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South America is a large continent situated mostly in the Southern Hemisphere (SH) with complex topography and diverse emissions sources. However, the atmospheric chemistry of this region has been historically understudied. Here, we employ the Multi‐Scale Infrastructure for Chemistry and Aerosols, a novel global circulation model with regional refinement capabilities and full chemistry, to explore the sources and distribution of the carbon monoxide (CO) tropospheric column in South America during 2019, and also to assess the effect that South American primary emissions have over the rest of the world. Most of the CO over South America can be explained either by non‐methane volatile organic compounds (NMVOC) secondary chemical production or by biomass burning emissions, with biomass burning as the main explanation for the variability in CO. Biomass burning in Central Africa is a relevant contributor to CO in all of the continent, including the southern tip. Biogenic emissions play a dual role in CO concentrations: they provide volatile organic compounds that contribute to the secondary CO production, but they also destroy OH, which limits the chemical production and destruction of CO. As a net effect, the lifetime of CO is extended to ∼120 days on average over the Amazon, while still being in the range of 30–60 days in the rest of South America. Plain Language Summary: We use the Multi‐Scale Infrastructure for Chemistry and Aerosols, a global model with regional refinement, to study the origins of carbon monoxide (CO) in South America during 2019. The main sources of CO are the secondary production from non‐volatile organic compounds and the biomass burning primary emissions. The main source of temporal variability in the whole are the biomass burning emissions. We show that biomass burning in central Africa is a relevant source of South American CO during all year in all of the continent, including the furthermost south. Key Points: We employ the Multi‐Scale infrastructure for Chemistry and Aerosols to quantify the budget of CO in South America during 2019Most of the variability in the CO burden is explained by the variability in biomass burning emissionsBiomass burning in Central Africa is a relevant contributor to CO in all of the continent, including the southern region [ABSTRACT FROM AUTHOR]

The Atmospheric Oxidation Capacity (AOC), often referred to as the self-cleansing ability of the atmosphere, considerably affects the concentrations of photochemical air pollutants. Despite substantial reductions in anthropogenic emissions of key chemical compounds in China, the mechanisms that determine the changes in the atmospheric oxidation capacity are still not sufficiently understood. Here, a regional chemical transport model is employed to quantify the sensitivity of air pollutants and photochemical parameters to specified emission reductions in China for conditions of January and July 2018 as representative. The model simulations show that, in winter, a 50 % decrease in nitrogen oxides (NOx) emissions leads to an 8–10 ppbv (15–20 %) increase in surface ozone concentrations across China. In summer, the ozone concentration decreases by 2–8 ppbv (3–12 %) in NOx-limited areas, while ozone increases by up to 12 ppbv (15 %) in volatile organic compounds (VOCs)-limited areas. This ozone increase is associated with a reduced NOx-titration effect and higher levels of hydroperoxyl (HO2) radical due to decreased aerosol uptake. With an additional 50 % reduction in anthropogenic VOCs emission, the predicted ozone concentration decreases by 5–12 ppbv (6–15 %) in the entire geographic area of China, with an exception in the areas, where the role of BVOCs is crucial to ozone formation. Further, the adopted reduction in NOx emission leads to an increase of AOC by 18 % in VOC-limited areas. This specific increase is associated with the combined effect of enhanced radical cycles associated with the photolysis of oxidized VOCs (OVOCs) and the oxidation of alkenes by hydroxyl (OH) radical and O3. A large reduction of daytime AOC in summer results from the reduction in anthropogenic VOCs emission, with a dominant contribution from the reaction of OH radical with reduced alkenes, followed by the reactions with depleted aromatics and OVOCs. This study highlights that photolysis of OVOCs and oxidation of alkenes in urban areas when NOx emission is reduced leads to an increase in O3. To mitigate ozone rises in urban areas, a joint reduction in the emission of NOx and specific VOCs species, including alkenes and aromatics and photodegradable OVOCs, should be implemented. [ABSTRACT FROM AUTHOR]

