Urban Plume Research Articles

Understudied High-Pollution Band along Long Island’s South Coastline Caused by the Interaction of New York City Urban Plumes and a Stagnant Sea Breeze Front

Understudied High-Pollution Band along Long Island’s South Coastline Caused by the Interaction of New York City Urban Plumes and a Stagnant Sea Breeze Front

Interpretable machine learning tools to analyze PM2.5 sensor network data so as to quantify local source impacts and long-range transport

Sensor networks provide spatially resolved information about the time variation of PM2.5 concentrations in urban areas around the world. With relatively simple improvements to the control of the temperature and humidity of incoming air, and with proper quality assurance and calibration protocols, a low cost monitor was developed that provides measurements that were highly correlated with a reference PM2.5 monitor. These have sufficient resolution and reliability to quantify small differences within an urban area. Nevertheless, many sites report similar concentrations and it can be difficult to interpret the results or distinguish local from regional effects. Generalized Additive Models are an effective Machine Learning method to distinguish the impact of factors across very different scales. As a type of Interpretable Machine Learning / eXplainable Artificial Intelligence, they provide direct information on the link between specific factors and PM2.5 concentrations. GAM simulations were developed for sensors located around Dhaka, Bangladesh, for both the dry and the wet seasons. The simulations show that the largest contributor to high PM2.5 concentration variations across both urban and peri-urban sites is the boundary layer height which represent the vertical mixing of the urban plume. Using Trajectory Cluster Concentration Impacts, the simulations showed that robust estimates of long-range transport could be obtained from measurements located within a polluted environment, and the model further showed that enhancements of more than 40μg/m3 were associated with air transport from the Indo-Gangetic Plain in the dry season. Finally, interaction maps of the effect of horizontal wind speed and direction showed that these could be associated with up to +/−20 % variation in PM2.5 from site to site. Most of the enhancements are related to very calm winds and appear to be more strongly associated with road emissions than with point sources. Overall, the sensor network shows that air is polluted throughout the Dhaka area and into the peripheries, and that a multipronged approach will be needed to improve air quality for its inhabitants.

Multiscale CFD analysis of urban air pollution dome and ventilation enhancement via an urban chimney

Air pollution episodes are critical environmental challenges in urban areas, primarily linked to the lack of ventilation during calm and stable atmospheric conditions, rather than a significant increase in emissions. The prevailing circulation pattern under these conditions is the Urban Heat Island Circulation (UHIC), which governs the characteristics of the urban pollution dome and the distribution of pollutants. This study aims to assess the UHIC's influence on spatial and temporal variations in pollutant concentrations and to investigate the efficiency of an Urban Chimney (UC) in improving air quality. Due to the interaction of micro- and meso-scale flows, a multi-scale analysis is required. For this purpose, a pressure-based CFD solver is adapted for analyzing airflow and pollutant dispersion in a multi-scale environment. A coordinate transformation method is used to account for meso-scale effects, and the resulting source terms are applied to the solver. The species transport equation is transformed to the new coordinate system, and the effects of species diffusion on enthalpy transport are included in the governing equations. The results demonstrate that UHIC reduces the average pollutant concentration and causes pollution peaks near ground level and at the city center. The interaction of micro-scale flow within the chimney with meso-scale UHIC impacts the flow field and pollutant concentration across the entire domain, decreasing the urban plume's vertical velocity by 40 percent and the mixing height at the city center by 20 percent. It also increases the vertical velocity within the UC by 23 percent. The UC can serve as a controlling tool for urban ventilation, and its capacity to remove pollutants increases sixfold when interacting with UHIC, reducing pollutant concentration citywide.

