Atmospheric Boundary Layer Research Articles

Influence of clouds on planetary boundary layer height: A comparative study and factors analysis

Influence of clouds on planetary boundary layer height: A comparative study and factors analysis

Effects of valley topography on ozone pollution in the Lanzhou valley: A numerical case study

Valley topography is recognized for its role in constraining pollutant dispersion, which frequently results in elevated pollutant concentrations within valley regions. However, the specific mechanisms by which valley topography influences daytime ozone (O3) production, nighttime O3 depletion, and diurnal variations in O3 concentrations remain inadequately understood. This study employs the online WRF-Chem air quality model to conduct sensitivity analyses, examining the effects of valley topography on summer O3 concentrations in Lanzhou, a valley city in western China. The results indicate that valley topography generally reduces surface temperature and wind speed while also lowering the planetary boundary layer height (PBLH), which ultimately leads to increased concentrations of primary pollutants. Nevertheless, the impact of valley terrain on O3 is time-dependent, with concentrations decreasing during the day and increasing at night. In Lanzhou, valley topography contributes to a 4.4% reduction in daytime O3 concentrations. This reduction is largely due to the blocking of solar radiation by surrounding mountains, which results in a 7% decrease in shortwave radiation (SR) and a 17% decline in PBLH, subsequently limiting O3 chemical production. Although valley topography restricts pollutant dispersion, the overall effect during the day is a net decrease in O3 levels. In contrast, nighttime valley topography leads to a notable 19.8% increase in O3 concentrations. This increase is driven by mountain breezes transporting O3-rich air from the slopes into urban areas, along with a 67.4% reduction in nitrogen oxide (NO) levels. Despite intensified NO titration within the valley, the combined effect of these local wind patterns contributes to elevated O3 concentrations at night. These findings offer valuable insights into the unique factors contributing to urban O3 pollution in areas with complex valley topography.

Effects of oncoming wind speed and turbulence intensity on wind characteristics of a simplified hill and ridge

Supertall buildings and long-span bridges are significantly affected by wind-induced vibrations, and the wind fields in mountainous areas are highly complex and influenced by the oncoming wind speed and turbulence intensity. To accurately determine the variation patterns of wind characteristics in mountainous areas under different oncoming wind speeds and turbulence intensities, large eddy simulation (LES) was employed to analyze wind fields over simplified hill and ridge models. By setting different basic wind speeds (5, 10, 15, and 20 m/s) and turbulence intensities (1%, 3%, 5%, and 10%) at the inlet, the variation patterns of wind characteristics over the simplified hill and ridge under atmospheric boundary layer inflow were studied, revealing wind field flow mechanisms. The results indicate that the wind characteristics on the leeward side of the simplified hill and ridge are significantly influenced by the oncoming wind speed and turbulence intensity. Increasing the oncoming wind speed and turbulence intensity leads to decreased wind profile deceleration, reduced changes in wind direction and attack angles, and increased wind speed amplification factor. In addition, the turbulence fluctuations, power spectra, and coherence function between two points on the leeward side increase with the oncoming wind speed and turbulence intensity. The turbulent integral scale decreases with an increase in the oncoming wind speed and turbulence intensity. As the wind speed and turbulence increase, the size of the recirculation bubble gradually decreases, with its center moving closer to the wall surface. In the vorticity field, the number of smaller-scale three-dimensional turbulent vortices near the hill and ridge increases. These differences in flow characteristics are the fundamental causes of the changes in wind characteristics. High wind speeds and turbulence intensities typically result in higher kurtosis and skewness, with significant non-Gaussian characteristics in the fluctuating wind. Traditional wind load design specifications for building architecture based on a Gaussian distribution may not be applicable to mountainous terrain. In practical engineering, the influences of the oncoming wind speed and turbulence intensity on wind characteristics should be fully considered.

