Low Planetary Boundary Layer Height Research Articles

Ozone pollution episodes and PBL height variation in the NYC urban and coastal areas during LISTOS 2019

In summer, frequent ozone (O3) episodes occur in New York City (NYC) and coastal areas along Long Island Sound (LIS) due to complex interplay of urban pollutants transport, land-sea breezes, and chemistry. The Long Island Sound Tropospheric Ozone Study (LISTOS) was carried out to address this issue; here we present an integrated analysis of the diurnal variation of planetary-boundary-layer height (PBLH) and O3 in the NYC urban and coastal area during LISTOS in summer 2019. A persistent O3 episode (July 28–31, 2019) is investigated which shows much higher O3 (90–120 ppb with 5-min average maximum) along the North-shore of Long Island (NLI) than the NYC urban and South-shore of Long Island (SLI). We evaluate the PBLH estimate with a multi-platform observation in the complex coastal environment, and compare the PBLH diurnal variation in the NYC urban to coastal areas. The results indicate lower PBLH in the mid-morning and noon in the NLI than the NYC urban area. With the simultaneously measured O3 and PBLH, the integrated O3 content or total content in the PBL defined as the O3 concentrations multiplied by PBLH show good consistency in the urban and NLI coast at noon on July 28–29, 2019. A dense O3 plume, rich in CO, aerosols, black carbon, HCHO and NOx appeared in the PBL as detected from the UMD Cessna research aircraft, satellite TROPOMI and ozonesonde; their coincidently large values imply the combined effects of urban plumes transport, lower PBLH and land-sea breezes in the NLI area. Finally, we evaluated the forecast of O3 and PBLH from the NOAA NAM-CMAQ (12 km grid) and WRF-Chem (1.3 km grid) models with the observations. Both models show good agreement with the observations at the NYC urban site; but at the NLI coastal site, the NAM-CMAQ model forecast indicates a timing bias for the maximum O3 at noon. These results provide insight into the impacts of the sea breeze and urban heat island on air quality.

Response of organic aerosol to Delhi's pollution control measures over the period 2011–2018

Some of the world's highest air pollution episodes occur in Delhi, India and studies have shown particulate matter (PM) is the leading air pollutant to cause adverse health effects on Delhi's population. It is therefore vital to chart sources of PM over long time periods to effectively identify trends, particularly as multiple air quality mitigation measures have been implemented in Delhi over the past 10 years but remain unevaluated. An automated offline aerosol mass spectrometry (AMS) method has been developed which has enabled high-throughput analysis of PM filters. This novel offline-AMS method uses an organic solvent mix of acetone and water to deliver high extraction recoveries of organic aerosol (OA) (95.4 ± 8.3%). Positive matrix factorisation (PMF) source apportionment was performed on the OA fraction extracted from PM10 filter samples collected in Delhi in 2011, 2015 and 2018 to provide snapshots of the responses of OA to changes in sources in Delhi. The nine factors of OA resolved by PMF group into four primary source categories: traffic, cooking, coal-combustion and burning-related (solid fuel or open burning). Burning-related OA made the largest contribution during the winter and post-monsoon, when total OA concentrations were at their highest. Annual mean burning-related OA concentrations declined by 47% between 2015 and 2018, likely associated with the 2015 ban on open waste burning and controls and incentives to reduce crop-residue burning. Compositional analysis of OA factors shows municipal waste burning tracers still present in 2018, indicating further scope to reduce burning-related OA. The closure of the two coal power stations, along with initiatives to decrease coal use in industry, businesses, and residential homes, resulted in a significant decrease (87%) in coal-combustion OA. This corresponds to a 17% reduction in total OA, which shows the effectiveness of these measures in reducing PM10. Increases in traffic OA appear to have been offset by the introduction of the Bharat stage emissions standards for vehicles as the increases do not reflect the rapid increase in registered vehicles. However, daytime restrictions on heavy goods vehicles (HGVs) entering the city is linked to large increases in PM10 during the winter and post-monsoon, likely because the large influx of diesel-engine HGVs during the early mornings and evenings is timed with a particularly low planetary boundary layer height that enhances surface concentrations.

