Turbulent Fluxes and Pollutant Mixing during Wintertime Air Pollution Episodes in Complex Terrain

Spatio-temporally differentiated impacts of temperature inversion on surface PM2.5 in eastern China

Temperature inversion (TI) is one of the meteorological conditions that significantly affect regional air quality. Knowledge gap regarding the impacts of TI on surface PM_2.5 in different topographies still existed. In the present study, the occurrence frequency, temperature lapse rate (TLR), depth, and the diurnal variations of TI, surface-based TI (SBTI), elevated TI (ElTI), and multiple layers of TIs (MultiTI) and their impacts on near-surface PM_2.5 concentrations over eastern China that covers a range of topographies and climates, are systematically investigated based on global reanalysis ERA5 and the nationwide monitoring PM_2.5 dataset from 2014 to 2020. TIs occurred mostly in the early morning. Different types of TIs present distinctive seasonal and spatial patterns. The majority of SBTIs and ElTIs occurred during nighttime in northern China and daytime in southern China, respectively, as the result of their formation mechanisms. SBTIs usually had larger TLR while ElTIs had deeper depth. SBTIs showed strong enhancement effects on PM_2.5 concentration over the study domain while ElTIs showed more obvious impacts on northern nocturnal PM_2.5. The peak time of PM_2.5 was found around 18:00-22:00 LST, and TLR and depth of TIs are thought to be more relevant to PM_2.5 peak concentration due to their coincident peak times. The strength of TIs is therefore more crucial in regulating PM_2.5 than its occurrence frequency. Based on statistical analysis, our study provided a large picture of the generic spatiotemporal patterns of TIs and illustrated the impacts of different TIs on surface PM_2.5 pollution on a diurnal basis. For a deeper understanding of the formation of PM_2.5 pollution, more attention needs to be paid to the nocturnal PM_2.5 not only at surface level but also at higher levels in the presence of TIs.

Coupled Air Quality and Boundary-Layer Meteorology in Western U.S. Basins during Winter: Design and Rationale for a Comprehensive Study

Wintertime episodes of high aerosol concentrations occur frequently in urban and agricultural basins and valleys worldwide. These episodes often arise following development of persistent cold-air pools (PCAPs) that limit mixing and modify chemistry. While field campaigns targeting either basin meteorology or wintertime pollution chemistry have been conducted, coupling between interconnected chemical and meteorological processes remains an insufficiently studied research area. Gaps in understanding the coupled chemical-meteorological interactions that drive high pollution events make identification of the most effective air-basin specific emission control strategies challenging. To address this, a September 2019 workshop occurred with the goal of planning a future research campaign to investigate air quality in Western U.S. basins. Approximately 120 people participated, representing 50 institutions and 5 countries. Workshop participants outlined the rationale and design for a comprehensive wintertime study that would couple atmospheric chemistry and boundary-layer and complex-terrain meteorology within western U.S. basins. Participants concluded the study should focus on two regions with contrasting aerosol chemistry: three populated valleys within Utah (Salt Lake, Utah, and Cache Valleys) and the San Joaquin Valley in California. This paper describes the scientific rationale for a campaign that will acquire chemical and meteorological datasets using airborne platforms with extensive range, coupled to surface-based measurements focusing on sampling within the near-surface boundary layer, and transport and mixing processes within this layer, with high vertical resolution at a number of representative sites. No prior wintertime basin-focused campaign has provided the breadth of observations necessary to characterize the meteorological-chemical linkages outlined here, nor to validate complex processes within coupled atmosphere-chemistry models.

