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The planetary boundary layer height (BLH) governs the vertical transport of mass, momentum and moisture between the surface and the free atmosphere, and thus its characterization is recognized as crucial for air quality, weather and climate. Although reanalysis products can provide important insight into the global view of BLH in a seamless way, the in situ observed BLH on a global scale remains poorly understood due to the lack of high-resolution (1-s or 2-s) radiosonde measurements. The present study attempts to establish a near-global BLH climatology at synoptic times (0000 and 1200 UTC) and in the daytime using high-resolution radiosonde measurements over 300 radiosonde sites worldwide for the period 2012 to 2019, which is then compared against the BLHs obtained from four reanalysis datasets, including ERA-5, MERRA-2, JRA-55, and NCEP-2. The variations of BLH exhibit large spatial and temporal dependence, and as a result the BLH maxima are generally discerned over the regions such as Western United States and Western China, in which the balloon launch times mostly correspond to the afternoon. The diurnal variations of BLH are revealed with a peak at 1700 Local Solar Time (LST). The most promising reanalysis product is ERA-5, which underestimates BLH by around 130 m as compared to radiosondes. In addition, MERRA-2 is a well-established product and has an underestimation of around 160 m. JRA-55 and NCEP-2 might produce considerable additional uncertainties, with a much larger underestimation of up to 400 m. The largest bias in the reanalysis data appears over the Western United States and Western China and it might be attributed to the maximal BLH in the afternoon when the boundary layer has grown up. Statistical analyses further indicate that the biases of reanalysis BLH products are positively associated with orographic complexity, as well as the occurrence of static instability. To our best knowledge, this study presents the first near-global view of high-resolution radiosonde derived BLH and provides a quantitative assessment of the four frequently used reanalysis products. [ABSTRACT FROM AUTHOR]

The Printed Optical Particle Spectrometer (POPS) is an advanced and small low-cost, light-weight, and high-sensitivity optical particle counter (OPC), particularly designed for deployed on unpiloted aerial vehicles (UAVs) and balloon sondes. We report the performance of the POPS against a reference scanning mobility particle sizer (SMPS) and an airborne passive cavity aerosol spectrometer probe (PCASP) while the POPS is operated on the ground and also while operated on a quadcopter drone, a DJI Matrice 200 V2. This is the first such documented test of the performance of a POPS instrument on a UAV. We investigate the root mean square difference (RMSD) and mean absolute difference (MAD) in particle number concentrations (PNCs) when operating on the ground and on the Matrice 200. When windspeeds are less than 2.6 m/s, we find only modest differences in the RMSDs and MADs of 2.4 % and 2.3 % respectively when operating on the ground, and to 5 % and 3 % when operating at 10m altitude. When windspeeds are greater than 2.6 m/s but less than 7.7 m/s the RMSDs and MADs increase to 10.2 % and 7.8 % respectively when operating on the ground, and 26.2 % and 19.1 %, respectively when operating at 10m altitude. No statistical difference in PNCs was detected when operating on the UAV in either ascent or descent. We also find size distributions of aerosols in the accumulation mode (here defined by diameter, d, where 0.1 = d = 1 µm) are relatively consistent between measurements at the surface and measurements at 10m altitude with RMSD and MAD of less than 21.6 % and 15.7 %, respectively. However, the differences between coarse mode (here defined by d > 1 µm) are universally larger than those measured at the surface with a RMSD and MAD approaching 49.5 % and 40.4 %. Our results suggest that the impact of the UAV rotors on the POPS does not unduly affect the performance of the POPS for wind speed less than 2.6 m/s, but when operating under higher wind speed of up to 7.6 m/s, larger discrepancies are noted. In addition to this, it appears that the POPS measures sub-micron aerosol particles more accurately than super-micron aerosol particles when airborne on the UAV. These measurements lay the foundations for determining the magnitude of potential errors that might be introduced into measured aerosol particle size distributions and concentrations owing to the turbulence created by the rotors on the UAV. [ABSTRACT FROM AUTHOR]