Satellite data geoprocessing to estimate PM2.5 over the Megalopolis of Central Mexico

The Megalopolis of Central Mexico experiences high levels above the Official Mexican Standard (NOM) of PM2.5, leading to various respiratory diseases ranging from acute symptoms to chronic illnesses such as asthma and lung cancer. It is crucial to measure PM2.5 levels accurately to warn the public about the risks of exposure to particulate matter. Unfortunately, the Megalopolis of Central Mexico has a shortage of monitoring sites, limiting data availability. This study addresses this issue using satellite data to develop a multiple linear regression model. Our model uses aerosol optical depth (AOD), relative humidity (RH), temperature (T), the planetary boundary layer height (PBLH), and the normalized difference vegetation index (NDVI) as independent variables to estimate PM2.5 concentrations in the region under study. The relationship between AOD and PM2.5 concentrations was found to be strongly influenced by RH and T. However, this effect is compensated for by a low PBLH (< 400 m), which enables AOD and PM2.5 measurements to be similar in magnitude. Our findings have important implications for estimating PM2.5 concentrations using satellite data. This study could help improve air quality monitoring in the Megalopolis of Central Mexico by providing more spatial and temporal data on particle concentrations in the atmosphere.

The Effects of Planetary Boundary Layer Features on Air Pollution Based on ERA5 Data in East China

The planetary boundary layer (PBL) structure and its evolution can significantly affect air pollution. Here, the PBL’s characteristics and their association with air pollution were analyzed in Hefei, east China, using ERA5 reanalysis data, weather observations and air pollutant measurements from 2016 to 2021. In the near-surface level, air pollution was directly influenced by ground meteorological conditions, and high PM2.5 was normally related to weak wind speed, northwest wind anomalies, low temperature and high relative humidity. Moreover, in the trajectory analysis, air masses from the north and the northwest with short length played an important role in the high PM2.5 with pollutant transport within the PBL. Furthermore, high PM2.5 showed a tight dependence on PBL stratification. There was high temperature and relative humidity and low wind speed and PBL height within all PBL altitudes in the polluted condition. Notably, vertical wind shear (VWS) and temperature gradient tended to be much weaker below 900 hPa, which created a deeply stable stratification that acted as a cap to upward-moving air. Such a PBL structure facilitated more stable stratification and enhanced the generation of air pollution. Finally, the stable stratification in the PBL was related to the special synoptic configuration for the high PM2.5 conditions, which included the block situation at the high level, the southerly wind anomalies at the middle level and the wild range of the uniform pressure field at the near-ground level. Therefore, air pollutant concentrations were regulated by ground factors, PBL structure and the synoptic situation. Our results provide a precise understanding of the role of PBL features in air pollution, which contributes to improving the assimilation method of the atmospheric chemistry model in east China.

Climatology of the planetary boundary layer height over China and its characteristics during periods of extremely temperature

Planetary boundary layer height (PBLH) plays a significant role in air quality and climate change. At present, there is a lack of study that focuses on the PBLH characteristics with high spatiotemporal resolution and the subregional variation over China. In this study, we investigated PBLH climatology and its relationship with meteorological parameters across China from 2000 to 2020 using the ERA5 datasets. The PBLH varied diurnally and peaked between 14:00 and 16:00 Beijing Time at an average height between 0.8 km and 1.5 km. The seasonal characteristics of the PBLH also varied among regions. In most regions, the PBLH at urban sites is highest in spring and summer, and lowest in autumn and winter. Interestingly, the PBLH at most urban sites in East China was highest in autumn and lowest in spring. Spatially, in warm seasons (spring and summer), the PBLH showed a strong north-to-south gradient. PBLH was positively correlated with temperature at 2 m, wind speed at 10 m, sensible heat flux, and latent heat flux at most urban sites. However, PBLH was negatively correlated with relative humidity, as well as with surface PM2.5 concentrations. The vertical profiles of PM2.5 and PBLH exhibited an opposite trend that the lower PBLH was accompanied by higher PM2.5 concentration below PBLH at the same time on different days both at Qingshen and Rongchang Stations. In addition, we investigated the relationship between high and low temperatures and PBLH, and found that most sites have a positive correlation between high and low temperatures and PBLH, while coastal cities have a negative correlation between PBLH and low temperatures. We further explored the PBLH characteristics during two periods of extremely high temperatures in July and August 2022 in the Sichuan Basin and extremely low temperatures in the north of Northeast China in January 2023. Compared with the same months during 2011–2020, the PBLH at 16:00 BJT in July (August) increased significantly with 0.68 km (1.24 km), 0.38 km (1.06 km), and 0.69 km (0.89 km) in Chengdu, Chongqing and Yibin, respectively. During the period of extremely low temperature, compared with the corresponding period of the past decade, the PBLH decreased by 45 m (23% reduction), 100 m (37% reduction) and 70 m (16% reduction) at Mohe, Qiqihar and Jixi sites. This study helps to better understand the characteristics of PBLH in China, and it can also provide a reference for studies focusing on the evolution of atmospheric boundary or changes in air quality under the background of climate change.

