free conditions based on the aerosol scattering coefficient and absorption coefficient in Wuhan City. The accuracy of the standard was determined to be 81.49% after testing, which indicates an accurate result. An error in aerosol optical properties may lead to an error in the calculation of aerosol radiative forcing, the earth’s energy budget, and climate prediction. Therefore, understanding of the aerosol properties during haze-free and haze-days will help policymakers to make new policies to control urban pollution and their effects on human health.

Optical and Physical Characteristics of the Lowest Aerosol Layers over the Yellow River Basin

Studying the presence of aerosols in different atmospheric layers helps researchers understand their impacts on climate change, air quality, and human health. Therefore, in the present study, the optical and physical properties of aerosol layers over the Yellow River Basin (YERB) were investigated using the CALIPSO Level 2 aerosol layer products from January 2007 to December 2014. The Yellow River Basin was divided into three sub-regions i.e., YERB1 (the plain region downstream of the YERB), YERB2 (the Loess Plateau region in the middle reaches of the YERB), and YERB3 (the mountainous terrain in the upper reaches of the YERB). The results showed that the amount (number) of aerosol layers (N) was relatively large (>2 layers) in the lower part of the YERB (YERB1), which was mainly caused by atmospheric convection. The height of the highest aerosol layer top (HTH) and the height of the lowest aerosol layers base (HB1) varied significantly with respect to the topography of the YERB. High and low values of aerosol optical depth (AOD) were observed over the YERB1 (plain area) and YERB3 (elevated area) regions, respectively. Population, economy, and agricultural activities might be the possible reasons for spatial variations in AOD. AOD values for the lowest aerosol layer were high—between 0.7 and 1.0 throughout the year—indicating that aerosols were mainly concentrated at the bottom layer of the atmosphere. In addition, the integrated volume depolarization ratio (0.15–0.2) and the integrated attenuated total color ratio (~0.1) were large during spring for the lowest aerosol layer due to the presence of dust aerosols. The thicknesses of the lowest aerosol layers (TL1) did not vary with respect to the topographic features of the YERB. Over the sub-regions of the YERB, a significant positive correlation between the AOD of the lowest aerosol layer (AOD1) and the thickness of the lowest aerosol layer (TL1) was found, which indicates that TL1 increases with the increase of AOD1. In the whole YERB, a positive linear correlation between the N and HTH was observed, whereas a negative correlation between N and the portion of AOD for the lowest aerosol layer (PAOD1) was found, which revealed that the large value of N leads to the small value of PAOD1. The results from the present study will be helpful to further investigate the aerosol behavior and their impacts on climate change, air quality, and human health over the YERB.

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.

Evaluation of the Aqua-MODIS C6 and C6.1 Aerosol Optical Depth Products in the Yellow River Basin, China

In this study, Aqua-Moderate Resolution Imaging Spectroradiometer (MODIS) Collection (C6) and C6.1 Dark Target aerosol optical depth (AOD) retrievals at 3 km (DT3K) and 10 km (DT10K), Deep Blue AOD retrievals at 10 km (DB10K), and combined DT and DB (DTB) AOD retrievals at 10 km resolutions were validated from 2002 to 2014 against ground-based sunphotometer AOD measurements obtained from the Chinese aerosol remote sensing network (CARSNET). The CARSNET AOD data were obtained for sites at Mt. Waliguan (MW), Lanzhou (LZ), Ulate (UL), and Zhengzhou (ZZ) located in the Yellow River basin (YERB) region, China. Errors and agreement between satellite and ground data were reported using Pearson’s correlation (R) and relative mean bias (RMB). Results showed that the DT3K C6.1 highest quality flag (QF = 3) AOD retrievals were well correlated with the sunphotometer AOD data, with an R of 0.82 and an RMB of 1.01. Overestimation and underestimation in DT AOD retrievals were observed for AOD > 1.1 and AOD < 1.1, respectively. A significant underestimation of 37% in DB10K AOD retrievals was observed across all the sites except ZZ, which was indicated by a low-value RMB (0.63). Spatial distribution maps showed high AOD values (>0.8) over the lower part of the YERB and low AOD values (<0.4) across the upstream part of the YERB. This might be due to a large number of aerosol emissions over the lower developed areas and a scarcity of aerosols over the upstream mountain areas. Overall, this study supports the use of DT10K C6.1 AOD retrievals over the western semi-arid and arid regions of the YERB and DTB10K AOD retrievals over the north-central water system and eastern plain regions of the YERB.

Characteristics and origins of air pollutants in Wuhan, China, based on observations and hybrid receptor models

To identify the characteristics of air pollutants and factors attributing to the formation of haze in Wuhan, this study analyzed the hourly observations of air pollutants (PM_2.5, PM₁₀, NO₂, SO₂, O₃, and CO) from March 1, 2013, to February 28, 2014, and used hybrid receptor models for a case study. The results showed that the annual average concentrations for PM_2.5, PM₁₀, NO₂, SO₂, O₃, and CO during the whole period were 89.6 μg m^-3, 134.9 μg m^-3, 54.9 μg m^-3, 32.4 μg m^-3, 62.3 μg m^-3, and 1.1 mg m^-3, respectively. The monthly variations revealed that the peak values of PM_2.5, PM₁₀, NO₂, SO₂, and CO occurred in December because of increased local emissions and severe weather conditions, while the lowest values occurred in July mainly due to larger precipitation. The maximum O₃ concentrations occurred in warm seasons from May to August, which may be partly due to the high temperature and solar radiation. Diurnal analysis showed that hourly PM_2.5, PM₁₀, NO₂, and CO concentrations had two ascending stages accompanying by the two traffic peaks. However, the O₃ concentration variations were different with the highest concentration in the afternoon. A case study utilizing hybrid receptor models showed the significant impact of regional transport on the haze formation in Wuhan and revealed that the mainly potential polluted sources were located in the north and south of Wuhan, such as Baoding and Handan in Hebei province, and Changsha in Hunan province.

