The variability of the mixing state of refractory black carbon aerosol (rBC) and the corresponding complicated light absorption capacity imposes great uncertainty for its climate forcing assessment. ...In this study, field observations using a single-particle soot photometer (SP2) were conducted to investigate the mixing state of rBC under different meteorological conditions at a rural site on the North China Plain. The results showed that the hourly mass concentration of rBC during the observation periods was 2.6±1.5 µg m.sup.-3 on average, with a moderate increase (3.1±0.9) during fog episodes. The mass-equivalent size distribution of rBC exhibited an approximately lognormal distribution with a mass median diameter (MMD) of 213 nm. We found that the count median diameter (CMD) of rBC particles during snowfall episodes was larger than that before snowfall, and the number of rBC particles with D.sub.c <121 nm were reduced by 28.4 % after snow. This may indicate that rBC-containing particles with small core sizes (D.sub.c) were much more effectively removed by snow with light snow intensity (0.23 mm h.sup.-1). Based on the Mie scattering theory simulation, the relative and absolute coating thicknesses of rBC-containing particles were estimated to be â¼1.6 and â¼52 nm for the rBC core with a mass-equivalent diameter (D.sub.c) of 170 to 190 nm, respectively, which indicates that most of the rBC-containing particles were thinly coated. Furthermore, a moderate light absorption enhancement (E.sub.abs =1.3) and relatively low absorption cross section (MAC = 5.5 m.sup.2 g.sup.-1) at 880 nm were observed at the Gucheng (GC) site in winter compared with other typical rural sites.
The vertical distribution of black carbon (BC), as well as its
mixing state, is of great concern due to BC's strong regional climatic and
environmental effects. In this study, vertical measurements ...were conducted
through a moveable container based on a meteorological tower in the Beijing urban area during June and July. A total of 112 vertical profiles (0–240 m),
including the concentrations of BC, O3, NOx and the optical
properties of aerosols, were obtained. Based on BC concentration, the
vertical profiles could be classified into four categories: uniform, gradual decrease, sharp decrease and sudden increase. The uniform type indicates
strong vertical mixing with similar pollutant concentrations along the
vertical direction. The gradual and sharp decrease types indicate stable
vertical conditions with higher pollutant concentrations on the ground and
lower concentrations at higher altitudes. Due to the strong radiation in
summer, the vertical profiles exhibited a clear diurnal variation in which
∼ 80 % of profiles were uniform during the daytime and
∼ 40 %–90 % of profiles were of the gradual and sharp decrease types
at night. O3 is an exception, and its concentration generally increases
with height, even under strong vertical mixing conditions. The size
distribution of the BC core varied slightly along the vertical direction, and
the coating thickness, denoted by the diameter ratio between the
BC-containing particle and BC core (Dp/Dc), of BC increased with
height under stable conditions. Although the coating thickness could
increase the absorption ability with an average absorption enhancement of
1.25 at 23:00 LT (local time: UTC+8), the vertical difference of Dp/Dc (2 %) was much
lower than that of BC concentration (∼ 35 %). The vertical
variation in absorption ability was mainly caused by the variation in BC
concentration. In addition, O3 and Dp/Dc occasionally
increased during 06:00–08:00 but remained stable during 08:00–10:00. Vertical
mixing and transportation from upper heights, such as the residual layer,
could significantly influence the pollutant properties on the surface during
early mornings. This study exhibits a continuous vertical picture of BC and
its mixing state in urban areas, which would be helpful for understanding
BC's regional environmental effect.
The attainment of suitable ambient air quality standards is a matter of great concern for successfully hosting the XXIV Olympic Winter Games (OWG). Transport patterns and potential sources of ...pollutants in Zhangjiakou (ZJK) were investigated using pollutant monitoring datasets and a dispersion model. The PM
2.5
concentration during February in ZJK has increased slightly (28%) from 2018 to 2021, mostly owing to the shift of main potential source regions of west-central Inner Mongolia and Mongolian areas (2015–18) to the North China Plain and northern Shanxi Province (NCPS) after 2018. Using CO as an indicator, the relative contributions of the different regions to the receptor site (ZJK) were evaluated based on the source-receptor-relationship method (SRR) and an emission inventory. We found that the relative contribution of pollutants from NCPS increased from 33% to 68% during 2019–21. Central Inner Mongolia (CIM) also has an important impact on ZJK under unfavorable weather conditions. This study demonstrated that the effect of pollution control measures in the NCPS and CIM should be strengthened to ensure that the air quality meets the standard during the XXIV OWG.