Using the COVID-19 lockdown to identify atmospheric processes and meteorology influences on regional PM2.5 pollution episodes in the Beijing-Tianjin-Hebei, China

A major tool for curtailing the spread of COVID-19 pandemic in China was a nationwide lockdown implemented by the Chinese authorities, which also led to significant reductions in anthropogenic emissions and fine particulate matter (PM2.5) concentrations. However, the lockdown measures did not prevent high PM2.5 pollution episodes (EPs). Three severe EPs were identified in the Beijing-Tianjin-Hebei (BTH) region during the lockdown. The integrated process rate (IPR) analysis tool in the Community Multiscale Air Quality (CMAQ) model was employed to identify and quantify the contributions of individual atmospheric processes to PM2.5 formation during the EPs in four representative cities (Beijing, Tianjin, Shijiazhuang, and Baoding) of the BTH. The influence of meteorological conditions on the accumulation and dispersion of PM2.5 during the EPs was also elucidated. The results of the IPR showed that emissions and aerosol processes were the dominant sources of net surface PM2.5 in Beijing and Tianjin, constituting a total of 86.2% and 92.9%, respectively, while emissions, horizontal transport, and aerosol processes dominated the net surface PM2.5 formation in Shijiazhuang and Baoding. In addition, the three pollution episodes in Beijing and Tianjin were primarily driven by local emissions, while the pollution events in Shijiazhuang and Baoding were attributed to the combined local emissions and regional transport. Also, the EPs were driven by low planetary boundary layer heights, low vertical export of PM2.5 from the boundary layer to the free troposphere, and substantial horizontal import, especially in Shijiazhuang and Baoding. Furthermore, the elevated PM2.5 concentrations during the EPs were greatly enhanced by unfavorable meteorological conditions, with the first two EPs being characterized with low ventilation coefficient (VC) values in the four representative cities. This study improves the understanding of buildup of PM2.5 during the pollution episodes, and the results provide insights for designing more effective emissions control strategies to mitigate future PM2.5 pollution episodes.

Significantly mitigating PM2.5 pollution level via reduction of NOx emission during wintertime

Despite considerable decreases in fine particulate matter (PM2.5) in Chinese megacities over the past decade, many second- and third-tier cities that distribute abundant industrial enterprises are still facing great challenges for PM2.5 further reduction under the recent policy background of eliminating heavily-polluted weather. In view of core effects of NOx on PM2.5, the deeper reductions of NOx in these cities are expected to break the plateau of PM2.5 decline, however, the link between NOx emission and PM2.5 mass loading is currently lacking. Herein, we progressively construct an evaluation system for PM2.5 productions based on daily NOx emissions in a typical industrial city (Jiyuan), considering a sequence of nested parameters involving evolutions of NO2 into nitric acid and then nitrate, and contributions of nitrate to PM2.5. The evaluation system was subsequently validated to better reproduce real increasing processes for PM2.5 pollution based on 19 pollution cases, with root mean square errors of 19.2 ± 16.4 %, suggesting the feasibility of developing NOx emission indicators linked to goals of mitigating atmospheric PM2.5. Additionally, further comparative results reveal that currently high NOx emissions in this industrial city severely hinder the achievement of atmospheric PM2.5 environmental capacity targets, especially in the scenarios of high initial PM2.5 level, low planetary boundary layer height and long pollution duration. It is anticipated that these methodologies and findings would supply guidelines for further regional PM2.5 mitigation, in which source-oriented NOx indicators could also provide some orientations for industrial cleaner production such as denitrification and low nitrogen combustion.