Measurement report: Atmospheric nitrate radical chemistry in the South China Sea influenced by the urban outflow of the Pearl River Delta

Abstract. The nitrate radical (NO3) is a critical nocturnal atmospheric oxidant in the troposphere, which widely affects the fate of air pollutants and regulates air quality. Many previous works have reported the chemistry of NO3 in inland regions of China, while fewer studies target marine regions. Here, we present a field measurement of the NO3 reservoir, dinitrogen pentoxide (N2O5), and related species at a typical marine site (Da Wan Shan Island) located in the South China Sea in the winter of 2021. Two patterns of air masses were captured during the campaign, including the dominant airmass from inland China (IAM) with a percentage of ∼ 84 %, and the airmass from eastern coastal areas (CAM) with ∼ 16 %. During the IAM period, the NO3 production rate reached 1.6 ± 0.9 ppbv h−1 due to the transportation of the polluted urban plume with high NOx and O3. The average nocturnal N2O5 and the calculated NO3 mixing ratios were 119.5 ± 128.6 and 9.9 ± 12.5 pptv, respectively, and the steady-state lifetime of NO3 was 0.5 ± 0.7 min on average, indicating intensive nighttime chemistry and rapid NO3 loss at this site. By examining the reaction of NO3 with volatile organic compounds (VOCs) and N2O5 heterogeneous hydrolysis, we revealed that these two reaction pathways were not responsible for the NO3 loss (< 20 %) since the NO3 reactivity (k(NO3)) towards VOCs was small (5.2×10-3 s−1) and the aerosol loading was low. Instead, NO was proposed to significantly contribute to nocturnal NO3 loss at this site, despite the nocturnal NO concentration always below the parts per billion by volume level and near the instrument detection limit. This might be from the local soil emission or something else. We infer that the nocturnal chemical NO3 reactions would be largely enhanced once without NO emission in the open ocean after the air mass passes through this site, thus highlighting the strong influences of the urban outflow to the downwind marine areas in terms of nighttime chemistry. During the CAM period, nocturnal ozone was higher, while NOx was much lower. The NO3 production was still very fast, with a rate of 1.2 ppbv h−1. With the absence of N2O5 measurement in this period, the NO3 reactivity towards VOCs and N2O5 uptake were calculated to assess NO3 loss processes. We showed that the average k(NO3) from VOCs (56.5 %, 2.6 ± 0.9 × 10−3 s−1) was higher than that from N2O5 uptake (43.5 %, 2.0 ± 1.5 × 10−3 s−1) during the CAM period, indicating a longer NO3 / N2O5 lifetime than that during IAM period. This study improves the understanding of the nocturnal NO3 budget and environmental impacts with the interaction of anthropogenic and natural activities in marine regions.

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.

New particle formation in the tropical free troposphere during CAMP2Ex: statistics and impact of emission sources, convective activity, and synoptic conditions

Abstract. Nucleation in the free troposphere (FT) and subsequent growth of new particles represent a globally important source of cloud condensation nuclei (CCN). Whereas new particle formation (NPF) has been shown to occur frequently in the upper troposphere over tropical oceans, there have been few studies of NPF at lower altitudes. In addition, the impact of urban emissions and biomass burning on the NPF in tropical marine FT remains poorly understood. In this study, we examine NPF in the lower and mid-troposphere (3–8.5 km) over the tropical ocean and coastal region using airborne measurements during the recent Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex). NPF was mostly observed above 5.5 km and coincided with elevated relative humidity (RH) and reduced condensation sink (CS), suggesting that NPF occurs in convective cloud outflow. The frequency of NPF increases with altitude, reaching ∼ 50 % above 8 km. An abrupt decrease in NPF frequency coincides with early monsoon transition and is attributed to increased CS resulting from reduced convective activity and more frequent transport of aged urban plumes. Surprisingly, a large fraction of NPF events in background air were observed in the early morning, and the NPF is likely made possible by very low CS despite low actinic flux. Convectively detrained biomass-burning plumes and fresh urban emissions enhance NPF as a result of elevated precursor concentrations and scavenging of pre-existing particles. In contrast, NPF is suppressed in aged urban plumes where the reactive precursors are mostly consumed, while CS remains relatively high. This study shows a strong impact of urban and biomass-burning emissions on the NPF in tropical marine FT. The results also illustrate the competing influences of different variables and interactions among anthropogenic emissions, convective clouds, and meteorology, which lead to NPF under a variety of conditions in tropical marine environments.