The region along the Taihang Mountains in the North China Plain (NCP) is characterized by serious fine particle pollution. To clarify the formation mechanism and controlling factors, an observational ...study was conducted to investigate the physical and chemical properties of the fine particulate matter in Jiaozuo city, China. Mass concentrations of the water-soluble ions (WSIs) in PM2.5 and gaseous pollutant precursors were measured on an hourly basis from December 1, 2017, to February 27, 2018. The positive matrix factorization (PMF) method and the FLEXible PARTicle (FLEXPART) model were employed to identify the sources of PM2.5. The results showed that the average mass concentration of PM2.5 was 111 μg/m3 during the observation period. Among the major WSIs, sulfate, nitrate, and ammonium (SNA) constituted 62% of the total PM2.5 mass, and NO3− ranked the highest with an average contribution of 24.6%. NH4+ was abundant in most cases in Jiaozuo. According to chemical balance analysis, SO42−, NO3−, and Cl− might be present in the form of (NH4)2SO4, NH4NO3, NH4Cl, and KCl. The liquid-phase oxidation of SO2 and NO2 was severe during the haze period. The relative humidity and pH were the key factors influencing SO42- formation. We found that NO3− mainly stemmed from homogeneous gas-phase reactions in the daytime and originated from the hydrolysis of N2O5 in the nighttime, which was inconsistent with previous studies. The PMF model identified five sources of PM2.5: secondary origin (37.8%), vehicular emissions (34.7%), biomass burning (11.5%), coal combustion (9.4%), and crustal dust (6.6%).
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The long‐range transport of Asian dust and its complex interaction with anthropogenic pollutants have a significant yet poorly quantified effect on the global climate. In this study, dust events were ...observed based on an optical particle counter with a polarization detection module at an urban site in North China. The temporal variations in the size distribution and morphological changes of dust particles were analyzed. The results demonstrate that the dust events were induced by a deep Siberian low‐pressure anomaly and/or Mongolian cyclone in the low troposphere. The morphology of dust particles, as indicated by the depolarization ratio (δ = s/(s + p), the ratio of the parallel signal to the total backscattering signal), varied significantly at different transport heights. A decreasing tendency was found when dust was trapped within the boundary layer over polluted areas, particularly when the atmospheric loading of pollutants and the relative humidity were high. During a typical polluted dust case, the δ value of mineral dust (Dp = 5 μm) decreased by 39%, as the conditions were stagnant in East China, consistent with the Cloud‐Aerosol LIDAR with Orthogonal Polarization results. The morphological variation was evaluated according to the aspect ratio, presuming that the dust particles were ellipsoidal. The interaction between dust particles and water vapor could lead to an increase of at least 6% in volume when the relative humidity increased up to 90%. An aerosol classification based on δ values and size information could be performed to estimate the contributions of different aerosol types to pollution days. The results should be helpful for evaluating satellite‐borne and ground‐based light detection and ranging data analyses.
Key Points
The occurrence of dust events in East Asia was related to a deep Siberian low‐pressure anomaly and/or a Mongolian cyclone
The single particle‐based depolarization ratio was observed to study the nonsphericity of dust particles
Environmental humidity and pollutant concentrations are the key factors that affect the production of polluted dust
In spring 2021, several strong dust export processes crossed over East Asia, including the combined transport of Mongolia Gobi and Inner Mongolia dust. Based on CAMS forecast systems and satellite ...sensors, we analyzed the transport mechanisms and impact on China. Our case studies illustrate that in all 4 dust storms, the initial phase of transport was strongly confined by a low-level Mongolian cyclone facilitated by a deep cold-core upper trough. Using a methodology based on MODIS Deep Blue AOD data enables us to estimate the dust optical depth (DOD) and the impact of dust plumes on different areas in China. The result shows that North China was the most affected area in all cases, second only to the source region, with a mean observed DOD >0.8 and depolarization ratio > 0.2. The vertical distribution of the aerosol frequency of occurrence (FoO) was yielded using the CALIOP aerosol classification result. Dust storms were the dominant factor within 6 km (peaked at 2–3 km), while a significant amount of transported smoke aerosols dominated the upper troposphere. Even in dust storms, anthropogenic aerosols still occupied a high proportion of China's atmosphere, especially in South China, indicating the great contribution of human activities. As a subdominant aerosol type, polluted dust showed a profile similar to that of polluted continental aerosols, with peak heights generally lower than that of pure dust, at approximately 1–2 km. The development of cyclone/anticyclone activities was mostly modulated by the atmospheric dynamic background. By discussing the climatic index related to dust storm occurrence in East Asia, we found that the high-frequency years in the 21st century have a satisfactory correspondence with the negative phase of the Pacific Decadal Oscillation (PDO) and the positive phase of the Southern Oscillation Index (SOI). These findings may have positive significance for dust forecasting in East Asia in the future.