The cities of Chengdu, Deyang, and Mianyang in the northwest Sichuan Basin are part of a rapidly developing urban agglomeration adjoining the eastern slopes of the Tibetan Plateau. Heavy air pollution events have frequently occurred over the cities in recent decade, but the effects of meteorological conditions on these pollution events are unclear. We explored the effects of weather systems on winter heavy air pollution from 1 January 2006 to 31 December 2012 and from 1 January 2014 to 28 February 2017. Ten heavy air pollution events occurred during the research period and eight of these took place while the region was affected by a dry low-pressure system at 700 hPa. When the urban agglomeration was in front of the low-pressure system and the weather conditions were controlled by a warm southerly air flow, and a strong temperature inversion appeared above the atmospheric boundary layer acting as a lid. Forced by this strong inversion layer, the local secondary circulation was confined within the atmospheric boundary layer and the horizontal wind speed in the lower troposphere was low. As a result, vertical mixing and horizontal dispersion in the atmosphere were poor, favoring the formation of heavy air pollution events. After the low-pressure system had transited over the region, the weather conditions in the urban agglomeration were controlled by a dry and cold air flow from the northwest at 700 hPa. The strong inversion layer gradually dissipated, the secondary circulation enhanced and uplifted, and the horizontal wind speed in the lower troposphere also increased, resulting in a sharp decrease in the concentration of air pollutants. The strong inversion layer above the atmospheric boundary layer induced by the low-pressure system at 700 hPa thus played a key role in the formation of heavy air pollution during the winter months in this urban agglomeration. This study provides scientific insights for forecasting heavy air pollution in this region of China. [ABSTRACT FROM AUTHOR]

The environmental impact of diesel-fueled buses can potentially be reduced by the adoption of alternative propulsion technologies such as lean-burn compressed natural gas (LB-CNG) or hybrid electric buses (HEB), and emissions control strategies such as a continuously regenerating trap (CRT), exhaust gas recirculation (EGR), or selective catalytic reduction with trap (SCRT). This study assessed the environmental costs and benefits of these bus technologies in Greater London relative to the existing fleet and characterized emissions changes due to alternative technologies. We found a >30% increase in CO2 equivalent (CO2e) emissions for CNG buses, a <5% change for exhaust treatment scenarios, and a 13% (90% confidence interval 3.8-20.9%) reduction for HEB relative to baseline CO2e emissions. A multiscale regional chemistry-transport model quantified the impact of alternative bus technologies on air quality, which was then related to premature mortality risk. We found the largest decrease in population exposure (about 83%) to particulate matter (PM2.5) occurred with LB-CNG buses. Monetized environmental and investment costs relative to the baseline gave estimated net present cost of LB-CNG or HEB conversion to be $187 million ($73 million to $301 million) or $36 million ($-25 million to $102 million), respectively, while EGR or SCRT estimated net present costs were $19 million ($7 million to $32 million) or $15 million ($8 million to $23 million), respectively. [ABSTRACT FROM AUTHOR]

Air pollution has been recognized as a global issue, through adverse effects on environment and health. While vertical atmospheric processes substantially affect urban air pollution, traditional epidemiological research using Land-Use Regression (LUR) modeling usually focused on ground-level attributes without considering upper-level atmospheric conditions. This study aimed to integrate Doppler LiDAR and machine learning techniques into LUR models (LURF-LiDAR) to comprehensively evaluate urban air pollution in Hong Kong, and to assess the complex interactions between vertical atmospheric processes and urban air pollution from long-term (i.e., annual) and short-term (i.e., two air pollution episodes) views in 2021. The results demonstrated significant improvements in model performance, achieving a CV R 2 of 0.81 (95 % CI: 0.75-0.86) for the long-term PM 2.5 prediction and 0.90 (95 % CI: 0.87-0.91) for a short-term one. Approximately 69 % of ground-level air pollution arose from the mixing of ground- and lower-level (105 m-225 m) particles, while 21 % was associated with upper-level (825 m-945 m) atmospheric processes. The identified transboundary air pollution (TAP) layer was located at ~900 m above the ground. The identified Episode one (E1: 7 Jan-22 Jan) was induced by the accumulation of local emissions under stable atmospheric conditions, whereas Episode two (E2: 13 Dec-24 Dec) was regulated by TAP under instable and turbulent conditions. Our improved air quality model is accurate and comprehensive with high interpretability for supporting urban planning and air quality policies., (Copyright © 2024 Elsevier B.V. All rights reserved.)