The impact of COVID-19 lockdowns on urban photochemistry as inferred from TROPOMI

Movement restrictions were imposed in 2020 to mitigate the spread of Covid-19. These lock-down episodes provide a unique opportunity to study the sensitivity of urban photochemistry to temporary emission reductions and test air quality models. This study uses Tropospheric Monitoring Instrument (TROPOMI) nitrogen dioxide/carbon monoxide (NO2/CO) ratios in urban plumes in combination with an exponential fitting procedure to infer changes in the NOx lifetime (τNOx) during Covid-19 lock-downs in the cities of Denver, Chicago, New York, Riyadh, Wuhan and Sao Paulo compared with the year before.The strict lockdown policy in Wuhan led to a 65–80% reduction in NO2, compared to 30–50% in the other cities that were studied. In New York and Wuhan, CO concentration was reduced by 10–15%, whereas over Riyadh, Denver, Chicago, and Sao Paulo the CO background concentration increased by 2–5 ppb. τNOx has been derived for calm (0.0 < U (m/s) < 3.5) and windy (5.0 < U (m/s) < 8.5) days to study the influence of wind speed. We find reductions in τNOx during Covid-19 lockdowns in all six megacities during calm days. The largest change in τNOx during calm days is found for Sao-Paulo (31.8 ± 9.0%), whereas the smallest reduction is observed over Riyadh (22 ± 6.6%). During windy days, reductions in τNOx are observed during Covid-19 lockdowns in New York and Chicago. However, over Riyadh τNOx is almost similar for windy days during the Covid-19 lockdown and the year before.Ground-based measurements and the Chemistry Land-surface Atmosphere Soil Slab (CLASS) model have been used to validate the TROPOMI-derived results over Denver. CLASS simulates an enhancement of ozone (O3) by 4 ppb along with reductions in NO (38.7%), NO2 (25.7%) and CO (17.2%) during the Covid-19 lockdown in agreement with the ground-based measurements. In CLASS, decreased NOx emissions reduce the removal of OH in the NO2 + OH reaction, leading to higher OH concentrations and decreased τNOx. The reduction in τNOx inferred from TROPOMI (28 ± 9.0%) is in agreement with CLASS. These results indicate that TROPOMI derived NO2/CO ratios provide useful information about urban photochemistry and that changes in photochemical lifetimes can successfully be detected.

Ground-based and AIRS carbon monoxide behavior at El Arenosillo observatory (Southwestern Europe)

Carbon monoxide (CO) observations registered at El Arenosillo observatory (Southwestern Europe), during three years (2019–2022) were used to explore levels, temporal variations and patterns. The seasonal mean hourly oscillated between 127 ± 21 and 97 ± 14 ppb, with extreme peaks measured under wildfire plume arrival. A remarkable monthly evolution was observed with peaks in cold months (135 ± 20 ppb in January) and minimum in summer (90 ± 11 ppb in June). Seasonal daily patterns showed a different nocturnal behavior depending on the season, with a clear maximum at 7:00–9:00 UTC, a decrease during diurnal time, and a minimum at 17:00–18:00 UTC. To determine the potential trends from 2002 to 2022, the observations of the AIRS (Atmospheric Infrared Sounder) instrument and the emissions from the global emission inventory of CAMS (Copernicus Atmosphere Monitoring Service) were used. A downward trend for emissions of −31% decade−1 was found, whereas the decrease from AIRS was −9.8% decade−1. The effort to reduce CO emissions is being countered by other processes. The CO behavior was investigated under weather conditions governed by the synoptic and mesoscale processes. Background levels (∼90 ppb) were obtained under Atlantic airflows, while an increase of ∼40% was observed with the impact of urban plumes from continental areas. CO even lower than 90 ppb was obtained under non-pure sea-land breezes, whereas, with pure breezes, reached peaks above 120 ppb, affecting the atmosphere at the surface and up to 1–1.2 km in the air. The extreme level was registered under wildfire plume arrival, with peaks exceeding 350 ppb. Climate change is modifying weather patterns and modulating breezes; in addition, it is also increasing the frequency and intensity of wildfires. These processes could be counteracting the efforts carried out to reduce emissions.