IMPLICATIONS

Wuhan city requires a 5% reduction of the annual mean of PM_2.5 concentration by the end of 2017. In order to accomplish this goal, Wuhan has adopted some measures to improve its air quality. This work has determined the main pollution sources that affect the formation of haze in Wuhan by transport. We showed that apart from the local emissions, north and south of Wuhan were the potential sources contributing to the high PM_2.5 concentrations in Wuhan, such as Baoding and Handan in Hebei province, Zhumadian and Jiaozuo in Henan province, and Changsha and Zhuzhou in Hunan province.

Optical properties of PM2.5 and the impacts of chemical compositions in the coastal city Xiamen in China

Continuous in situ measurements of optical properties of fine aerosols (PM2.5) were conducted in the urbanized coastal city Xiamen in Southeast China from November 2013 to January 2014. PM2.5 samples were also collected and chemical compositions including organic carbon (OC), elemental carbon (EC) and water-soluble inorganic ions were determined to investigate the impacts of chemical compositions on aerosol optical properties. Average values of scattering coefficient (bscat), absorption coefficient (babs), extinction coefficient (bext) and single scattering albedo (SSA) were 164.0Mm(-1), 22.4Mm(-1), 187.0Mm(-1) and 0.88, respectively. bscat, babs and bext showed obvious bi-modal diurnal variations with high values in the morning and at night while low value in the early afternoon, whereas SSA exhibited an opposite diurnal variation. Average bscat and babs were largest in the wind direction of southwest and were larger with slower wind. babs was mainly affected by EC, while bscat was affected by ammonium, sulfate, nitrate and OC. The IMPROVE formula was applied to estimate bext based on the chemical species. Results shows that ammonium sulfate was the largest contributor, accounting for 36.4% of bext, followed by organic matter (30.6%), ammonium nitrate (20.1%), EC (9.0%) and sea salt (3.9%). The deterioration in visibility was mainly led by increases in secondary aerosols including sulfate and nitrate. Backward trajectories analysis showed that during the sampling period Xiamen was significantly affected by the air masses originating from the Northern and Northeastern areas. Air masses from the Northern associated with relative higher bext and less relative contribution from ammonium sulfate and more relative contribution from ammonium nitrate, organic matter and sea salt.

Measurement and Study of Lidar Ratio by Using a Raman Lidar in Central China

We comprehensively evaluated particle lidar ratios (i.e., particle extinction to backscatter ratio) at 532 nm over Wuhan in Central China by using a Raman lidar from July 2013 to May 2015. We utilized the Raman lidar data to obtain homogeneous aerosol lidar ratios near the surface through the Raman method during no-rain nights. The lidar ratios were approximately 57 ± 7 sr, 50 ± 5 sr, and 22 ± 4 sr under the three cases with obviously different pollution levels. The haze layer below 1.8 km has a large particle extinction coefficient (from 5.4e-4 m(-1) to 1.6e-4 m(-1)) and particle backscatter coefficient (between 1.1e-05 m(-1)sr(-1) and 1.7e-06 m(-1)sr(-1)) in the heavily polluted case. Furthermore, the particle lidar ratios varied according to season, especially between winter (57 ± 13 sr) and summer (33 ± 10 sr). The seasonal variation in lidar ratios at Wuhan suggests that the East Asian monsoon significantly affects the primary aerosol types and aerosol optical properties in this region. The relationships between particle lidar ratios and wind indicate that large lidar ratio values correspond well with weak winds and strong northerly winds, whereas significantly low lidar ratio values are associated with prevailing southwesterly and southerly wind.

Measurement and analysis of surface aerosol optical properties over urban Nanjing in the Chinese Yangtze River Delta

Aerosol optical properties including aerosol scattering coefficient (σsp), absorption coefficient (σap), single scattering albedo (SSA), PM2.5 mass concentration and their relationship with meteorological factors were measured and analyzed from 1st March to 30th April 2011. The observations were conducted at an urban-industrial site in Nanjing located in the Yangtze River Delta (YRD) region, China. The averaged σsp at 550 nm is found to be 329.3±321.6 Mm(-1) and occurred most frequently within the range of 100-200 Mm(-1). σap at 532 nm is measured to be 28.1±17.6 Mm(-1), which is remarkably smaller than most of the measurements made over megacities in China and nearly equivalent to the observations made at rural sites in the YRD. The mean SSA at 532 nm is 0.89±0.08 with the occurrence frequency of higher SSA (>0.9) accounting for about 60%, indicating that scattering-related aerosols are dominant over Nanjing during the study period. A bi-peak diurnal variation of σap and σsp is found with the maximum during late at night and early morning and the minimum in the afternoon, but the diurnal pattern of SSA is just the opposite to this. Significant correlation coefficients are noticed between different aerosol optical properties. σsp and σap showed a clear negative correlation with wind speed, temperature, and visibility. During the study period, the lower northwesterly and southeasterly winds are attributed to an increase in the aerosol optical properties. The σsp and σap increased significantly during severe haze and dust episodes caused by the accumulation of pollutants from both local and regional sources under favorable weather conditions.