While previous studies have investigated haze events over Southeast Asia (SEA), local and transboundary contributions of various emission sources to haze months over the entire SEA have yet to be assessed comprehensively and systematically. We utilized the Particle Source Apportionment Technique (PSAT) to quantify the spatial local, transboundary, and sectoral contributions to PM 2.5 over SEA during the haze months of 2015-2019. Results show that local emission contributions accounted for 56.1 % ~ 94.2 % of PM 2.5 in Indonesia, Philippines, Vietnam, and Thailand. Transboundary contributions (23.1 % ~ 57.6 %) from Indonesia notably influenced maritime SEA. Vietnam (15.6 % ~ 39.1 %) and super-regional (17.0 % ~ 34.3 %) contributions outside the SEA exerted remarkable impacts on mainland SEA. Among different sectors, fire emissions contributed the most to PM 2.5 over maritime SEA (23.0 % ~ 68.6 %) during the studied haze months, whereas residential and other emissions were the main contributors to mainland SEA (27.2 % ~ 36.7 %). Regarding the source species, primary PM 2.5 accounted for the majority of PM 2.5 . VOC and SO 2 composed most of the secondary PM 2.5 due to massive VOC emissions in the region and the priority reaction of NH 3 with sulfuric acid (H 2 SO 4 ) to form ammonium sulfate. Besides, the intensified haze months in Oct 2015 and Sep 2019 were characterized by more intensive fire emissions in the region and the climatic variability-induced meteorological effects that provided favorable condition for transboundary air pollution (56.9 % and 44.9 %, respectively, for maritime SEA, as well as 46.0 % and 37.7 %, respectively, for mainland SEA in the two studied haze months). The haze months can be attributed to the notable drought conditions amidst global climatic phenomena such as El Niño and positive Indian Ocean Dipole (IOD) in Oct 2015 and Sep 2019, respectively., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Air pollution and extreme heat have been responsible for more than a million deaths in China every year, especially in densely urbanized regions. While previous studies intensively evaluated air pollution episodes and extreme heat events, a limited number of studies comprehensively assessed atmospheric hot-and-polluted-episodes (HPE) - an episode with simultaneously high levels of air pollution and temperature - which have potential adverse synergic impacts on human health. This study focused on the Pearl River Delta (PRD) region of China due to its high temperature in summer and poor air quality throughout a year. We employed geostatistical downscaling to model meteorology at a spatial resolution of 1 km, and applied a machine learning algorithm (XGBoost) to estimate a high-resolution (1 km) daily concentration of particulate matter with an aerodynamic diameter ≤2.5 μm (PM 2.5 ) and ozone (O 3 ) for June to October over 20 years (2000-2019). Our results indicate an increasing trend (∼50%) in the frequency of HPE occurrence in the first decade (2000-2010). Conversely, the annual frequency of HPE occurrence reduced (16.7%), but its intensity increased during the second decade (2010-2019). The northern cities in the PRD region had higher levels of PM 2.5 and O 3 than their southern counterparts. During HPEs, regional daily PM 2.5 exceeded the World Health Organization (WHO) and Chinese guideline levels by 75% and 25%, respectively, while the O 3 exceeded the WHO O 3 standard by up to 69%. Overall, 567,063 (95% confidence interval (CI): 510,357-623,770) and 52,231 (95%CI: 26,116-78,346) excessive deaths were respectively attributable to exposure to PM 2.5 and O 3 in the PRD region. Our findings imply the necessity and urgency to formulate co-benefit policies to mitigate the region's air pollution and heat problems., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