Identification of the roles of urban plume and local chemical production in ozone episodes observed in Long Island Sound during LISTOS 2018: Implications for ozone control strategies

Long Island Sound (LIS) frequently experiences ozone (O3) exceedance events that surpass national ambient air quality standards (NAAQS) due to complex driving factors. The underlying mechanisms governing summertime O3 pollution are investigated through collaborative observations from lidar remote sensing and ground samplers during the 2018 LIS Tropospheric O3 Study (LISTOS). Regional transport and local chemical reactions are identified as the two key driving factors behind the observed O3 episodes in LIS. An enhanced laminar structure is observed in the O3 vertical structure in the atmospheric boundary layer (i.e., 0–2 km layer) for the case dominated by regional transport. An O3 formation regime shift is found in ozone-precursor sensitivity (OPS) for the O3 exceedance event dominated by regional transport with NOx-enriched air mass transport from the New York City (NYC) urban area to LIS. Furthermore, the Integrated Process Rate (IPR) analysis demonstrates that transport from the NYC urban area contributed 40% and 27.1% of surface O3 enhancement to the cases dominated by regional transport and local production, respectively. This study provides scientific evidence to uncovers two key processes that govern summertime O3 pollution over LIS and can help to improve emission control strategies to meet the attainment standards for ambient O3 levels over LIS and other similar coastal areas.

Performance of summertime temperature and wind fields under different background winds in Kowloon, Hong Kong

The increasingly frequent urban extreme heat events are adversely affecting human health and the economy worldwide. In the last three decades, Kowloon, Hong Kong, one of the most densely populated urban areas in the world, has witnessed a rapid warming rate of 0.31 °C/decade. An in-depth study of the wind and temperature environment is needed to explore the urban warming mechanisms in Kowloon. Using semi-idealised Weather Research and Forecasting (WRF) simulations, we investigated the wind flow fields over the Kowloon area under various background wind conditions. The evolution and interaction of urban heat island circulation, sea–land breeze circulation and mountain circulation under calm weather conditions were also studied. We examined weather photos of Kowloon to identify cloud locations and evaluate our predicted urban plumes. We also explored the association between background wind and spatial air temperature distribution in Kowloon. Northeasterly wind conditions can lead to high temperatures at the foothills of mountains situated northeast of Kowloon; a streamline analysis showed that heat advection accounts for this phenomenon during the day, while a possible foehn-like wind plays an important role at night. These insights into the wind and thermal environment in Kowloon can contribute to the interpretation and prediction of extreme heat events.

Diagnosis of photochemical O3 production of urban plumes in summer via developing the real-field IRs of VOCs: A case study in Beijing of China