Impact of thermal structure of planetary boundary layer on aerosol pollution over urban regions in Northeast China

This study investigated the characteristics of aerosol pollution in three provincial capitals (Shenyang, Changchun, and Harbin) in Northeast China and the impact of the thermal structure of the planetary boundary layer (PBL) using surface air quality monitoring data and meteorological observations and sounding data from January 1 to January 20, 2020. The number of pollution days in Shenyang, Changchun, and Harbin were 13, 17, and 19, respectively, and the number of heavy pollution days was 4, 3, and 13, respectively. Harbin suffered the heaviest and longest pollution with the smallest diurnal variation between daytime low and nighttime high PM2.5. The atmosphere in Harbin was more stable than that in Shenyang and Changchun, and PM2.5 concentration in the three cities tended to increase with decreasing planetary boundary layer height (PBLH). The lower winter air temperature in Northeast China led to a shallower PBL and weaker ventilation coefficient, which in turn led to an increase in PM2.5 concentration, indicating that air temperature is the key meteorological factor affecting local pollution in Northeast China. The analysis of two typical pollution events showed that a warm advection aloft suppressed the development of the PBL in Shenyang. In contrast, the suppressed effect of warm advection on the PBL was weakened in Harbin, which originally had a low PBLH due to cold weather. Moreover, the daytime convective boundary layer in Harbin developed 1–2 h later than that in Shenyang; the delayed development of PBL during the daytime in Harbin was also conducive to poor air quality.

Chemical characteristics and sources of PM2.5 in the urban environment of Seoul, Korea

In Seoul, Korea, episodes of high concentrations of PM2.5 (particulate matter ≤2.5 μm) continue to occur despite the enforcement of air quality standards. Seoul has several sources of pollution owing to its high population density. Therefore, this study evaluated the chemical characteristics of PM2.5, including inorganic ions and various gases, focusing on those with concentrations exceeding the standard limit. The study was conducted from June 1 to August 22, 2018, in Seongbuk, and from March 29 to May 31, 2019, in Jungnang. Water-soluble inorganic ions of PM2.5 were measured every 30 min using a particle-into-liquid sampler (PILS) combined with two ion chromatographs. In Seongbuk, ammonium sulfate was mainly caused by ammonium-poor conditions. In Jungnang, ammonium nitrate production occurred mainly due to ammonium-rich conditions. In both regions, during the occurrence of concentrations of PM2.5 exceeding the standard limit, the proportion of nitrate among inorganic ions was the highest. In Jungnang, two emission sources located in the west and southwest were identified using a conditional probability function. Contaminants from the southwest had high concentrations of nitrates, presumably due to atmospheric stagnation and nitrate mixing in lower planetary boundary layer height due to an decrease in surface temperatures at dawn. Pollutants from the west had high concentrations of sulfates. These are generated through photochemical reactions from industrial complexes. Cluster analysis confirmed that 27.2% of the air was stagnant and flowed from the south. In most cases, air pollutants originated from western Korea and coastal China.

Estimation of daytime planetary boundary layer height (PBLH) over the tropics and subtropics using COSMIC-2/FORMOSAT-7 GNSS–RO measurements

The advantage provided by the unprecedented data density and deeper penetration of Global Navigation Satellite System (GNSS) Radio Occultation (RO) based refractivity profiles from COSMIC-2/FORMOSAT-7 mission over the tropical and subtropical regions is leveraged to get a seasonal picture of daytime planetary boundary layer height (PBLH) distribution from 2 years of data (from October 2019 to September 2021) using the traditional method and a new approach proposed in this study. Traditional PBLH retrieval relies on the minimum refractivity gradient (MRG). In the new method, preference is given to the lowest significant peak in refractivity gradient (if present) rather than the MRG alone for determining PBLH. Comparison with the MRG method shows the highest decrease in seasonal mean PBLH over the oceanic ITCZ regions (by 500–700 m) which is in line with studies reporting low PBLH from in situ observations. The lowest decrease was observed over subtropical oceanic subsidence regions with marine stratocumulus clouds (MSC region; ~100–150 m) where the MRG method was thoroughly validated. The seasonal scale PBLH uncertainty (standard deviation) decreased by ~300 m over the ITCZ regions in the proposed method; it partially increased over the offshore MSC regions by ~100 m indicating detection of infrequent cases of marine boundary layer decoupled from stratocumulus clouds aloft which is not possible in the traditional method. The overestimation and higher uncertainty in PBLH derived from the traditional method over deep convective regions are attributed to the erroneous detection of seasonally varying cloud tops, subsidence base, and elevated water vapour layers as PBL top. An eastward shift by more than 5o longitude is observed in the location of PBLH maxima (seasonal mean) over the southeastern Pacific Ocean in the proposed method (with respect to the traditional method) for all seasons. The proposed method shows a better comparison with daytime PBLH from ERA5 reanalyses over the oceans but there is a partial overestimation of PBLH by ERA5 over continental regions in the summer hemisphere.