Pathways for wintertime deposition of anthropogenic light-absorbing particles on the Central Andes cryosphere

Ice and snow in the Central Andes contain significant amounts of light-absorbing particles such as black carbon. The consequent accelerated melting of the cryosphere is not only a threat from a climate perspective but also for water resources and snow-dependent species and activities, worsened by the mega-drought affecting the region since the last decade. Given its proximity to the Andes, emissions from the Metropolitan Area of Santiago, Chile, are believed to be among the main contributors to deposition on glaciers. However, no evidence backs such an assertion, especially given the usually subsident and stable conditions in wintertime, when the snowpack is at its maximum extent. Based on high-resolution chemistry-transport modeling with WRF-CHIMERE, the present work shows that, for the month of July 2015, up to 40% of black carbon dry deposition on snow or ice covered areas in the Central Andes downwind from the Metropolitan area can be attributed to emissions from Santiago. Through the analysis of aerosol tracers we determine (i) that the areas of the Metropolitan Area where emissions matter most when it comes to export towards glaciers are located in Eastern Santiago near the foothills of the Andes, (ii) the crucial role of the network of Andean valleys that channels pollutants up to remote locations near glaciers, following gentle slopes. A direct corollary is that severe urban pollution, and deposition of impurities on the Andes, are anti-correlated phenomena. Finally, a two-variable meteorological index is developed that accounts for the dynamics of aerosol export towards the Andes, based on the zonal wind speed over the urban area, and the vertical diffusion coefficient in the valleys close to ice and snow covered terrain. Numerous large urban areas are found along the Andes so that the processes studied here can shed light on similar investigations for other glaciers-dependent Andean regions.

Emissions vs. turbulence and atmospheric stability: A study of their relative importance in determining air pollutant concentrations

Air pollution in the urban environment is a major concern. The ambient concentrations depend on the levels of transboundary imported pollution, the intensity of local sources and the prevailing atmospheric conditions. This work studies the relative impact of two atmospheric variables-atmospheric stability and regional scale turbulence-in determining the air pollution concentrations. We considered a setting (downtown Haifa, Israel) impacted by a large variety of sources, emitting pollutants with different chemical attributes and atmospheric life times. We found that traffic accounts for most of the locally produced pollution in the study location. However, the meteorological factors can overwhelm its impact and dictate the concentrations. The switch from stable to convective conditions and the more vigorous daytime wind are associated with a premature end of the morning peak concentrations that result from rush hour emissions of NO_x, Black Carbon (BC) and ultra-fine particles. It results in daytime concentration which are lower than (winter) or equal to (summer) those at night, in spite of the much lower night-time traffic volumes. Similar, albeit weaker, impact was detected in the benzene and toluene concentrations. Sources outside the study area are responsible for most of the CO, PM₁ and PM_2.5 concentrations but during winter nights, characterised by strong atmospheric stability and low turbulence, their concentrations are elevated due to the local emissions. We developed a diagnostic statistical nonlinear model for the pollutant concentrations, which points to a stronger association of the atmospheric stability with the concentrations during stable conditions but turbulence dominating during convective conditions. Our findings explain the relatively low overall concentrations of locally emitted pollutants in the study area but warn of the potential for high concentrations during night-time in locations with comparable meteorological conditions.

In vivo and in vitro inflammatory responses to fine particulate matter (PM2.5) from China and California

Ambient PM2.5 was collected during the winter season from Taiyuan, Shanxi, China; Jinan, Shandong, China; and Sacramento, California, USA, and used to create PMSX, PMSD, and PMCA extracts, respectively. Time-lag experiments were performed to explore the in vivo and in vitro toxicity of the PM extracts. In vivo inflammatory lung responses were assessed in BALB/c mice using a single oropharyngeal aspiration (OPA) of PM extract or vehicle (CTRL) on Day 0. Necropsies were performed on Days 1, 2, and 4 post-OPA, and pulmonary effects were determined using bronchoalveolar lavage (BAL) and histopathology. On Day 1, BAL neutrophils were significantly elevated in all PM- versus CTRL-exposed mice, with PMCA producing the strongest response. However, histopathological scoring showed greater alveolar and perivascular effects in PMSX-exposed mice compared to all three other groups. By Day 4, BAL neutrophilia and tissue inflammation were resolved, similar across all groups. In vitro effects were examined in human HepG2 hepatocytes, and U937 cells following 6, 24, or 48 h of exposure to PM extract or DMSO (control). Luciferase reporter and quantitative polymerase chain reaction assays were used to determine in vitro effects on aryl hydrocarbon receptor (AhR) activation and gene transcription, respectively. Though all three PM extracts activated AhR, PMSX produced the greatest increases in AhR activation, and mRNA levels of cyclooxygenase-2, cytochrome P450, interleukin (IL)-8, and interleukin (IL)-1β. These effects were assumed to result from a greater abundance of polycyclic aromatic hydrocarbons (PAHs) in PMSX compared to PMSD and PMCA.