This study mainly developed an estimate method for photochemical ozone (O3) production from urban plumes in hot season, through simulating O3 evolution from precursors locally emitted and determining the real-field O3 increment reactivity (IR) of volatile organic compounds (VOCs) based on the box chemical model. Our simulation on June-2019 indicated that Beijing local emissions produced O3 at the rate of 0.7–9.2 ppb/h and led to an O3 increase of 48.9 ppb during 05:00–18:00, accounting for 68.3% of the observed O3 increase. The maximum level and production rate of simulated O3 showed a linear response to VOCs, therefore we can use VOCs levels in urban plumes to quantify O3 formation in summer. The IR (g O3 formed per g VOCs) was calculated on the actual precursor and meteorology condition of this megacity, 0.12–4.90 g/g for individual VOCs and 1.49 g/g for comprehensive TVOCs. The weighted average of individual IRs agreed well with that of TVOCs, but these IRs were 34.5% of MIR values that were widely used in references. It's noteworthy that these IRs had greater sensitivity to precursor levels, and broadly remained stable under the fixed VOCs:NOx. Considering the synchronous reductions of precursors in Beijing, we applied these IRs to quantify chemical O3 evolution from Beijing local emissions in summer of recent years, declining from 63.5 ppb in 2016 to 44.0 ppb in 2020 for June. The contributions of the diagnosed chemical O3 to Beijing O3 better matched with the atmospheric transport paths on daily basis, higher than 100% when the transport paths starting from the clean neighbor cities, but lower to 45%–66% when the transport paths originating from the highly-polluted neighbor cities. This consistence indicated the reliability of our IR calculation method for quickly estimating chemical O3 production of urban plumes in summer.

Towards sector-based attribution using intra-city variations in satellite-based emission ratios between CO2 and CO

Abstract. Carbon dioxide (CO2) and air pollutants such as carbon monoxide (CO) are co-emitted by many combustion sources. Previous efforts have combined satellite-based observations of multiple tracers to calculate their emission ratio (ER) for inferring combustion efficiency at the regional to city scale. Very few studies have focused on combustion efficiency at the sub-city scale or related it to emission sectors using space-based observations. Several factors are important for interpreting and deriving spatially resolved ERs from asynchronous satellite measurements, including (1) variations in meteorological conditions given the mismatch in satellite overpass times, (2) differences in vertical sensitivity of the retrievals (i.e., averaging kernel profiles), (3) interferences from the biosphere and biomass burning, and (4) the mismatch in the daytime variations of CO and CO2 emissions. In this study, we extended an established emission estimate approach to arrive at spatially resolved ERs based on retrieved column-averaged CO2 (XCO2) from the Snapshot Area Mapping (SAM) mode of the Orbiting Carbon Observatory-3 (OCO-3) and column-averaged CO from the TROPOspheric Monitoring Instrument (TROPOMI). To evaluate the influences of the confounding factors listed above and further attribute intra-urban variations in ERs to certain sectors, we leveraged a Lagrangian atmospheric transport model with an urban land cover classification dataset and reported ERCO values from the sounding level to the overpass and city level. We found that the differences in overpass times and averaging kernels between OCO and TROPOMI strongly affect the estimated spatially resolved ERCO. Specifically, a time difference of &gt;3 h typically led to dramatic changes in wind directions and urban plume shapes, thereby making the calculation of accurate sounding-specific ERCO difficult. After removing such cases from consideration and applying a simple plume shift method when necessary to account for changes in wind direction and speed, we discovered significant contrasts in combustion efficiencies between (1) two megacities versus two industry-oriented cities and (2) different regions within a city, based on six nearly coincident overpasses per city. Results suggest that the ERCO impacted by heavy industry in Los Angeles is slightly lower than the overall city-wide value (&lt;10 ppb-CO/ppm-CO2). In contrast, the ERCO related to heavy industry in Shanghai is much higher than Shanghai's city mean and more aligned with the city means of two selected industry-oriented cities in China (approaching 20 ppb-CO/ppm-CO2). Although investigations based on a larger number of satellite overpasses are needed, our unique approach (i.e., without using sector-specific information from emission inventories) provides new insights into assessing combustion efficiency within a city from future satellite missions, such as those that will map column CO2 and CO concentrations simultaneously with high spatiotemporal resolutions.