While levels of particulate matters in the Pearl River Delta Region (PRD) show a significant reduction, ozone (O 3 ) has an opposite increasing trend, becoming the critical air quality target in this decade. Emission control strategies are typically formulated sector by sector, spatial variability in emissions reductions and health impacts of air pollutants may not be taken into account, affecting the overall effectiveness of control strategies. This study proposes an adjoint-based optimization framework to facilitate health-oriented O 3 control over PRD. The location-specific adjoint sensitivity coefficients, which reflect the spatiotemporal influences from emissions of nitrogen dioxide (NO x ) on O 3 health impacts, are combined with metaheuristic algorithms to minimize the O 3 -related premature mortalities over receptor regions. Using the proposed optimization methodology, the regional O 3 health benefits under current emission reduction policy can be increased by 16-27%. The results show that relatively larger NO x emissions reductions occurred at highly developed and populated areas. Particularly, significant reductions in NO x emissions are observed at Shenzhen and urban Guangzhou. Furthermore, implementing regional NO x emissions abatement has advantages to achieve an overall O 3 health benefits for all cities. The interregional influences of NO x emissions abatement between cities indicate a promising strategy of health-oriented O 3 control in PRD., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

While fine particulate matters are decreasing in the Pearl River Delta (PRD) region, the regional ozone (O 3 ) shows an increasing trend that affects human health, leading to an urgent need for scientific understanding of source-receptor relationship between O 3 and its precursor emissions given the changing background composition. We advanced and applied an adjoint air quality model to map contributions of individual O 3 precursor emission sources [nitrogen oxides (NO x ) and volatile organic compound (VOC)] at each location to annual regional O 3 concentrations and to identify the possible dominant influential pathways of emission sources to O 3 at different spatiotemporal scales. Additionally, we introduced the novel adjoint sensitivity approach to assess the relationship between precursor emissions and O 3 -induced premature mortality. Adjoint results show that Shenzhen was a major source contributor to regional O 3 throughout all seasons, of which 49.4% (3.8%) were from its NO x (VOC) emissions. Local emissions (within PRD) contributed to 83% of the regional O 3 whereas only ~54% of the estimated ~4000 regional O 3 -induced premature mortalities. The discrepancy between these two contributions was because O 3 -induced mortalities are dependent on not only O 3 concentration, but incident rate and population density. We also found that a city with low O 3 -induced mortalities could have significant emission contributions to health impact in the region since the transport pathways could be through transport of local O 3 or through transport of O 3 precursors that form regional O 3 thereafter. It is therefore necessary to formulate emission control policies from both air quality and public health perspectives, and it is also critical to have better understanding of influential pathways of emission sources to O 3 ., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare no competing financial interest.