•All 4 dust storms in 2021 were initialized by Mongolian cyclones.•Pollutants occupied a high proportion of China's atmosphere even in strong storms.•PDO and SOI can be stable indicators of East Asia dust storms.
The complex microphysical characteristics of black carbon aerosol (BC) results in large uncertainties in the evaluation of BC's climatic and environmental effects. In this study, the microphysical ...properties were investigated in fresh BC emitted from traffic sources (gasoline vehicle and diesel) and aged BC emitted from solid fuel burning sources (coal, cotton stem, rice stem and mulberry). For the BC core size distribution, the number size distribution of traffic sources peaks at a lower value than that of aged solid fuel burning sources. Although traffic sources emit a greater number of small BC cores, traffic sources could also emit BC aerosols with large BC cores (diameter larger than 400 nm) not found in solid fuel sources. The coating thickness of BC from traffic sources is significantly lower than that of aged BC from solid fuel sources. Such differences in size distribution and coating thickness cause large variations in other microphysical parameters, such as light absorption and hygroscopicity, for these two types of BC. The mass absorption cross section (MAC) of aged BC from cotton stems reached 11.69 m2/g, nearly 2 times higher than the MAC (6.08 m2/g) of fresh BC from gasoline vehicles. The hygroscopicity of aged BC from solid fuel burning is also much higher than that of fresh BC from traffic. The critical supersaturation needed to activate BC to become cloud droplets is 0.03% for the former and 0.42% for the latter. This study reports the BC emission characteristics of traffic sources and emphasizes the large property difference between fresh traffic emission BC and aged solid fuel burning BC. The microphysical properties of BC should be carefully considered to better resolve BC concentrations and optical properties in climatic models.
•BC number size distribution from traffic peaks at lower value than that from solid fuel burning.•The absorption ability of aged BC from solid fuel burning could be nearly two times higher than that from traffic.•The effective density of aged BC exhibited a unimodal pattern peaking at 0.9–1.4 g/cm3.
Atmospheric loading of black carbon aerosols influences air quality and visibility, and their mixing state and aerosol chemistry were sensitive to both emission scenarios and regional transport. In ...this study, the aerodynamic size-resolved mixing state of refractory black carbon aerosols (rBC) was investigated using a novel tandem system consisting of an aerodynamic aerosol classifier (AAC), a single particle soot photometer (SP2), and single-particle-aerosol mass spectrometry (SPA-MS) during the XXIV Olympic Winter Games (OWG) at an urban site in Beijing. Particles with aerodynamic diameter (Dae) of 200 nm and 300 nm were selected by AAC to focus on the effect of relative differences in morphology on the coating thickness, coating compositions, and optical properties of rBC particles. We found that rBC-containing particles with Dae of 300 nm were characterized by a rBC core with a count median diameter (CMD) of 108 ± 4 nm that typically have regular morphology (χ:mean∼1), higher relative coating thickness (RCT: mean ∼1.9), and strong light absorption enhancement (Eabs: mean ∼1.5). The rBC-containing particles with Dae = 200 nm normally had irregular shapes (χ:mean∼1.4) and weak Eabs (Eabs: mean ∼1.37). Classification based on air mass clustering and air pollution level revealed that the coating thickness of rBC particles only exhibited unimodal patterns under clean air conditions and distinct bimodal distributions under heavy pollution conditions, implying multiple origins of rBC particles, even during OWG periods. The coating thickness and Eabs of rBC particles were greater when influenced by the higher RH air from east. Furthermore, the chemical compositions of the rBC coating quantitatively determined by SPA-MS were used to estimate the possible origins of the Dae = 300 nm, featured by a thicker coating thickness and stronger light absorption enhancement. Our results showed that rBC particles were primarily mixed with organics, nitrate, and sulfate during the XXIV OWG period. The organic components have a limited role in increasing rBC coatings under polluted conditions, while the alkali metal ions (K and Na) associated with traffic emissions and the secondary inorganic species favor the formation of thick coatings. Higher RH makes limited contributions to rBC mixed with sulfate and organics only (BCOC*_S) and pure BC. The relatively fresh BC could directly mix with organics and sulfate at low RH levels while evolving to mix with nitrate at high RH conditions. In addition to high concentrations of locally emitted precursors and high RH that tend to form thick coatings of nitrate-related rBC aerosols, secondary transport from the west and southwest also contributes to rBC aerosols mixed with nitrate with relatively thick coatings.