Temporal variation of surface O3, NOx and PM10 and associated human health risk in Gabès suburban (South-Eastern Tunisia): multi-scales impact of sources and meteorology

The study of daily and hourly average concentrations variability of surface O3, NO, NO2 and PM10 was carried out at the proximity of a crossroad (≈23,000 vehicles per day) located in suburban Gabès, Tunisia. It was performed during 2017–2019 “winter/spring” seasonal transitions and spring seasons. Depending on multi-scale sources and meteorology, it showed variable values ​​within each studied annual period and the years. Statistical approaches based on factorial analysis of correspondences (FAC) and principal component analysis (PCA) revealed three main components of pollution; local retraced by NOx, regional by O3 and synoptic by PM10. Furthermore, they have refined the significant impacts of Saharan dust advections on high recorded PM10 and NOx (by drained urban plume) and of both deep depressions (cut-off lows) and Azores anticyclonic events on the excess of surface O3 concentrations. The investigation of the air quality health index (AQHI) related to the selected pollutants highlighted the significant effect of PM10. It has revealed that more than 53% of the total days were retraced by a moderate air quality index level linked to the negative impact of PM10. Besides, for such a pollutant, more than 20% of the total days were unhealthy for sensitive groups. Even for ordinary people, almost 3% of the total days were shown to be very unhealthy to hazardous.

Fast Nocturnal Heterogeneous Chemistry in a Coastal Background Atmosphere and Its Implications for Daytime Photochemistry

AbstractNitryl chloride (ClNO2) is an important chlorine reservoir in the atmosphere that affects the oxidation capacity by producing first‐ and second‐generation radicals, but it has been rarely observed in the world. Here, online measurements of both N2O5 and ClNO2 at a coastal background site in the Pearl River Delta (PRD) in southern China, a region with high atmospheric oxidation capacity in the daytime, are presented. We observed continuous accumulation of elevated ClNO2 at night, while N2O5 was generally low. The maximum concentrations of N2O5 and ClNO2 were 1.4 and 1.7 ppbv, respectively, associated with a transported urban plume with high levels of O3 and NO2. The average product of N2O5 uptake coefficient and ClNO2 yield (γ × f = 0.02, with average γ of 0.027 and f of 0.71) was roughly twice those in urban areas in China, indicating active N2O5 heterogenous chemistry and effective ClNO2 formation in the background atmosphere. The production rate of Cl radicals (pCl) from the photolysis of ClNO2 was estimated to be an order of magnitude higher than those at coastal and rural sites in the literature. Such levels of Cl radical production could have considerable contribution to the atmospheric oxidation capacity after sunrise. In addition, NO2 produced by ClNO2 photolysis in the morning accounted for approximately 10% of the NO2 increase within several hours after sunrise. Thus, high nocturnal ClNO2 production and its following daytime photochemistry should be considered seriously in regional air quality modeling in the PRD and regions with similar conditions.

Impact of wind speed on urban heat and pollution islands

Large urban areas are typically subjected to excessive levels of heat and pollution. The formation of an urban-scale plume would arise from releasing heat in a stable atmosphere. The presence of wind may disrupt the pattern of the plume. This study explores the effect of wind intensity and ambient stability on the plume structure and pollution dispersion numerically. A metropolitan has been selected as the urban, and the average heat and pollution release rate is calculated based on a typical megacity. It has been found that for buoyancy ratios (the ratio between the horizontal velocity of the urban plume to wind speed) smaller than 0.5, the wind is the governing mechanism. For buoyancy ratio higher than 2, the urban heat island is governing mechanism. In stable conditions and the ratios smaller than 0.5, the urban average temperature difference reduces by about 0.125 K for a unit increase in wind speed. When the ratios are less than 0.2, there are 0.5 K temperature differences between urban and rural, and 0.072 μg/m3 averaged pollution for all stability conditions. It is also found that for ratios higher than 0.5, any increase in wind velocity does not reduce the pollution concentration significantly. For values smaller than 0.5, any increase in wind speed would result in a considerable drop in pollution concentration if the radiative effect of pollutants is ignored.