Simulation and modeling of wind farms in baroclinic atmospheric boundary layers

Simulation and modeling of wind farms in baroclinic atmospheric boundary layers

Effects of Attached Afterbody Shapes on Wind Forces on Rectangular Tall Buildings

ABSTRACTThe knowledge of aerodynamic forces has important implications for the wind‐resistant design of tall buildings. At present, the aerodynamic characteristics of conventional rectangular‐shaped tall buildings have been extensively studied, while those of irregular‐shaped tall buildings have received relatively less attention. Therefore, in the current study, several tall building models are designed to investigate the aerodynamic characteristics of a rectangular tall building with and without attaching different shapes of afterbody. The influences of the afterbody shapes (i.e., semicircle, rectangle, and triangle attached to a rectangular tall building called the basic model in this paper), atmospheric boundary layer (ABL) flows, and models' aspect ratios on the aerodynamic forces are analyzed and discussed in detail based on wind tunnel test of pressure measurements on the models. It is shown that the attached afterbodies can significantly increase the local fluctuating lift coefficients and base fluctuating moment coefficients comparing to those of the basic model. Moreover, a larger aspect ratio causes the increase of local fluctuating lift coefficients and base fluctuating moment coefficients of the basic model attached with rectangular afterbody in suburban terrain. On the other hand, the above increases are strongly attenuated or even suppressed by the ABL flow over urban terrain. The results of this study are expected to provide useful information for the wind‐resistant design of irregular‐shaped tall buildings.

Investigating uncertainties in air quality models used in GMAP/SIJAQ 2021 field campaign: General performance of different models and ensemble results

The international field campaign, GMAP/SIJAQ 2021, was conducted in Korea from October 18th to November 25th to enhance the performance and validation of the Geostationary Environment Monitoring Spectrometer (GEMS) products algorithm and obtain a better understanding of the current air pollution status of the Korean Peninsula. Five chemical transport models (CTMs), including CMAQ, CMAQ-GIST, CAMx, WRF-Chem, and WRF GEOS-Chem, were utilized during the campaign to assist in organizing the observation plan and identifying changes in pollutant concentrations and their spatiotemporal distribution in Korea following the Korea–United States Air Quality (KORUS-AQ) 2016. In this study, we evaluated the forecasting performance, strengths, and limitations of these five CTMs and their ensemble in simulating air quality. Intensive measurement data and intercomparisons were employed to explain discrepancies between observed and simulated results. A comparison of the CTM ensemble results for PM2.5 and various gaseous pollutants between the current GMAP/SIJAQ 2021 and previous KORUS-AQ 2016 campaigns showed the R-value for the total mass PM2.5 concentration increased from 0.88 to 0.94. This improvement is related to CTM updates, including the emission inventory and better reproductions of the concentrations of gaseous species. However, the models consistently underestimated carbon monoxide (CO) concentrations, similar to the results from KORUS-AQ. This finding still suggests a further challenge that requires consideration of missing anthropogenic sources. The results of the ensemble model agreed well with the chemical composition of PM2.5 observed at the intensive monitoring station. However, for NO3− and NH4+, discrepancies were primarily due to inaccuracies in the meteorological inputs, such as precipitation, relative humidity (RH), and nighttime planetary boundary layer height (PBLH) in the CTMs. Hence, all models overestimated the concentration of elemental carbon (EC), therefore, it is necessary to revise EC emissions in the SIJAQv2 inventory, as these apply to unusual levels recorded in Seoul during the reference year of 2018.

Elucidating key factors in regulating budgets of ozone and its precursors in atmospheric boundary layer

The vertical variations and key drivers of ozone and its precursors, namely NOx and VOCs, in the atmospheric boundary layer, have vital impacts on surface ozone budgets but are poorly understood so far. Using online gradient measurements from a 356 m tower, we obtained continuous vertical profiles of ozone and its precursors, which exhibited strong gradients throughout the day. In the daytime, the vertical gradients of ozone precursors are significantly regulated by reactions with OH radicals. At night, our observations confirmed more intense VOC reactions with NO3 radicals in the residual layer than in the boundary layer. Additionally, we found that residual layer entrainment could contribute to over half of the boundary-layer ozone enhancements in the morning periods. Our results underscore the importance of considering vertical changes of ozone and its precursors in the atmospheric boundary layer when developing future ozone mitigation strategies.