Increasing the model resolution is expected to be one way for improving air quality forecasts in urban areas. In this study, we evaluate the model performance in a large city at various resolutions to examine the best resolution for air pollution simulations. The comparison with measurements at a station near the traffic emissions shows the advantage of using high resolutions for capturing the extreme values. The statistical evaluation indicates that the highest model resolution (33 m) provides the best results for NOX concentration distributions near the traffic roads, while the improvement for roadside O3 with decreasing grid spacing stops at a certain point. The best model performance for the areas with a distance to the pollution sources is with the resolution of 100–300 m, at which the transport errors are equivalent to the emission biases. Plain Language Summary: As the increasing needs in the air quality forecasting in large cities, there is a trend in decreasing the model grid spacing to obtain more detailed pollutants distributions between neighborhoods or at street levels. To determine at which resolution the model can obtain the best representation of the pollutants' concentrations, we evaluate the model performance at different resolutions taking Hong Kong as an illustration. The analysis shows that the improvement with increasing model resolution is not monotonic for the areas far away from the intense emissions; however, the model with the highest resolution (33 m) reproduces the best results for the short‐lived species near the pollution sources. Key Points: Increasing horizontal resolution to 33 m improves the prediction of NOX near the traffic emissionsThe threshold of the model resolution is around 300 m for areas with a distance to the pollution sourcesThe changes of model performances with varied resolutions are different for NOX and O3 [ABSTRACT FROM AUTHOR]

Atmospheric oxidation capacity (AOC) characterizes the ability of the atmosphere to scavenge air pollutants. However, the processes involved in China, where anthropogenic emissions have changed dramatically in the past decade, are not fully understood. A detailed analysis of different parameters that determine the AOC in China is presented on the basis of numerical simulations performed with the regional chemical–meteorological Weather Research and Forecasting model with Chemistry (WRF-Chem). The model shows that the aerosol effects related to extinction and heterogeneous processes produce a decrease in surface ozone of approximately 8–10 ppbv in NO x -limited rural areas and an increase of 5–10 ppbv in VOC-limited urban areas. In this latter case, the ozone increase is noticeable for aerosol concentrations ranging from 20 to 45 µ g m -3 in July 2018. The ozone reduction in NO x -sensitive regions is due to the combined effect of nitrogen dioxide and peroxy radical uptake on particles and of the light extinction by aerosols, which affects the photodissociation rates. The ozone increase in VOC-sensitive areas is attributed to the uptake of NO 2 by aerosols, which is offset by the reduced ozone formation associated with HO 2 uptake and with aerosol extinction. Our study concludes that more than 90 % of the daytime AOC is due to the reaction of the hydroxyl radical with VOCs and carbon monoxide. In urban areas, during summertime, the main contributions to daytime AOC are the reactions of OH with alkene (30 %–50 %), oxidized volatile organic compounds (OVOCs) (33 %–45 %), and carbon monoxide (20 %–45 %). In rural areas, the largest contribution results from the reaction of OH with alkenes (60 %). Nocturnal AOC is dominantly attributed to the reactions with the nitrate radical (50 %–70 %). Our results shed light on the contribution of aerosol-related NO x loss and the high reactivity of alkenes for photochemical pollution. With the reduction in aerosols and anthropogenic ozone precursors, the chemistry of nitrogen and temperature-sensitive VOCs will become increasingly important. More attention needs to be paid to the role of photodegradable OVOCs and nocturnal oxidants in the formation of secondary pollutants. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence, which produces chaotic motions in the planetary boundary layer, can inhibit mixing between fast-reacting species produced or released at different locations. This segregation process modifies the effective rate at which reactions occur between these species and is not appropriately accounted for in coarse-resolution models, since these models assume complete mixing of tracers within each grid box. Here, we present a few examples of large-eddy simulations (LES) applied to chemically reactive species in a forested area with high emissions of biogenic hydrocarbons, an urban area rich in anthropogenic emissions, and a maritime area with high emissions of reduced sulfur species. [ABSTRACT FROM AUTHOR]