Overview: On the transport and transformation of pollutants in the outflow of major population centres – observational data from the EMeRGe European intensive operational period in summer 2017

Abstract. Megacities and other major population centres (MPCs) worldwide are major sources of air pollution, both locally as well as downwind. The overall assessment and prediction of the impact of MPC pollution on tropospheric chemistry are challenging. The present work provides an overview of the highlights of a major new contribution to the understanding of this issue based on the data and analysis of the EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales) international project. EMeRGe focuses on atmospheric chemistry, dynamics, and transport of local and regional pollution originating in MPCs. Airborne measurements, taking advantage of the long range capabilities of the High Altitude and LOng Range Research Aircraft (HALO, https://www.halo-spp.de, last access: 22 March 2022), are a central part of the project. The synergistic use and consistent interpretation of observational data sets of different spatial and temporal resolution (e.g. from ground-based networks, airborne campaigns, and satellite measurements) supported by modelling within EMeRGe provide unique insight to test the current understanding of MPC pollution outflows. In order to obtain an adequate set of measurements at different spatial scales, two field experiments were positioned in time and space to contrast situations when the photochemical transformation of plumes emerging from MPCs is large. These experiments were conducted in summer 2017 over Europe and in the inter-monsoon period over Asia in spring 2018. The intensive observational periods (IOPs) involved HALO airborne measurements of ozone and its precursors, volatile organic compounds, aerosol particles, and related species as well as coordinated ground-based ancillary observations at different sites. Perfluorocarbon (PFC) tracer releases and model forecasts supported the flight planning, the identification of pollution plumes, and the analysis of chemical transformations during transport. This paper describes the experimental deployment and scientific questions of the IOP in Europe. The MPC targets – London (United Kingdom; UK), the Benelux/Ruhr area (Belgium, the Netherlands, Luxembourg and Germany), Paris (France), Rome and the Po Valley (Italy), and Madrid and Barcelona (Spain) – were investigated during seven HALO research flights with an aircraft base in Germany for a total of 53 flight hours. An in-flight comparison of HALO with the collaborating UK-airborne platform Facility for Airborne Atmospheric Measurements (FAAM) took place to assure accuracy and comparability of the instrumentation on board. Overall, EMeRGe unites measurements of near- and far-field emissions and hence deals with complex air masses of local and distant sources. Regional transport of several European MPC outflows was successfully identified and measured. Chemical processing of the MPC emissions was inferred from airborne observations of primary and secondary pollutants and the ratios between species having different chemical lifetimes. Photochemical processing of aerosol and secondary formation or organic acids was evident during the transport of MPC plumes. Urban plumes mix efficiently with natural sources as mineral dust and with biomass burning emissions from vegetation and forest fires. This confirms the importance of wildland fire emissions in Europe and indicates an important but discontinuous contribution to the European emission budget that might be of relevance in the design of efficient mitigation strategies. The present work provides an overview of the most salient results in the European context, with these being addressed in more detail within additional dedicated EMeRGe studies. The deployment and results obtained in Asia will be the subject of separate publications.

Spatial variability of forward modelled attenuated backscatter in clear-sky conditions over a megacity: Implications for observation network design.

