Characteristics of atmospheric black carbon and its wet scavenging in Nanning, South China

Based on in-situ measurement of black carbon (BC) and carbon monoxide (CO), the characteristics of BC emissions and wet scavenging were comprehensively investigated in Nanning, South China. The average annual BC concentration was 1.02 ± 0.53 μg m-3 with higher pollution levels during winter. In winter, a higher net BC/CO (ΔBC/ΔCO) ratio of 3.3 ± 0.3 ng m-3 ppb-1 along with an increased absorption Ångström exponent (AAE) and BC mass from biomass burning (BCbb), indicated a significant contribution of biomass burning to BC emissions. However, emissions from the traffic sector consistently exerted a dominant influence throughout the year. Cluster analysis of backward trajectories identified three types of air masses with distinct origins. Cluster #1 originated from Guangxi province and its vicinity, intermittently influencing the sampling site throughout the year with varying effects between winter and summer. This air mass brought in clean sea breeze in summer whereas transported a higher proportion of BCbb to the site during wintertime due to local open biomass burning. Cluster #3 primarily arrived in autumn and winter (October-December) from polluted central China, resulting in substantially high BC mass at the site. Cluster #2 coincided with the period (January-March) when extensive surface open biomass burning events occurred in Southeast Asia (SEA) regions. These BC aerosols in cluster#2 initially rose to higher altitudes above SEA before being regionally transported, but were significantly scavenged by clouds and precipitation during vertical uplift. The remaining BC exhibited a notably lower BC loss rate on relative humidity (RH) of -0.01 ng m-3 ppb-1 %-1 compared to cluster #1 (-0.03) and cluster #3 (-0.06), corresponding to an average BC transport efficiency of 0.85, 0.73, and 0.53, respectively. Nonetheless, air masses in cluster #2 could still transport considerably high BC mass to Nanning due to dry conditions and less wet scavenging along trajectory pathways. These findings provide valuable insights for policymakers and government officials in regulating and mitigating BC pollution in South China.

Fog scavenging of particulate matters in air pollution events: Observation and simulation in the Yangtze River Delta, China

Stable atmospheric boundary layer is conducive to the accumulation of atmospheric pollution and the occurrence of fog, and fog has a removal effect on air pollution. In this study, we use the observation and WRF-Chem (Weather Research and Forecasting Model with Chemistry) simulation to analyze the factors affecting the removal efficiency in a continuous fog and haze episode from November 26 to 28, 2018 in Jiangsu Province, such as fog thickness and duration. The results show that the WRF-Chem simulation well reproduces the boundary layer characteristics in the stages of fog formation, development and dissipation. The atmospheric boundary layer provides favorable conditions for the maintenance of fog and air pollution. The inversion layer, with the maximum intensity of 3 °C per 100 m, creates favorable thermal conditions, and the water vapor advection is also conducive to the fog maintenance. The ground observation verifies the wet scavenging of PM_2.5 during dense fog events. The scavenging effect is related to the fog duration, and the correlation is positive when the fog is just formed and negative when the fog is dissipating. The PM_2.5 concentration decreases from 159 μg m^-3 to 38 μg m^-3 after the fog lasts for 11 h. The fog has a remarkable scavenging effect on PM_2.5 in the vertical direction, due to the deposition effect of fog droplets on the pollutant particles. The PM_2.5 concentration on the ground is lower than the vertical average in the fog area, and the scavenging effects during the dense fog periods on November 27 and 28 are 47.7 μg m^-3 and 36.1 μg m^-3, respectively. The fog duration is mostly concentrated in 3-17 h. When the duration of fog is 4-8 h, the scavenging effect on PM_2.5 reaches the strongest, with an average PM_2.5 concentration decrease of >70 μg m^-3.

Molecular characterization and spatial distribution of dicarboxylic acids and related compounds in fresh snow in China

Low molecular weight organic compounds are ubiquitous in the atmosphere. However, knowledge on their concentrations and molecular distribution in fresh snow remains limited. Here, twelve fresh snow samples collected at eight sites in China were investigated for dicarboxylic acids and related compounds (DCRCs) including oxocarboxylic acids and α-dicarbonyls. Dissolved organic carbon (DOC) concentrations in the snow samples ranged from 0.99 to 14.6 mg C L^-1. Concentrations of total dicarboxylic acids were from 225 to 1970 μg L^-1 (av. 650 μg L^-1), while oxoacids (28.3-173, av. 68.1 μg L^-1) and dicarbonyls (12.6-69.2, av. 31.3 μg L^-1) were less abundant, accounting for 4.6-8.5% (6.2%), 0.45-1.4% (0.73%), and 0.12-0.88% (0.46%) of DOC, respectively. Molecular patterns of dicarboxylic acids are characterized by a predominance of oxalic acid (C₂) (95.0-1030, av. 310 μg L^-1), followed by phthalic (Ph) (9.69-244, av. 69.9 μg L^-1) or succinic (C₄) (23.8-163, av. 63.7 μg L^-1) acid. Higher concentrations of Ph in snow from Beijing and Tianjin than other urban and rural regions suggest significant emissions from vehicular exhausts and other fossil fuel combustion sources in megacities. C₂ constituted 40-54% of total diacids, corresponding to 1.5-2.6% of snow DOC. The total measured DCRCs represent 5.5-10% of snow DOC, which suggests that there are large amounts of unknown organics requiring further investigations. The spatial distributions of diacids exhibited higher loadings in megacities than rural and island sites. Molecular distributions of diacids indicated that the photochemical modification was restrained under the weak solar radiation during the snow events, while anthropogenic primary sources had a more significant influence in megacities than rural areas and islands.