Temperature inversions in severe polluted days derived from radiosonde data in North China from 2011 to 2016

Temperature inversion tends to inhibit the transfer of momentum, heat and moisture in the atmospheric boundary layer, which is often accompanied by severe air pollution. Recently, severe haze pollution has frequently occurred in North China. In this study, the characteristics of temperature inversion on severe polluted days (SPDs) in Beijing were investigated by using radiosonde data with standard pressure levels from 2011 to 2016. Both surface-based inversion (SI) and elevated inversion (EI) were analyzed. 93% of the SPDs were accompanied by temperature inversion, most of which occurred in wintertime. Annual frequency of SI (F_SI) and EI (F_EI) showed slight fluctuations with mean value of 0.18 and 0.67, respectively. Overall, the annual SI was stronger and deeper than annual EI. Seasonally, the SI was most frequent (0.39) in autumn, in contrast to EI that occurred most frequently (0.95) in summer. Both SI and EI were weakest in summer and strongest in winter. Average monthly SI strength was about 0.38 °C in summer and 2.40 °C in winter, average monthly EI strength was about 0.64 °C in summer and 2.20 °C in winter. The average monthly SI and EI were deepest in winter and shallowest in summer. SI depth were 778 m and 221 m in winter and summer, EI were 630 m and 336 m in winter and summer. The substantially strong liner relationship was found between seasonal inversion strength and PM_2.5 concentration, and the inversion strength was found to be better compared with the inversion depth at predicting the PM_2.5 concentration during SPDs. Obvious lower air outflow and turbulent kinetic energy were found in SPDs compared to non-SPDs, which indicated weaker turbulence in SPDs. Future efforts should focus on accurate model simulations of temperature inversions in SPDs.

Differential pulmonary effects of wintertime California and China particulate matter in healthy young mice

Airborne particulate matter (PM) is associated with adverse cardiorespiratory effects. To better understand source-orientated PM toxicity, a comparative study of the biological effects of fine PM (diameter≤2.5μm, PM_2.5) collected during the winter season from Shanxi Province, China, and the Central Valley, California, United States, was conducted. The overarching hypothesis for this study was to test whether the chemical composition of PM on an equal mass basis from two urban areas, one in China and one in California, can lead to significantly different effects of acute toxicity and inflammation in the lungs of healthy young mice. Male, 8-week old BALB/C mice received a single 50μg dose of vehicle, Taiyuan PM or Sacramento PM by oropharyngeal aspiration and were sacrificed 24h later. Bronchoalveolar lavage, ELISA and histopathology were performed along with chemical analysis of PM composition. Sacramento PM had a greater proportion of oxidized organic material, significantly increased neutrophil numbers and elevated CXCL-1 and TNF-α protein levels compared to the Taiyuan PM. The findings suggest that Sacramento PM_2.5 was associated with a greater inflammatory response compared to that of Taiyuan PM_2.5 that may be due to a higher oxidice. Male, 8-week old BALB/C mice received a single 50μg dose of vehicle, Taiyuan PM or Sacramento PM by oropharyngeal aspiration and were sacrificed 24h later. Bronchoalveolar lavage, ELISA and histopathology were performed along with chemical analysis of PM composition. Sacramento PM had a greater proportion of oxidized organic material, significantly increased neutrophil numbers and elevated CXCL-1 and TNF-α protein levels compared to the Taiyuan PM. The findings suggest that Sacramento PM_2.5 was associated with a greater inflammatory response compared to that of Taiyuan PM_2.5 that may be due to a higher oxidized state of organic carbon and copper content.