Impact of the Indian Ocean Dipole Mode on Planetary Boundary Layer Ozone in China

AbstractThe Indian Ocean Dipole (IOD) mode exerts distinct impacts on the climate in China and can further affect tropospheric ozone. Using long‐term GEOS‐Chem simulations, we found distinct changes in planetary boundary layer ozone throughout China during positive and negative phases of IOD. In summer, ozone shows synchronized increases except in southern China during positive IOD; the ozone increases are dominated by chemical production and transport in northern and western China, respectively. The increased precursor from biogenic emissions contributed to ozone chemical formation in the northern region, and the increased precipitation and decreased solar radiation hindered ozone production in southern China. Ozone changes show good symmetry over most regions during negative IOD. In autumn, the ozone reduction in southern China shares the same reason as summer, while the chemical increase over northern China is affected more by changes in solar radiation and relative humidity than in the precursor emission.

Light Absorption Properties of Brown Carbon Aerosol During Winter at a Polluted Rural Site in the North China Plain

Brown carbon aerosols (BrC), a subfraction of organic aerosols, significantly influence the atmospheric environment, climate and human health. The North China Plain (NCP) is a hotspot for BrC research in China, yet our understanding of the optical properties of BrC in rural regions is still very limited. In this study, we characterize the chemical components and light absorption of BrC at a rural site during winter in the NCP. The average mass concentration of PM1 is 135.1 ± 82.3 μg/m3; organics and nitrate are the main components of PM1. The absorption coefficient of BrC (babs,BrC) is 53.6 ± 45.7 Mm−1, accounting for 39.5 ± 10.2% of the total light absorption at 370 nm. Diurnal variations reveal that the babs,BrC and organics are lower in the afternoon, attributed to the evolution of planetary boundary layers. BrC is mainly emitted locally, and both the aqueous phase and the photooxidation reactions can increase babs,BrC. Notably, the babs,BrC is reduced when RH > 65%. During foggy conditions, reactions in the aqueous phase facilitate the formation of secondary components and contribute to the bleaching of BrC. This process ultimately causes a decrease in both the absorption Ångström exponent (AAE) and the mass absorption efficiency (MAE). In contrast, the babs,BrC, along with AAE and MAE, rise significantly due to substantial primary emissions. This study enhances our understanding of the light absorption of BrC in rural polluted regions of the NCP.

Effects of Inflow Turbulent Coherent Structures on a Small-Scale Wind Turbine in the Atmospheric Boundary Layer of Long Corridor Terrains: An Example Study in the Hexi Corridor

Considering its Venturi effect, long corridor terrain is considered as one of the outstanding wind-energy resources, but has not been developed due to unclear inflow turbulence effects on wind turbines. To advance the wind-power technology in long corridors and reveal the physics behind it, we conducted a series of data-fusion analyses combining field measurements in the Hexi Corridor, Gansu, China, and numerical simulations and summarize the effort here. In this study, the classical spectrum for the atmospheric boundary layer for wind-energy research, the IEC Kaimal spectrum (also known as IECKwind, International Elector technical Commission 61400-1) cannot describe the wind profile in corridors. Here, we proposed a new spectrum to describe the wind in the Hexi Corridor by modifying the IEC Kaimal spectrum, and we call it the IECK-HX spectrum. The results demonstrate that it preferentially resides at lower frequencies (larger spatial scales) compared with IECKwind. Large-scale motions (LSMs) and very large-scale motions (VLSMs) coexist in the Hexi Corridor atmospheric boundary layer, which can be well described by the IECK-HX spectrum, and these two dominate the large fluctuations in the power, thrust, and blade root flapwise moment of wind turbines, while the high-frequency small-scale coherent structures mainly cause high-frequency fluctuations in wind-turbine load. Furthermore, IEC Kaimal spectrum can not describe the VLSMs, and the generated flow presents significant fluctuations and thereby reduces the power output in high wind-speed regions. This study provides fundamental support for wind-energy scientists and engineers who are interested in wind energy in corridor terrains. Published by the American Physical Society 2024