The atmospheric oxidation capacity (AOC) characterizes the ability of the atmosphere to scavenge air pollutants. However, it is not well understood in China, where anthropogenic emissions have changed dramatically in the past decade. A detailed analysis of different parameters that determine the AOC in China is presented on the basis of numerical simulations performed with the regional chemical-meteorological model WRF-Chem. The model results, with the aerosol effects of extinction and heterogeneous processes taken into account, show that the presence of aerosols leads to a decrease in surface ozone of approximately 8-10 ppbv in NOx-limited rural areas and an increase of 5-10 ppbv in VOC-limited urban areas. The ozone reduction in NOx-sensitive regions is due to the combined effect of nitrogen dioxide and peroxy radical uptake on particles and of the light extinction by aerosols, which affects the photodissociation rates. The ozone increase in VOC-sen- sitive areas is attributed to the uptake of NO2 by aerosols, which is offset by the reduced ozone formation associated with HO2 uptake and with the aerosol extinction. Our study concludes that more than 90% of the daytime AOC is due to the reaction of the hydroxyl radical with VOCs and carbon monoxide. In urban areas, during summertime, the main contributions to daytime AOC are the reactions of OH with alkene (30-50%), oxidized volatile organic compounds (OVOCs) (33-45%), and carbon monoxide (20-45%). In rural areas, the largest contribution results from the reaction of OH with alkenes (60%). Nocturnal AOC is dominantly attributed to the nitrate radical (50-70%). Our results shed light on the contribution of aerosol-related NOx loss and the high reactivity of alkenes for photochemical pollution. With the reduction of aerosols and anthropogenic ozone precursors, the chemistry of nitrogen and temperature-sensitive VOCs will become increasingly important. More attention needs to be paid to the role of photodegradable OVOCs and nocturnal oxidants in the formation of secondary pollutants. [ABSTRACT FROM AUTHOR]

Amplitude modulation (AM) quantifies the top-down interactions between the large-scale motions (LSMs) in the outer layer and the near-ground turbulence structures. They are important to the momentum transport and pollutant dispersion in urban atmospheric surface layers (ASLs). The dataset of large-eddy simulation over a densely built region in Kowloon Peninsula, Hong Kong, therefore, is adopted to investigate the AM of small-scale eddies by LSMs in the ASL over real urban morphology. Alike its smooth-wall counterpart, the small-scale eddies are (positively) amplitude modulated by the LSMs in most regions of the roughness sublayer (RSL). However, negative AM is unexpectedly found in the RSL on the building windward side in this study, illustrating the heterogeneity of the urban surface and the flow dynamics being affected aloft. In addition, strong sweep (u′ > 0 and w′ < 0) and ejection (u′ < 0 and w′ > 0) dominate the flows, respectively, in the positive and negative AM zones. In the positive AM zones, the large-scale sweep (uL′ > 0) leads to the surplus in the small-scale turbulence kinetic energy (TKE), while the large-scale ejection (uL′ < 0) brings a TKE deficit to the small-scale eddies. By contrast, the large-scale sweeps result in a TKE deficit to the small-scale eddies and the large-scale ejections result in a TKE surplus in the negative AM zones. These findings could help elucidate the AM over different building designs and urban morphology in cities, promoting the momentum transport and pollutant dispersion via proper city planning. [ABSTRACT FROM AUTHOR]

To perform realistic high-resolution air quality modeling in a polluted urban area, the WRF (Weather Research and Forecasting) model is used with an embedded large-eddy simulation (LES) module and with online chemistry. As an illustration, a numerical experiment is conducted in the megacity Hong Kong, which is characterized by multi-type pollution sources as well as complex topography. The multi-resolution simulations from mesoscale to LES scales are evaluated by comparing to ozone sounding profiles and surface observations. The comparisons show that both mesoscale and LES simulations reproduce well the mean concentrations of the chemical species and their diurnal variations at the background stations. However, the mesoscale simulations largely underestimate the NOX concentrations and overestimate O3 at the roadside stations due to the coarse representation of the traffic emissions. The LES simulations improve the agreement with the measurements near the road traffic, and the LES with the highest spatial resolution (33.3 m) provides the best results. The LES simulations show more detailed structures in the spatial distributions of chemical species than the mesoscale simulations, highlighting the capability of LES to resolve high-resolution photochemical transformations in urban areas. Compared to the mesoscale model results, the LES simulations show similar evolutions in the profiles of the chemical species as a function of the boundary layer development over a diurnal cycle. [ABSTRACT FROM AUTHOR]