Dust-planetary boundary layer interactions amplified by entrainment and advections

Mineral dust contributes to more than half of the global aerosol loading. However, the radiative impacts of dust aerosols on planetary boundary layer (PBL) structure have not been explored sufficiently. During a typical dust storm event over Tarim Basin, dust aerosols exhibit a well-mixed distribution during the daytime in spite of a shallow layer of dust particles accumulated at higher altitudes. By contrast, nocturnal dust plumes are located near the surface due to stable stratification. We demonstrate that these differentiated vertical distributions determine the spatial heterogeneity of dust loading, radiative fluxes and PBL height variations. Dust aerosols cause daytime PBL suppression and nighttime PBL promotion through modulating surface and atmospheric radiative budgets. Specifically, dust-induced cooling effect within PBL directly inhibits the daytime PBL development. PBL suppression effect is then amplified by entrainment processes resulting in excessively low PBL height, especially for dust particles below but near the PBL top. Dust plumes weaken updrafts from PBL and downdrafts of the free atmosphere, which further reduce the entrainment mixing through attenuating horizontal and vertical advection, and eventually amplify PBL suppression. At night, near-surface dust aerosols stimulate a warm and unstable lower atmosphere, generate warm advection heating and promote the PBL development. Our study highlights the importance of specifying entrainment parameters and nighttime advection activities in quantifying the dust-PBL interactions.

Collective influences of boundary layer process and synoptic circulation on particulate pollution: A new study in changsha-zhuzhou-xiangtan urban agglomeration of central china

Particulate matter pollution is increasingly serious in Changsha-Zhuzhou-Xiangtan urban agglomeration (CZT) due to rapid industrialization and urbanization. Particulate matter pollution and weather conditions are closely related. In this study, the relationship between the large-scale weather circulation types (CTs), horizontal recirculation wind field, boundary layer structure and particulate matter pollution (PM2.5) in CZT was systematically investigated. Firstly, the Allwine and Whiteman (AW) wind field classification method and Richardson method were used to calculate the horizontal recirculation index and the planetary boundary layer height (PBLH) in CZT. By analyzing the relationship among the recirculation index, the PBLH and the particle concentration, it was found that the recirculation process in CZT could be divided into horizontal recirculation and “pseudo recirculation”. Then, the circulation patterns affecting the CZT were divided into nine types (CT1-CT9) by using T-mode PCA method based on 925 hpa geopotential data. The CZT was prone to recirculation under the control of high pressure rear (CT4), weak high pressure edge (CT5) and southwest vortex (CT7, CT8). It was found that CT4 and CT5 accompanied with lower PBLH and poorer vertical diffusion conditions characterized by high particle concentration. While CT7 and CT8 accompanied with higher PBLH and better vertical diffusion conditions characterized by low particle concentration. Finally, the effect of CTs, recirculation index and boundary layer structure on paticle matter concentration were assessed during two recirculation processes in December 2016 and August 2017, repectively. The analysis was complemented with FLEXPART-WRF model simulations, which confirmed that the recirculation and “pseudo recirculation” can be effectively distinguished by introducing the PBLH, combined with the large-scale weather circulation.