Recommended coupling to global meteorological fields for long-term tracer simulations with WRF-GHG

Abstract. Atmospheric transport models are often used to simulate the distribution of greenhouse gases (GHGs). This can be in the context of forward modeling of tracer transport using surface–atmosphere fluxes or flux estimation through inverse modeling, whereby atmospheric tracer measurements are used in combination with simulated transport. In both of these contexts, transport errors can bias the results and should therefore be minimized. Here, we analyze transport uncertainties in the commonly used Weather Research and Forecasting (WRF) model coupled with the greenhouse gas module (WRF-GHG), enabling passive tracer transport simulation of CO2 and CH4. As a mesoscale numerical weather prediction model, WRF's transport is constrained by global meteorological fields via initialization and at the lateral boundaries of the domain of interest. These global fields were generated by assimilating various meteorological data to increase the accuracy of modeled fields. However, in limited-domain models like WRF, the winds in the center of the domain can deviate considerably from these driving fields. As the accuracy of the wind speed and direction is critical to the prediction of tracer transport, maintaining a close link to the observations across the simulation domain is desired. On the other hand, a link that is too close to the global meteorological fields can degrade performance at smaller spatial scales that are better represented by the mesoscale model. In this work, we evaluated the performance of strategies for keeping WRF's meteorology compatible with meteorological observations. To avoid the complexity of assimilating meteorological observations directly, two main strategies of coupling WRF-GHG with ERA5 meteorological reanalysis data were tested over a 2-month-long simulation over the European domain: (a) restarting the model daily with fresh initial conditions (ICs) from ERA5 and (b) nudging the atmospheric winds, temperatures, and moisture to those of ERA5 continuously throughout the simulation period, using WRF's built-in four-dimensional data assimilation (FDDA) in grid-nudging mode. Meteorological variables and simulated mole fractions of CO2 and CH4 were compared against observations to assess the performance of the different strategies. We also compared planetary boundary layer height (PBLH) with radiosonde-derived estimates. Either nudging or daily restarts similarly improved the meteorology and GHG transport in our simulations, with a small advantage of using both methods in combination. However, notable differences in soil moisture were found that accumulated over the course of the simulation when not using frequent restarts. The soil moisture drift had an impact on the simulated PBLH, presumably via changing the Bowen ratio. This is partially mitigated through nudging without requiring daily restarts, although not entirely alleviated. Soil moisture drift did not have a noticeable impact on GHG performance in our case, likely because it was dominated by other errors. However, since the PBLH is critical for accurately simulating GHG transport, we recommend transport model setups that tie soil moisture to observations. Our method of frequently re-initializing simulations with meteorological reanalysis fields proved suitable for this purpose.

Aerosol Vertical Turbulent Mass Flux Retrievals Through Novel Remote Sensing Algorithm

