Mixing State of Black Carbon Aerosol in a Heavily Polluted Urban Area of China: Implications for Light Absorption Enhancement

<div><p>Black carbon (BC) is important for climate forcing, and its effects on the Earth's radiative balance remain a major uncertainty in climate models. In this study, we investigated the mixing state of refractory black carbon (rBC) and aerosol optical properties in a polluted atmosphere at Xi’an, western China. The average rBC mass concentration was 9.9 <b>μ</b>g m⁻³ during polluted periods, 7.6 times higher than that in clean periods. About 48.6% of the rBC was internally-mixed or coated with nonrefractory materials during polluted periods; this was 27% higher than in clean periods. Correlation analysis between the number fraction of thickly-coated rBC particles (<i>f</i>_BC) and the major particulate species indicate that organics may be the primary contributor to rBC coatings during polluted periods. The average mass absorption cross section of rBC (MAC_BC) particles at <b>λ =</b> 870 nm was 7.6 <b>±</b> 0.02 m² g⁻¹ for the entire campaign. The MAC_BC showed a positive correlation with <i>f</i>_BC, and the enhancement of MAC_BC due to internal mixing was 1.8 times. These observations suggest that an enhancement of BC absorption by a factor of ∼2 could be appropriate for climate models associated with high PM_2.5 levels.</p><p>Copyright 2014 American Association for Aerosol Research</p></div

Atmospheric Fossil Fuel CO₂ Traced by Δ¹⁴C in Beijing and Xiamen, China: Temporal Variations, Inland/Coastal Differences and Influencing Factors

One year of atmospheric Δ¹⁴CO₂ were observed in 2014 in the inland city of Beijing and coastal city of Xiamen, China, to trace temporal CO_2<i>ff</i> variations and to determine the factors influencing them. The average CO_2<i>ff</i> concentrations at the sampling sites in Beijing and Xiamen were 39.7 ± 36.1 ppm and 13.6 ± 12.3 ppm, respectively. These contributed 75.2 ± 14.6% and 59.1 ± 26.8% to their respective annual ΔCO₂ offsets over background CO₂ concentrations. Significantly (<i>p</i> < 0.05) high CO_2<i>ff</i> values were observed in winter in Beijing. We did not find any significant differences in CO_2<i>ff</i> values between weekdays and weekends. Diurnal CO_2<i>ff</i> variations were plainly evident, with high values between midnight and 4:00, and during morning and afternoon rush hours. The sampling site in the inland city of Beijing displayed much higher CO_2<i>ff</i> inputs and overall temporal variations than the site in the coastal city of Xiamen. The variations of CO_2<i>ff</i> at both sites were controlled by a combination of emission sources, topography, and atmospheric dispersion. In particular, diurnal observations at the urban site in Beijing showed that CO_2<i>ff</i> was easily accumulated under the southeast wind conditions

Double-Edged Role of VOCs Reduction in Nitrate Formation: Insights from Observations during the China International Import Expo 2018

Aerosol nitrate (NO3–) constitutes a significant component of fine particles in China. Prioritizing the control of volatile organic compounds (VOCs) is a crucial step toward achieving clean air, yet its impact on NO3– pollution remains inadequately understood. Here, we examined the role of VOCs in NO3– formation by combining comprehensive field measurements conducted during the China International Import Expo (CIIE) in Shanghai (from 10 October to 22 November 2018) and multiphase chemical modeling. Despite a decline in primary pollutants during the CIIE, NO3– levels increased compared to pre-CIIE and post-CIIENO3– concentrations decreased in the daytime (by −10 and −26%) while increasing in the nighttime (by 8 and 30%). Analysis of the observations and backward trajectory indicates that the diurnal variation in NO3– was mainly attributed to local chemistry rather than meteorological conditions. Decreasing VOCs lowered the daytime NO3– production by reducing the hydroxyl radical level, whereas the greater VOCs reduction at night than that in the daytime increased the nitrate radical level, thereby promoting the nocturnal NO3– production. These results reveal the double-edged role of VOCs in NO3– formation, underscoring the need for transferring large VOC-emitting enterprises from the daytime to the nighttime, which should be considered in formulating corresponding policies