Air pollution has become an adverse environmental problem in China, resulting in serious public health impacts. This study advanced and applied the CMAQ adjoint model to quantitatively assess the source-receptor relationships between surface ozone (O 3 ) changes over different receptor regions and precursor emissions across all locations in China. Five receptor regions were defined based on the administrative division, including northern China (NC), southern China (SC), Pearl River Delta region (PRD), Yangtz River Delta region (YRD), and Beijing-Tianjin-Hebei region (BTH). Our results identified the different influential pathways of atmospheric processes and emissions to O 3 pollution. We found that the atmospheric processes such as horizontal and vertical advection could offset the O 3 removal through chemical reactions in VOC-limited areas inside the receptor regions. In addition, O 3 pollution can be induced by transport of O 3 directly or its precursors. Our results of relative source contributions to O 3 show that transboundary O 3 pollution was significant in SC, NC and YRD, while the O 3 pollution in PRD and BTH were more contributed by local sources. Anhui, Hubei and Jiangsu provinces were the three largest source areas of NO x and VOC emissions to O 3 in SC (>52%) and YRD (>69%). NO x and VOC emissions from Tianjin and Beijing were the largest contributors to O 3 in NC (>34%) and BTH (>51%). PRD was the dominant source areas of NO x (>89%) and VOC emissions (~98%) to its own regional O 3 ., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Personal exposure and ambient fine particles (PM 2.5 ) measurements for 13 adult subjects (ages 19-57) were conducted in Hong Kong between April 2014 and June 2015. Six to 21 personal samples (mean = 19) per subject were obtained throughout the study period. Samples were analyzed for mass by gravimetric analysis, and 19 elements (from Na to Pb) were analyzed using X-Ray Fluorescence. Higher subject-specific correlations between personal and ambient sulfur (r s  = 0.92; p < 0.001) were found as compared to PM 2.5 mass (r s  = 0.79; p < 0.001) and other elements (0.06 < r s  < 0.86). Personal vs. ambient sulfur regression yielded an average exposure factor (F pex ) of 0.73 ± 0.02, supporting the use of sulfur as a surrogate to estimate personal exposure to PM 2.5 of ambient origin (E a ). E a accounted for 41-82% and 57-73% of total personal PM 2.5 exposures (P) by season and by subject, respectively. The importance of both E a and non-ambient exposures (E na , 11.2 ± 5.6 μg/m 3 ; 32.5 ± 10.9%) are noted. Mixed-effects models were applied to estimate the relationships between ambient PM 2.5 concentrations and their corresponding exposure variables (E a , P). Higher correlations for E a (0.90; p < 0.001) than for P (0.58; p < 0.01) were found. A calibration coefficient < 1 suggests an attenuation of 22% (ranging 16-28%) of the true effect estimates when using average ambient concentrations at central monitoring stations as surrogates for E a . Stationary ambient data can be used to assess population exposure only if PM exposure is dominated by E a ., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Personal monitoring for fine particulate matter (PM 2.5 ) was conducted for adults (48 subjects, 18-63years of age) in Hong Kong during the summer and winter of 2014-2015. All filters were analyzed for PM 2.5 mass and constituents (including carbonaceous aerosols, water-soluble ions, and elements). We found that season (p=0.02) and occupation (p<0.001) were significant factors affecting the strength of the personal-ambient PM 2.5 associations. We applied mixed-effects models to investigate the determinants of personal exposure to PM 2.5 mass and constituents, along with within- and between-individual variance components. Ambient PM 2.5 was the dominant predictor of (R 2 =0.12-0.59, p<0.01) and the largest contributor (>37.3%) to personal exposures for PM 2.5 mass and most components. For all subjects, a one-unit (2.72μg/m 3 ) increase in ambient PM 2.5 was associated with a 0.75μg/m 3 (95% CI: 0.59-0.94μg/m 3 ) increase in personal PM 2.5 exposure. The adjusted mixed-effects models included information extracted from individual's activity diaries as covariates. The results showed that season, occupation, time indoors at home, in transit, and cleaning were significant determinants for PM 2.5 components in personal exposure (R 2 β =0.06-0.63, p<0.05), contributing to 3.0-70.4% of the variability. For one-hour extra time spent at home, in transit, and cleaning an average increase of 1.7-3.6% (ammonium, sulfate, nitrate, sulfur), 2.7-12.3% (elemental carbon, ammonium, titanium, iron), and 8.7-19.4% (ammonium, magnesium ions, vanadium) in components of personal PM 2.5 were observed, respectively. In this research, the within-individual variance component dominated the total variability for all investigated exposure data except PM 2.5 and EC. Results from this study indicate that performing long-term personal monitoring is needed for examining the associations of mass and constituents of personal PM 2.5 with health outcomes in epidemiological studies by describing the impacts of individual-specific data on personal exposures., (Copyright © 2018 Elsevier B.V. All rights reserved.)