AbstractIntegrated measurements of aerosol, radiation, cloud, and turbulent transport in the planetary boundary layer (PBL) are essential for understanding and modeling climate and air quality. Here, we developed a new technique for the identification of convective turbulent regions and deriving the vertical distribution of aerosol turbulent mass fluxes within PBL. The algorithm uses retrievals from coherent Doppler lidars and a high spectral resolution lidar. The technique was applied to study particle mass fluxes over 2 months (November–December 2020) during the campaign conducted at the DOE Atmospheric Radiation Measurement Southern Great Plains (SGP) site in Lamont, Oklahoma. The algorithm developed here is capable of continuously deriving vertically resolved (curtains) aerosol mass fluxes. Our data analysis shows that at the site, the 30‐min averaged fluxes at 135 m above the surface were mainly positive (upward) at ∼1 μg m−2 s−1, suggesting that the surface is the primary source of the particle mass supplied to the boundary layer at the SGP site. Analyses of the individual case studies have revealed that not all the derived fluxes can be linked to surface emissions. Both positive and negative values in a range of ±5 μg m−2 s−1 can be caused by convective thermals interacting between the residual layer and the mixed layer and by rotation of the horizontal wind with the height. Large erroneous negative fluxes can also be caused by drizzling/precipitating clouds. We anticipate that the application of the current technique will lead to a more realistic representation of aerosol mass budgets and bidirectional mixing rates.

Oceanic eddy with submesoscale edge drives intense air-sea exchanges and beyond

Oceanic mesoscale eddies influence air-sea interaction and atmosphere dynamics through ventilating heat and moisture upward. However, whether the sea surface temperature (SST) gradient on the eddy edge could affect the heat and moisture release is still unknown because of the limited observations and coarse-resolution climate models. Using high-resolution atmospheric simulations, this study compares the atmospheric response to the mesoscale (~ 40 km) and submesoscale (~ 4 km) SST gradients at the edge of an eddy. Results show that submesoscale SST gradient drives stronger surface heat and moisture fluxes, enhancing the vertical mixing intensity by 2–3 times within and above the marine atmospheric boundary layer. As a result, one local precipitation event is found to be an order of magnitude larger overlying the eddy. Our findings highlight the importance of resolving oceanic submesoscale features for accurately predicting atmosphere dynamics and precipitation over the ocean.

In situ airborne measurements of atmospheric parameters and airborne sea surface properties related to offshore wind parks in the German Bight during the project X-Wakes

Abstract. Between 14 March 2020 and 11 September 2021, meteorological measurement flights were conducted above the German Bight in the framework of the project X-Wakes. The scope of the measurements was to study the transition of the wind field and atmospheric stability from the coast to the sea, to study the interaction of wind park wakes, and to study the large-scale modification of the marine atmospheric boundary layer by the presence of wind parks. In total, 49 measurement flights were performed with the research aircraft Dornier 128 of the Technische Universität (TU) Braunschweig during different seasons and different stability conditions. Seven of the flights in the time period from 24 to 30 July 2021 were organised using a second research aircraft, the Cessna F406 of TU Braunschweig. The instrumentation of both aircraft consisted of a nose boom with sensors for measuring the wind vector, temperature and humidity and, additionally, a surface temperature sensor. The Dornier 128 was further equipped with a laser scanner for deriving sea state properties and two downward-looking cameras in the visible and infrared wavelength range. The Cessna F406 was additionally equipped with shortwave and longwave broadband radiation sensors for measuring upward and downward solar and terrestrial radiation. A detailed overview of the aircraft, sensors, data post-processing and flight patterns is provided here. Further, averaged profiles of atmospheric parameters illustrate the range of conditions. The potential use of the dataset has been already shown by the first few publications. The data of both aircraft are publicly available on the world data centre PANGAEA at https://doi.org/10.1594/PANGAEA.955382 (Rausch et al., 2023a).

Influence of Large Eddy Generation Mechanisms on the Turbulent Flux Transport in the Unstable Atmosphere Boundary Layer

The turbulent transport dissimilarity between momentum and scalars and the transport similarity among scalars have been widely investigated in unstable atmospheric boundary layers (ABLs). Although buoyancy and mechanically driven turbulence, along with variations in scalar sources and sinks, are recognized as key factors influencing transport similarity, the specific roles of local thermal plume-generated and nonlocal bulk shear-generated large eddies under varying stability conditions are less explored. This study utilized over four years of eddy covariance data sampled 50 m above a complex suburban canopy to characterize the influence of buoyancy and wind shear on flux transport similarity in an unstable ABL. The time threshold τ method was applied to detect large coherent events, with wind shear enhancing their intensity, while buoyancy primarily affected the ejection–sweep asymmetry of scalars. The dynamics between buoyancy and wind shear were analyzed through separate momentum, heat, and joint transport events. The results show that strong wind shear enhances nonlocal large eddies, reducing momentum–heat transport similarity, whereas strong buoyancy supports localized turbulence. As stability varies, the shift between nonlocal and local eddies alters the trends in co-transport duration and intensity, revealing distinct patterns in the water vapor intensity from that of the sensible heat owing to local sources and sinks.