Sensors that measure the attenuated backscatter coefficient (e.g., automatic lidars and ceilometers [ALCs]) provide information on aerosols that can impact urban climate and human health. To design an observational network of ALC sensors for supporting data assimilation and to improve prediction of urban weather and air quality, a methodology is needed. In this study, spatio‐temporal patterns of aerosol‐attenuated backscatter coefficient are modelled using Met Office numerical weather prediction (NWP) models at two resolutions, 1.5 km (UKV) and 300 m (London Model [LM]), for 28 clear‐sky days and nights. Initially, attenuated backscatter coefficient data are analysed using S‐mode principal component analysis (PCA) with varimax rotation. Four to seven empirical orthogonal functions (EOFs) are produced for each model level, with common EOFs found across different heights (day and night) for both NWP models. EOFs relate strongly to orography, wind, and emissions source location, highlighting these as critical controls of attenuated backscatter coefficient spatial variability across the megacity. Urban–rural differences are largest when wind speeds are low and vertical boundary‐layer dynamics can more effectively distribute near‐surface aerosol emissions vertically. In several night‐time EOFs, gravity‐wave features are found for both NWP models. Increasing the horizontal resolution of native ancillaries (model input parameters) and improving the urban surface scheme in the LM may enhance the urban signal in the EOFs. PCA output, with agglomerative Ward cluster analysis (CA), minimises intra‐group variance. The UKV and LM CA shape and size results are similar and strongly related to orography. PCA‐CA is a simple, but adaptable methodology, allowing close alignment with observation network design goals. Here, CA is used with wind roses to suggest the optimised ALC deployment is one in the city to observe the urban plume and others surrounding the city, with priority given to cluster size and frequency of upwind advection.

Rapid growth of anthropogenic organic nanoparticles greatly alters cloud life cycle in the Amazon rainforest.

Aerosol-cloud interactions remain uncertain in assessing climate change. While anthropogenic activities produce copious aerosol nanoparticles smaller than 10 nanometers, they are too small to act as efficient cloud condensation nuclei (CCN). The mechanisms responsible for particle growth to CCN-relevant sizes are poorly understood. Here, we present aircraft observations of rapid growth of anthropogenic nanoparticles downwind of an isolated metropolis in the Amazon rainforest. Model analysis reveals that the sustained particle growth to CCN sizes is predominantly caused by particle-phase diffusion-limited partitioning of semivolatile oxidation products of biogenic hydrocarbons. Cloud-resolving numerical simulations show that the enhanced CCN concentrations in the urban plume substantially alter the formation of shallow convective clouds, suppress precipitation, and enhance the transition to deep convective clouds. The proposed nanoparticle growth mechanism, expressly enabled by the abundantly formed semivolatile organics, suggests an appreciable impact of anthropogenic aerosols on cloud life cycle in previously unpolluted forests of the world.

Unraveling the interaction of urban emission plumes and marine breezes involved in the formation of summertime coastal high ozone on Long Island

The different mechanisms for high ozone cases at the north shorevs.the south shore of Long Island, New York are influenced by the interplay of urban plumes with different coastal meteorological flow patterns.

Accelerated toluene degradation over forests around megacities in southern China

Toluene is a typical anthropogenic pollutant that has profound impacts on air quality, climate change, and human health, but its sources and sinks over forests surrounding megacities remain unclear. The Nanling Mountains (NM) is a large subtropical forest and is adjacent to the Pearl River Delta (PRD) region, a well-known hotspot for toluene emissions in southern China. However, unexpectedly low toluene concentrations (0.16 ± 0.20 ppbv) were observed at a mountaintop site in NM during a typical photochemical period. A backward trajectory analysis categorized air masses received at the site into three groups, namely, air masses from the PRD, those from central China, and from clean areas. The results revealed more abundant toluene and its key oxidation products, for example, benzaldehyde in air masses mixed with urban plumes from the PRD. Furthermore, a more than three times faster degradation rate of toluene was found in this category of air masses, indicating more photochemical consumption in NM under PRD outflow disturbance. Compared to the categorized clean and central China plumes, the simulated OH peak level in the PRD plumes (15.8 ± 2.2 × 106 molecule cm−3) increased by approximately 30% and 55%, respectively, and was significantly higher than the reported values at other background sites worldwide. The degradation of toluene in the PRD plumes was most likely accelerated by increased atmospheric oxidative capacity, which was supported by isoprene ozonolysis reactions. Our results indicate that receptor forests around megacities are not only highly polluted by urban plumes, but also play key roles in environmental safety by accelerating the degradation rate of anthropogenic pollutants.