Impact of aerosol-boundary layer interactions on PM2.5 pollution during cold air pool events in a semi-arid urban basin.

Global emission reductions still must address winter fine particulate matter (PM2.5) pollution in urban basins with enclosed terrains and frequent cold air pool (CAP) events when the temperatures within the basin are colder than above it. The effects of urban basin aerosol-boundary layer interactions on PM2.5 pollution during CAP events remain unclear. Intensive boundary layer observations in January 2021 and numerical models were used to investigate this issue in the semi-arid urban Lanzhou Basin of China. The results showed that CAPs formed because of the synoptic weather system that exacerbated the warming over the basin. The CAPs in this experiment were characterized by stronger temperature inversion (TI) layers in the vertical direction and lower relative humidity, lower wind speed, and weaker turbulence at the bottom of the basin compared to other conditions. The strong TI layers below the top of the basin inhibited the vertical dispersion of pollutants in the basin and concentrated the PM2.5 within a height of 0.3 km from the bottom of the basin. During CAP events, the proportion of elemental carbon in PM2.5 increased, whereas that of secondary inorganic species decreased. Aerosol absorption increased faster than scattering during CAP events. Therefore, the mean single scattering albedo decreased from 0.85 during non-CAP periods to 0.81 during CAP events. Radiosonde-sounding observations and numerical simulations indicated that aerosols accumulating in the lower basin heated the atmosphere during the daytime and facilitated boundary layer development via the "stove effect" (absorption aerosol heats lower atmosphere to promote boundary layer development). No significant "dome effect" (absorption aerosol heats the upper boundary layer to suppress boundary layer development) occurred during the two CAP events. These findings provide a theoretical basis for scientifically-guided PM2.5 pollution control in winter in isolated urban basins.

Wintertime vertical distribution of black carbon and single scattering albedo in a semi-arid region derived from tethered balloon observations.

The vertical distribution of atmospheric aerosols plays an essential role in aerosol-radiation and aerosol-cloud interactions. Because of strong light absorption, the radiative effects of black carbon (BC) are highly sensitive to its vertical distribution; the lack of high-resolution observations is the reason for their poor quantification. We used a tethered balloon platform to acquire high-resolution vertical profiles of BC, particle number concentration, and meteorological parameters in the semi-arid region of Northwest China in December 2018. A total of 112 BC profiles were classified into four vertical distribution categories, which were determined by local emissions, regional transport, vertical mixing due to the ABL evolution, and topography. BC profiles with peaks near or above the atmospheric boundary layer (ABL) accounted for 57% of the profiles. Vertical single scattering albedo (SSA) profiles were subsequently calculated using the profiles of BC and particle size distribution. The vertical SSA distribution is generally modulated by BC profiles. The diurnal variations of the BC and SSA profiles were summarized using a boundary-layer normalization method. In the ABL, BC decreased and SSA increased with increasing height at 02:00, 08:00, and 20:00, while both BC and SSA exhibited a uniform distribution at 14:00. The SSA decreased above the ABL at 14:00, which might have had a profound impact on ABL development. These results provide a better understanding of the vertical BC and SSA distributions, which can also be used to reduce uncertainties in estimating the BC radiative effects.

High contribution of vehicle emissions to fine particulate pollutions in Lanzhou, Northwest China based on high-resolution online data source appointment.

The quantitative estimation of urban particulate matter (PM) sources is essential but limited because of various reasons. The hourly online data of PM2.5, organic carbon (OC), elemental carbon (EC), water-soluble ions, and elements from December 2019 to November 2020 was used to conduct PM source appointment, with an emphasis on the contribution of vehicle emissions to fine PM pollution in downtown Lanzhou, Northwest China. Vehicle emissions, secondary formation, mineral dust, and coal combustion were found to be the major PM sources using the positive matrix factorization model. The seasonal mean PM2.5 were estimated to be 72.8, 39.2, 24.3, and 43.6 µg·m-3 and vehicle emissions accounted for 35.7%, 25.8%, 30.0%, and 56.6% in winter, spring, summer, and autumn, respectively. Vehicle emissions were the largest source of PM considering the high PM pollution in winter and its significantly large contribution in autumn. Furthermore, the contribution of vehicle emissions increased with increasing PM in winter and autumn. Vehicle emissions were also the most important source of EC, accounting for 70.3%, 91.0%, 83.5%, and 93.7% of the total EC in winter, spring, summer, and autumn, respectively. With the reduction in industrial emissions and increase in vehicle numbers in China in recent years, vehicle emissions are going to be the largest source of urban PM pollution. To sustainably improve air quality in Lanzhou and other Chinese cities, efforts should be made to control vehicle emissions such as promoting clean-energy vehicles and encouraging public transportation.