Nonlinear changes in urban heat island intensity, urban breeze intensity, and urban air pollutant concentration with roof albedo

This study systematically examines how the urban heat island (UHI) and urban breeze circulation (UBC) respond to an increase in roof albedo (αr) and its influence on urban air pollutant dispersion. For this, idealized ensemble simulations are performed using the Weather Research and Forecasting (WRF) model. The increase in αr from 0.20 to 0.65 decreases the UHI intensity, UBC intensity, and urban planetary boundary layer (PBL) height in the daytime (from 1200 to 1700 LST) by 47%, 36%, and 6%, respectively. As both UBC intensity and urban PBL height decrease, the daytime urban near-surface passive tracer concentration increases by 115%. The daytime UHI intensity, UBC intensity, and urban tracer concentration nonlinearly change with αr: For 0.10 ≤ αr < 0.80, the rates of changes in the UHI intensity, UBC intensity, and urban tracer concentration with αr overall increase as αr increases. For αr ≥ 0.80, the daytime roof surface temperature is notably lower than the daytime urban near-surface air temperature, the UHI intensity, UBC intensity, and urban tracer concentration very slightly changing with αr. This study provides insights into the associations between changes in roof surface temperature and roof surface energy fluxes with αr and those in UHI intensity.

Simulation of the Neutral Atmospheric Flow Using Multiscale Modeling: Comparative Studies for SimpleFoam and Fluent Solver

The reproduced planetary boundary layer (PBL) wind is commonly applied in downscaled simulations using commercial CFD codes with Reynolds-averaged Navier–Stokes (RANS) turbulence modeling. When using the turbulent inlets calculated by numerical weather prediction models (NWP), adjustments of the turbulence eddy viscosity closures and wall function formulations are concerned with maintaining the fully developed wind profiles specified at the inlet of CFD domains. The impact of these related configurations is worth discussing in engineering applications, especially when commercial codes restrict the internal modifications. This study evaluates the numerical performances of open-source OpenFOAM 2.3.0 and commercial Fluent 17.2 codes as supplementary scientific comparisons. This contribution focuses on the modified turbulence closures to incorporate turbulent profiles produced from mesoscale PBL parameterizations and the modified wall treatments relating to the roughness length. The near-ground flow features are evaluated by selecting the flat terrains and the classical Askervein benchmark case. The improvement in near-ground wind flow under the downscaled framework shows satisfactory performance in the open-source CFD platform. This contributes to engineers realizing the micro-siting of wind turbines and quantifying terrain-induced speed-up phenomena under the scope of wind-resistant design.

The atmospheric boundary layer: a review of current challenges and a new generation of machine learning techniques

Atmospheric boundary layer (ABL) structure and dynamics are important aspects to consider in human health. The ABL is characterized by a high degree of spatial and temporal variability that hinders their understanding. This paper aims to provide a comprehensive overview of machine learning (ML) methodologies, encompassing deep learning and ensemble approaches, within the scope of ABL research. The goal is to highlight the challenges and opportunities of using ML in turbulence modeling and parameterization in areas such as atmospheric pollution, meteorology, and renewable energy. The review emphasizes the validation of results to ensure their reliability and applicability. ML has proven to be a valuable tool for understanding and predicting how ABL spatial and seasonal variability affects pollutant dispersion and public health. In addition, it has been demonstrated that ML can be used to estimate several variables and parameters, such as ABL height, making it a promising approach to enhance air quality management and urban planning.