Impacts of Changes in Meteorological Conditions During COVID-19 Lockdown on PM2.5 Concentrations over the Jing-Jin-Ji Region

The Chinese government triggered the immediate implementation of a lockdown policy in China following the outbreak of the COVID-19 pandemic, leading to drastic decreases in air pollutant emissions. However, concentrations of PM2.5 and other pollutants increased during the COVID-19 lockdown over the Jing-Jin-Ji region compared with those averaged over 2015-2019, and two PM2.5 pollution events occurred during the lockdown. Using the ERA5 reanalysis data, we found that the Jing-Jin-Ji region during the COVID-19 lockdown was characterized by higher relative humidity, lower planetary boundary layer height, and anomalous updraft. These conditions were favorable for condensation and the secondary formation of aerosols and prevented turbulent diffusion of pollutants. Furthermore, we conducted sensitivity tests using the WRF-Chem model and found that ρ(PM2.5) increased by 20-55 μg·m-3(60%-170%) over the middle region of Jing-Jin-Ji during the COVID-19 lockdown due to changes in meteorological conditions. Furthermore, the enhanced aerosol chemistry and unfavorable diffusion conditions were identified as the key factors driving increases in PM2.5 concentrations during the lockdown. Planetary boundary layer height and relative humidity may become the important factors in forecasting PM2.5 pollution events over the Jing-Jin-Ji region under the background of emission reduction.

Temporal Variation of NO2 and HCHO Vertical Profiles Derived from MAX-DOAS Observation in Summer at a Rural Site of the North China Plain and Ozone Production in Relation to HCHO/NO2 Ratio

We performed a comprehensive and intensive field experiment including ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurement at Raoyang (115°44′ E, 38°14′ N; 20 m altitude) in summer (13 June–20 August) 2014. The NO2 and HCHO profiles retrieved by MAX-DOAS take on different vertical distribution shapes, with the former declining with the increasing altitude and the latter having an elevated layer. The average levels of vertical column densities (VCDs) and near-surface volume mixing ratios (VMRs) were 1.02 ± 0.51 × 1016 molec·cm−2 and 3.23 ± 2.70 ppb for NO2 and 2.32 ± 0.56 × 1016 molec·cm−2 and 5.62 ± 2.11 ppb for HCHO, respectively. The NO2 and HCHO levels are closely connected with meteorological conditions, with the larger NO2 VCDs being associated with lower temperature, higher relative humidity (RH) and lower planetary boundary layer height (PBLH). With respect to the diurnal variations of vertical distribution, the NO2 in the residual layer gradually disappeared from 1.2 km height to the surface during the period of 7:00–11:00 Beijing time (BJ), and the near-surface NO2 had larger VMRs in the early morning and evening than in the later morning and afternoon. An elevated HCHO layer was observed to occur persistently with the lifted layer height rising from ~0.5 km to ~1.0 km before 10:00 BJ; the near-surface HCHO VMRs gradually increased and peaked around 10:00 BJ. The ratios of HCHO to NO2 (RHCHO-NO2) were generally larger than two in the boundary layer from 11:00 BJ until 19:00 BJ, the time period when ozone photochemistry was most active. Thus, ozone (O3) production was mainly in the NOx-limited regime during the observation campaign, which was closely related to relatively high temperatures and low RH. The O3 production regimes also changed with the wind’s direction. These results are significant to reveal the formation mechanism of O3 pollution and develop strategies for controlling the O3 photochemical pollution over the North China Plain.

Modulation of Wintertime Canopy Urban Heat Island (CUHI) Intensity in Beijing by Synoptic Weather Pattern in Planetary Boundary Layer

AbstractStudying the spatiotemporal variations of the urban heat island (UHI) effect and its cause is important towards understanding urban climate change, planning and green development, and disaster mitigation. In this paper, by using surface observations and reanalysis data with objective classification of synoptic weather patterns (SWPs), we analyze the associations between canopy UHI intensity (CUHI) and SWPs in the planetary boundary layer (PBL) and their potential drivers during wintertime of the period 2012–2017. Six dominant types of SWP are identified as follows: In the case of Types 3, 4, and 6, weak high‐pressure systems exist to the south of Beijing, resulting in weak southerly winds with low PBL height, large cloud coverage and high relative humidity (RH). These conditions are generally conducive to a strengthening of the CUHII. In contrast, under Types 1, 2, and 5, high‐pressure systems are located to the northwest of Beijing, and the associated strong northwesterly flows of dry and cold air strengthen the boundary layer mixing process, resulting in large wind speed and low RH. This is conducive to a weakening of the CUHII. In general, our work reveals the impacts of SWPs on the strength of CUHII mainly via the modulation of local weather conditions at diurnal and interannual scales, while spatial pattern of CUHII is largely dominated by local climate zones. Our findings have implications for CUHII forecasts, as well as impact assessments and policymaking in the context of UHI‐related energy conservation in winter over high‐density urban areas on the synoptic scale.