•rBC with an aerodynamic diameter (Dae) of 300 nm typically have regular morphology.•rBC of regular shape(Dae = 300 nm)has thicker coatings and stronger absorption enhancement.•During the OWG, rBC particles were primarily mixed with organics, nitrate and sulfate.•rBC particles had multiple sources, even in the OWG periods.•The organic components have a limited role in increasing rBC coatings under polluted conditions.
The mixing state of black carbon aerosols (BC) has the largest uncertainty for climate forcing evaluation. In this study, a tandem observation, combining a centrifugal particle mass analyzer (CPMA) ...and a single particle soot photometer (SP2), was conducted to investigate the mixing state of BC in a coastal city in eastern China. First, particles with specific masses (1.0 fg, 2.1 fg, 4.5 fg, 9.7 fg, and 20.1 fg) were selected using CPMA and consecutively measured by SP2 to determine the corresponding BC mass (MBC) of the particle. The mass ratio (MR) of the coating matter to the BC core was used to indicate the coating thickness and aging degree of BC. The results showed that the MR value obtained from the CPMA-SP2 system was well correlated with the mixing state condition derived from the traditional lag-time method. However, the lag-time method tends to misclassify thickly to thinly coated BC as particles with larger masses. The MR generally increased as the mass of BC-containing particles increased. MR showed a clear diurnal pattern with a minimum at 8:00 and a maximum at 13:00 due to photochemical process during the daytime. The larger BC-containing particles had a much weaker diurnal pattern because other secondary processes may contribute more to the coatings for large BC-containing particles. The MR maintained a high value when the air mass was stagnant in the local area with high pollutant concentrations and high relative humidity (RH), while the MR was at its lowest value when the air mass was derived from the marine area with high RH and low pollutant concentrations. These observations suggest that the increase in MR is the result of the combined effect of high RH and high pollutant concentrations.
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•Traditional lag-time method tends to misclassify thickly to thinly coated BC as particles with larger masses.•The increase of BC's coating needs the joint action of pollutants precursors, aging time and humidity.•The coating thickness of BC with larger masses had weaker diurnal pattern.
The effect of the nonsphericity of mineral dust aerosols on its deposition and transport was investigated based on model simulation for a typical dust event over northern China from April 6 to 12, ...2018. The settling velocity related to morphological change in dust size was considered in Nested Air Quality Prediction Modeling System (NAQPMS) to simulate the dust spatial distribution. Comparison of these results with observations showed that the model reproduced the temporal variability in the mass concentration of particles along the dust plume pathway. The most frequently reported aspect ratio (λ) was 1.7 ± 0.2 for Asian dust aerosols. Changing the nonsphericity of the particle from typical prolate ellipsoids (λ = 1.7) to spherical ellipsoids (λ = 1) caused an ~3% decrease in the surface dust concentration on average. For particles with diameters >5 μm, nonsphericity caused a change in the surface dust concentration up to 10%, especially at the periphery of the dust source region. The overall effects on the fine dust (<2.5 μm) were not significant. A sensitivity study using a more extreme nonspherical shape (λ = 2) showed that the differences in PM10 concentration were evident, and the surface dust concentration increased by 15 ± 5% as a result of an ~10% decrease in settling velocity. These results confirmed that the effect of the variability in the nonsphericity of Asian dust particles on their regional transport highly depended on synoptical and pollution conditions, and the adoption of a deposition value that changes over time due to this morphological variability could improve the performance of dust modeling and the assessment of climate effects on a global scale, especially for transboundary processes.
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•The spherical treatment of the dusts gravity settling in chemical transport models underestimate their atmospheric loading.•Most chemical global models neglect the irregularity shape of dust particles from the beginning of their life.•The adoption of a deposition value that reflects dust irregularity could improve the assessment of climate effects.
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