Tropospheric ozone (O3) concentration is increasing in China along with dramatic changes in precursor emissions and meteorological conditions, adversely affecting human health and ecosystems. O3 is formed from the complex nonlinear photochemical reactions from nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs). Although the mechanism of O3 formation is rather clear, describing and analyzing its changes and formation potential at fine spatial and temporal resolution is still a challenge today. In this study, we briefly summarized and evaluated different approaches that indicate O3 formation regimes. We identify that atmospheric oxidation capacity (AOC) is a better indicator of photochemical reactions leading to the formation of O3 and other secondary pollutants. Results show that AOC has a prominent positive relationship to O3 in the major city clusters in China, with a goodness of fit (R2) up to 0.6. This outcome provides a novel perspective in characterizing O3 formation and has significant implications for formulating control strategies of secondary pollutants. [ABSTRACT FROM AUTHOR]

Using the CAM‐chem Model, we simulate the response of chemical species in the free troposphere to scenarios of primary pollutant emission reductions during the COVID‐19 pandemic. Zonally averaged ozone in the free troposphere during Northern Hemisphere spring and summer is found to be 5%–15% lower than 19‐yr climatological values, in good agreement with observations. About one third of this anomaly is attributed to the reduction scenario of air traffic during the pandemic, another third to the reduction scenario of surface emissions, the remainder to 2020 meteorological conditions, including the exceptional springtime Arctic stratospheric ozone depletion. For the combined emission reductions, the overall COVID‐19 reduction in northern hemisphere tropospheric ozone in June is less than 5 ppb below 400 hPa, but reaches 8 ppb at 250 hPa. In the Southern Hemisphere, COVID‐19 related ozone reductions by 4%–6% were masked by comparable ozone increases due to other changes in 2020. Plain Language Summary: The reduction in the emissions of primary air pollutants during the 2020 COVID‐19 pandemic has generated perturbations in the chemical state of the atmosphere. A global Earth system model that accounts for chemical, physical, and dynamical processes in the atmosphere and for the coupling between the atmosphere, the ocean and the land surface, indicates that the abundance of tropospheric ozone was significantly reduced during the pandemic in response to realistic scenarios of reduced emissions of primary pollutants associated with restrictions of air traffic and economic activities. These simulated findings are consistent with observed ozone anomalies during the summer of 2020. Key Points: The ozone concentration in the northern extratropical free troposphere was 5%–15% lower in May and June 2020 relative to climatologyA third of this anomaly is attributed to meteorological conditions including stratospheric Arctic air with abnormally low ozoneThe assumed reduction in surface and aircraft emissions associated with the COVID‐19 pandemic can explain an ozone anomaly of 4%–8% [ABSTRACT FROM AUTHOR]

We use the global Community Earth System Model to investigate the response of secondary pollutants (ozone O3, secondary organic aerosols SOA) in different parts of the world in response to modified emissions of primary pollutants during the COVID‐19 pandemic. We quantify the respective effects of the reductions in NOx and in volatile organic carbon (VOC) emissions, which, in most cases, affect oxidants in opposite ways. Using model simulations, we show that the level of NOx has been reduced by typically 40% in China during February 2020 and by similar amounts in many areas of Europe and North America in mid‐March to mid‐April 2020, in good agreement with space and surface observations. We show that, relative to a situation in which the emission reductions are ignored and despite the calculated increase in hydroxyl and peroxy radicals, the ozone concentration increased only in a few NOx‐saturated regions (northern China, northern Europe, and the US) during the winter months of the pandemic when the titration of this molecule by NOx was reduced. In other regions, where ozone is NOx‐controlled, the concentration of ozone decreased. SOA concentrations decrease in response to the concurrent reduction in the NOx and VOC emissions. The model also shows that atmospheric meteorological anomalies produced substantial variations in the concentrations of chemical species during the pandemic. In Europe, for example, a large fraction of the ozone increase in February 2020 was associated with meteorological anomalies, while in the North China Plain, enhanced ozone concentrations resulted primarily from reduced emissions of primary pollutants. Plain Language Summary: With the reduction in economic activities following the COVID‐19 pandemic outbreak in early 2020, most emissions of air pollutants (i.e., nitrogen oxides [NOx], carbon monoxide [CO], sulfur dioxide [SO2], volatile organic carbon [VOC], black carbon [BC], organic carbon [OC]) have decreased substantially during several months in different regions of the world. This unintended global experiment offered a glimpse into a potential future in which air quality would be improved. Here, a global atmospheric model is used to assess the changes in the chemical composition of the atmosphere during the pandemic period and in the related chemical processes that lead to the formation of ozone (O3) and secondary organic aerosols (SOA). The study illustrates the nonlinearity of the air quality response to reduced NOx and VOC emissions, which depends on the chemical environment including the background level of nitrogen oxides. Meteorological variability can lead to anomalies in the concentration of chemical species with magnitudes that are as large or even larger than the perturbations due to COVID‐induced changes in the emissions. Key Points: During the COVID‐19 lockdown, the atmospheric concentration of primary pollutants (NOx, VOCs, CO, SO2) was considerably reducedThe concentration of secondary pollutants increased in NOx‐saturated areas and decreased in NOx‐limited areasThe response of the chemical system depends on the relative changes in NOx and VOC emissions, and is affected by weather variability [ABSTRACT FROM AUTHOR]