Surface and aloft NO2 pollution over the greater Tokyo area observed by ground-based and MAX-DOAS measurements bridged by kilometer-scale regional air quality modeling

To advance our understanding of surface and aloft nitrogen dioxide (NO2) pollution, this study extensively evaluated NO2 concentrations simulated by the regional air quality modeling system with a 1.3 km horizontal grid resolution using the Atmospheric Environmental Regional Observation System ground-based observation network and aloft measurements by multi-axis differential optical absorption spectroscopy (MAX-DOAS) over the greater Tokyo area. Observations are usually limited to the surface level, and gaps remain in our understanding of the behavior of air pollutants above the near-surface layer, particularly within the planetary boundary layer (PBL). Therefore, MAX-DOAS measurement was used, which observes scattered sunlight in the ultraviolet/visible range at several elevation angles between the horizon and zenith to determine the aloft NO2 pollution averaged over 0–1 km. In total, four MAX-DOAS measurement systems at Chiba University (35.63°N, 140.10°E) systematically covered the north, east, west, and south directions to capture the aloft NO2 pollution over the greater Tokyo area. The target period was Chiba-Campaign 2015 conducted during 9–23 November 2015. The evaluations showed that the air quality modeling system can generally capture the observed behavior of both surface and aloft NO2 pollution in terms of spatial and temporal coverage. The diurnal variation, which typically showed an increase from evening to early morning without daylight and a decrease during the daytime, was also captured by the model. During Chiba-Campaign 2015, two cases of episodic higher NO2 concentration were identified: one during the nighttime and another during the daytime as different diurnal patterns. These were related to a stagnant wind field, with the latter also connected to a lower PBL height in cloudy conditions. Comparison of the modeled daily-averaged surface and aloft NO2 concentrations showed that aloft NO2 concentration exhibited a strong linear correlation with surface NO2 concentration, with the aloft (0–1 km) value scaled to 0.4–0.5-fold the surface value, irrespective of whether the day was clean or polluted. This scaling value was lower during the nighttime and higher during the daytime. Based on this synergetic analysis of surface and aloft observation bridged by a kilometer-scale fine-resolution modeling simulation, this study contributes to fostering understanding of aloft NO2 pollution.

Planetary Boundary Layer Height Modulates Aerosol—Water Vapor Interactions During Winter in the Megacity of Delhi

AbstractThe Indo‐Gangetic Plain (IGP) is one of the dominant sources of air pollution worldwide. During winter, the variations in planetary boundary layer (PBL) height, driven by a strong radiative thermal inversion, affect the regional air pollution dispersion. To date, measurements of aerosol‐water vapor interactions, especially cloud condensation nuclei (CCN) activity, are limited in the Indian subcontinent, causing large uncertainties in radiative forcing estimates of aerosol‐cloud interactions. We present the results of a one‐month field campaign (February‐March 2018) in the megacity, Delhi, a significant polluter in the IGP. We measured the composition of fine particulate matter (PM1) and size‐resolved CCN properties over a wide range of water vapor supersaturations. The analysis includes PBL modeling, backward trajectories, receptor models and fire spots to elucidate the influence of PBL and air mass origins on aerosols. The aerosol properties depended strongly on PBL height and a simple power‐law fit could parameterize the observed correlations of PM1 mass, aerosol particle number and CCN number with PBL height, indicating PBL induced changes in aerosol accumulation. The low inorganic mass fractions, low aerosol hygroscopicity and high externally mixed weakly CCN‐active particles under low PBL height (100 m) indicated the influence of PBL on aerosol aging processes. In contrast, aerosol properties did not depend strongly on air mass origins or wind direction, implying that the observed aerosol and CCN are from local emissions. An error function could parameterize the relationship between CCN number and supersaturation throughout the campaign.