Comparing MJO Intensity over the Tropical Western Pacific during Mega and Equatorial La Niña Winters

Abstract This study investigated variations in the Madden–Julian oscillation (MJO) behavior between two types of La Niña winters: mega and equatorial La Niñas. Results of this work show that in contrast to mega conditions, accompanied by more abundant intraseasonal column-integrated moisture anomalies over the planetary boundary layer, intensities of intraseasonal outgoing longwave radiation anomalies are stronger over the tropical western Pacific (WP) at MJO phases 5–6 under equatorial conditions. The occurrence of the moisture anomaly change averaged over the tropical WP is supported by a distinct moisture tendency difference. Moisture budget and multiscale interaction diagnoses underscore the pivotal effect of an area-averaged vertical gradient change in low-frequency background moisture in driving such tendency change. Considering notable MJO convection variations, the teleconnections associated with MJO phases 5–6 over East Asia (EA) were also explored, including warmer surface air temperature anomalies and stronger positive geopotential height anomalies at 200 hPa on the intraseasonal time scale under equatorial conditions. Results from a linear baroclinic model demonstrate that such teleconnection changes could be effectively explained by the linear response to the MJO’s diabatic heating anomaly. Roles of mean state and MJO itself variations in the linear response change are also discussed. These findings provide novel insights into MJO activity and offer potential improvements for subseasonal forecasting in EA. Significance Statement The relationship between El Niño–Southern Oscillation (ENSO) and MJO has been extensively explored recently. However, variations in the MJO behavior and its climate effects under mega and equatorial La Niña winters have not been thoroughly investigated. In this work, we utilized reanalysis datasets and an idealized linear baroclinic model to address these questions. We observed more robust and abundant intraseasonal convection activity and planetary boundary layer–integrated moisture anomaly averaged over the tropical WP at MJO phases 5–6 under equatorial conditions than mega conditions, which is closely linked with the vertical gradient change in low-frequency background moisture. Additionally, MJO-related teleconnections also exhibit some changes. This work may provide a new perspective on understanding the relationship between the MJO and ENSO and offer potential improvements for the subseasonal forecast.

The Effects of Air Quality and the Impact of Climate Conditions on the First COVID-19 Wave in Wuhan and Four European Metropolitan Regions

This paper investigates the impact of air quality and climate variability during the first wave of COVID-19 associated with accelerated transmission and lethality in Wuhan in China and four European metropolises (Milan, Madrid, London, and Bucharest). For the period 1 January–15 June 2020, including the COVID-19 pre-lockdown, lockdown, and beyond periods, this study used a synergy of in situ and derived satellite time-series data analyses, investigating the daily average inhalable gaseous pollutants ozone (O3), nitrogen dioxide (NO2), and particulate matter in two size fractions (PM2.5 and PM10) together with the Air Quality Index (AQI), total Aerosol Optical Depth (AOD) at 550 nm, and climate variables (air temperature at 2 m height, relative humidity, wind speed, and Planetary Boundary Layer height). Applied statistical methods and cross-correlation tests involving multiple datasets of the main air pollutants (inhalable PM2.5 and PM10 and NO2), AQI, and aerosol loading AOD revealed a direct positive correlation with the spread and severity of COVID-19. Like in other cities worldwide, during the first-wave COVID-19 lockdown, due to the implemented restrictions on human-related emissions, there was a significant decrease in most air pollutant concentrations (PM2.5, PM10, and NO2), AQI, and AOD but a high increase in ground-level O3 in all selected metropolises. Also, this study found negative correlations of daily new COVID-19 cases (DNCs) with surface ozone level, air temperature at 2 m height, Planetary Boundary PBL heights, and wind speed intensity and positive correlations with relative humidity. The findings highlight the differential impacts of pandemic lockdowns on air quality in the investigated metropolises.