Monitoring and modeling/predicting air pollution are crucial to understanding the links between emissions and air pollution levels, to supporting air quality management, and to reducing human exposure. Yet, current monitoring networks and modeling capabilities are unfortunately inadequate to understand the physical and chemical processes above ground and to support attribution of sources. We highlight the need for the development of an international stereoscopic monitoring strategy that can depict three-dimensional (3D) distribution of atmospheric composition to reduce the uncertainties and to advance diagnostic understanding and prediction of air pollution. There are three reasons for the implementation of stereoscopic monitoring: 1) current observation networks provide only partial view of air pollution, and this can lead to misleading air quality management actions; 2) satellite retrievals of air pollutants are widely used in air pollution studies, but too often users do not acknowledge that they have large uncertainties, which can be reduced with measurements of vertical profiles; and 3) air quality modeling and forecasting require 3D observational constraints. We call on researchers and policymakers to establish stereoscopic monitoring networks and share monitoring data to better characterize the formation of air pollution, optimize air quality management, and protect human health. Future directions for advancing monitoring and modeling/predicting air pollution are also discussed. [ABSTRACT FROM AUTHOR]

Global and regional chemical transport models of the atmosphere are based on the assumption that chemical species are completely mixed within each model grid box. However, in reality, these species are often segregated due to localized sources and the influence of topography. In order to investigate the degree to which the rates of chemical reactions between two reactive species are reduced due to the possible segregation of species within the convective boundary layer, we perform large-eddy simulations (LESs) in the mountainous region of Hong Kong Island. We adopt a simple chemical scheme with 15 primary and secondary chemical species, including ozone and its precursors. We calculate the segregation intensity due to inhomogeneity in the surface emissions of primary pollutants and due to turbulent motions related to topography. We show that the inhomogeneity in the emissions increases the segregation intensity by a factor of 2–5 relative to a case in which the emissions are assumed to be uniformly distributed. Topography has an important effect on the segregation locally, but this influence is relatively limited when considering the spatial domain as a whole. In the particular setting of our model, segregation reduces the ozone formation by 8 %–12 % compared to the case with complete mixing, implying that the coarse-resolution models may overestimate the surface ozone when ignoring the segregation effect. [ABSTRACT FROM AUTHOR]

We employed direct numerical simulations to estimate the error on chemical calculation in simulations with regional chemical-transport models induced by neglecting subgrid chemical segregation due to inefficient turbulent mixing in an urban boundary layer with strong and heterogeneously distributed surface emissions. In simulations of initially segregated reactive species with an entrainment-emission configuration with an A–B–C second-order chemical scheme, urban surface emission fluxes of the homogeneously emitted tracer A result in a very large segregation between the tracers and hence a very large overestimation of the effective chemical reaction rate in a complete-mixing model. This large effect can be indicated by a large Damköhler number (Da) of the limiting reactant. With heterogeneous surface emissions of the two reactants, the resultant normalised boundary-layer-averaged effective chemical reaction rate is found to be in a Gaussian function of Da , and it is increasingly overestimated by the imposed rate with an increased horizontal scale of emission heterogeneity. Coarse-grid models with resolutions commensurable to regional models give reduced yet still significant errors for all simulations with homogeneous emissions. Such model improvement is more sensitive to the increased vertical resolution. However, such improvement cannot be seen for simulations with heterogeneous emissions when the horizontal resolution of the model cannot resolve emission heterogeneity. This work highlights particular conditions in which the ability to resolve chemical segregation is especially important when modelling urban environments. [ABSTRACT FROM AUTHOR]