Characterizing vertical distribution patterns of PM2.5 in low troposphere of Shanghai city, China: Implications from the perspective of unmanned aerial vehicle observations

The vertical distribution patterns of PM2.5 (particulate matters with diameters ≤2.5 μm) are crucial for understanding the aggregation, dispersion, and regional transport of PM2.5. However, due to the measurement limitation, the vertical observational data o is difficult to be obtained and is relatively insufficient at present stage. With this consideration, two electric-fueled rotary-wing unmanned aerial vehicles (UAVs) are used in this study to reveal the vertical distribution of PM2.5 in Shanghai of China. Totally 53 vertical profiles of PM2.5 concentration and meteorological factors are collected to reveal the vertical distribution pattern of PM2.5. The results indicate that PM2.5 vertical concentration, especially the maximum of the PM2.5 vertical concentration, shows a good correlation with surface PM2.5 concentration, which means it is possible to use surface PM2.5 concentration to extrapolate PM2.5 vertical concentration. In terms of meteorological factors, low wind speed and northerly (or northwesterly) wind direction significantly contribute to the accumulation of high concentration PM2.5 in Shanghai during the winter. Furthermore, the low height of the planetary boundary layer (PBL) facilitates the accumulation of pollutants near the ground, but it is not imperative for the vertically high concentration of PM2.5, since vertically high PM2.5 concentration exists within high PBL as well. It has been concluded that low PBL height is not mandatory for well vertical mixing within it, for the vertically even distribution of PM2.5 occurred under both low and high PBL. In terms of the effect the temperature, although temperature makes a significant contribution to the inverse distribution of PM2.5, no significant linear correlation was found between temperature and PM2.5 vertical distribution. The findings in this study could help to understand the vertical distribution patterns of PM2.5 during the winter, and provide more evidence of how other factors impact these patterns.

Evaluating the impacts of burning biomass on PM2.5 regional transport under various emission conditions

The fine particulate matter (PM2.5) emitted by burning biomass has become the main source of pollution in cities; this pollution seriously threatens the ecosystem and inhabitants' health. A major challenge in dealing with this issue is the uncertainty regarding the influence of burning biomass on PM2.5 regional transport. In this study, Harbin-Changchun Megalopolis is the research area. Using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model combined with satellite data and PM2.5 monitoring data, we quantitatively analyzed the regional transport of PM2.5 from burning biomass upwind of each city under different emission conditions. Conditions at burn sites, especially emission intensity and meteorological factors, as well as transport distance play significant roles in the regional transport of PM2.5. Higher emission intensity aggravated the concentration of downwind PM2.5, at most 19.7 μg ⋅ m−3. Shorter transport distance strengthened the impact of biomass burning on downstream PM2.5 by weakening elimination, which could be up to 96.8 μg ⋅ m−3. Moreover, meteorological factors at fire points were closely related to the transport of PM2.5. First, lower planetary boundary layer height could enhance the transport of PM2.5 from the burning biomass by inhibiting vertical diffusion, and the enhancement could be up to 46.1 μg ⋅ m−3. Second, compared to strong wind, light wind caused the weaker dilution, enhancing PM2.5 regional transport by as much as 32.5 μg ⋅ m−3. Third, relatively humidity at 30%‐40% had the strongest effect in facilitating the transport of PM2.5 from burning biomass. We conclude that comprehensively considering these three factors, namely the emission intensity, transport distance and meteorological factors at burn sites can facilitate the cross-regional development of accurate prediction models and effective pollution control measures.

Fog - low stratus (FLS) regimes on Corsica with wind and PBLH as key drivers

The French Mediterranean island of Corsica is already today confronted with a clear tendency towards water shortage, leading not only to socio-economical, but also to ecological problems. A potential, but not very widespread source of water is the presence of near-ground clouds, mostly fog. In this study, we investigate fog-low stratus (FLS) frequencies in Corsica, derived from a data set of Meteosat Second Generation SEVIRI, whereby a distinction between fog and low stratus is hardly feasible using remote sensing data. The FLS frequency was studied with respect to its interaction with distinct locally-generated wind and its dependence on the planetary boundary layer height (PBLH) obtained by ERA5 reanalysis (the fifth generation of the European Centre for Medium-Range Weather Forecasts, ECMWF). Results show that radiation FLS is formed in coastal areas at sunrise, with low PBLH. On the other hand, in the interior of the island at sunset, a maximum of advection FLS is formed, fostered by locally-generated and related transport of moisture. On the east side of the island, FLS frequency is lower throughout the year due to frequent lee situations. This situation is reinforced by reduced synoptic moisture transport by westerly winds, so that westerly exposed slopes benefit from moisture input by FLS formation.