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Outdoor air pollution adversely affects human health and is estimated to be responsible for five to ten per cent of the total annual premature mortality in the contiguous United States 1 – 3 . Combustion emissions from a variety of sources, such as power generation or road traffic, make a large contribution to harmful air pollutants such as ozone and fine particulate matter (PM 2.5 ) 4 . Efforts to mitigate air pollution have focused mainly on the relationship between local emission sources and local air quality 2 . Air quality can also be affected by distant emission sources, however, including emissions from neighbouring federal states 5 , 6 . This cross-state exchange of pollution poses additional regulatory challenges. Here we quantify the exchange of air pollution among the contiguous United States, and assess its impact on premature mortality that is linked to increased human exposure to PM 2.5 and ozone from seven emission sectors for 2005 to 2018. On average, we find that 41 to 53 per cent of air-quality-related premature mortality resulting from a state’s emissions occurs outside that state. We also find variations in the cross-state contributions of different emission sectors and chemical species to premature mortality, and changes in these variations over time. Emissions from electric power generation have the greatest cross-state impacts as a fraction of their total impacts, whereas commercial/residential emissions have the smallest. However, reductions in emissions from electric power generation since 2005 have meant that, by 2018, cross-state premature mortality associated with the commercial/residential sector was twice that associated with power generation. In terms of the chemical species emitted, nitrogen oxides and sulfur dioxide emissions caused the most cross-state premature deaths in 2005, but by 2018 primary PM 2.5 emissions led to cross-state premature deaths equal to three times those associated with sulfur dioxide emissions. These reported shifts in emission sectors and emission species that contribute to premature mortality may help to guide improvements to air quality in the contiguous United States. This analysis of the exchange of air pollution amongst the contiguous United States finds that, on average, around half of the early deaths caused by a state’s air pollution occurs outside that state, with different contributions by different emission sectors and chemical species.

Crop residue burning contributes to poor air quality and imposes a health burden on India. Despite government bans and other interventions, this practice remains widespread. Here we estimate the impact of changes in agricultural emissions on air quality across India and quantify the potential benefit of district-level actions using an adjoint modeling approach. From 2003 to 2019, we find that agricultural residue burning caused 44,000–98,000 particulate matter exposure-related premature deaths annually, of which Punjab, Haryana, and Uttar Pradesh contribute 67–90%. Due to a combination of relatively high downwind population density, agricultural output, and cultivation of residue-intensive crops, six districts in Punjab alone contribute to 40% of India-wide annual air quality impacts from residue burning. Burning two hours earlier in Punjab alone could avert premature deaths up to 9600 (95% CI: 8000–11,000) each year, valued at 3.2 (95% CI: 0.49–7.3) billion US dollars. Our findings support the use of targeted and potentially low-cost interventions to mitigate crop residue burning in India, pending further research regarding cost-effectiveness and feasibility. We quantify the air quality and health impacts of Indian crop residue burning and trace these impacts back to individual burning events by hour and district. We find that small interventions, such as burning one hour earlier in the day, may provide broad public health benefits

Climate policies that target greenhouse gas emissions can improve air quality by reducing co-emitted air pollutant emissions. However, the extent to which climate policy could contribute to the targets of reducing existing pollution disparities across different populations remains largely unknown. We quantify potential air pollution exposure reductions under U.S. federal carbon policy, considering implications of resulting health benefits for exposure disparities across U.S. racial/ethnic groups. We focus on policy cases that achieve reductions of 40-60% in 2030 economy-wide carbon dioxide (CO 2 ) emissions, when compared with 2005 emissions. The 50% CO 2 reduction policy case reduces average fine particulate matter (PM 2.5 ) exposure across racial/ethnic groups, with greatest benefit for non-Hispanic Black (−0.44 μg/m 3 ) and white populations (−0.37 μg/m 3 ). The average exposure disparity for racial/ethnic minorities rises from 12.4% to 13.1%. Applying an optimization approach to multiple emissions reduction scenarios, we find that no alternate combination of reductions from different CO 2 sources would substantially mitigate exposure disparities. Results suggest that CO 2 -based strategies for this range of reductions are insufficient for fully mitigating PM 2.5 exposure disparities between white and racial/ethnic minority populations; addressing disparities may require larger-scale structural changes. U.S. federal climate policies can reduce air pollutant emissions and associated health impacts from fine particulate matter. However, near-term CO 2 reductions alone are insufficient to address racial/ethnic disparities in pollution exposure.

We present a comprehensive simulation of tropospheric chlorine within the GEOS-Chem global 3-D model of oxidant–aerosol–halogen atmospheric chemistry. The simulation includes explicit accounting of chloride mobilization from sea salt aerosol by acid displacement of HCl and by other heterogeneous processes. Additional small sources of tropospheric chlorine (combustion, organochlorines, transport from stratosphere) are also included. Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl, HOCl, ClNO3, ClNO2, and minor species, is produced by the HCl+OH reaction and by heterogeneous conversion of sea salt aerosol chloride to BrCl, ClNO2, Cl2, and ICl. The model successfully simulates the observed mixing ratios of HCl in marine air (highest at northern midlatitudes) and the associated HNO3 decrease from acid displacement. It captures the high ClNO2 mixing ratios observed in continental surface air at night and attributes the chlorine to HCl volatilized from sea salt aerosol and transported inland following uptake by fine aerosol. The model successfully simulates the vertical profiles of HCl measured from aircraft, where enhancements in the continental boundary layer can again be largely explained by transport inland of the marine source. It does not reproduce the boundary layer Cl2 mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in the daytime, low at night); the model is too high at night, which could be due to uncertainty in the rate of the ClNO2+Cl- reaction, but we have no explanation for the high observed Cl2 in daytime. The global mean tropospheric concentration of Cl atoms in the model is 620 cm−3 and contributes 1.0 % of the global oxidation of methane, 20 % of ethane, 14 % of propane, and 4 % of methanol. Chlorine chemistry increases global mean tropospheric BrO by 85 %, mainly through the HOBr+Cl- reaction, and decreases global burdens of tropospheric ozone by 7 % and OH by 3 % through the associated bromine radical chemistry. ClNO2 chemistry drives increases in ozone of up to 8 ppb over polluted continents in winter.

Quasi-horizontal chemical plumes in the free troposphere can preserve their concentrated structure for over a week, enabling transport on intercontinental scales with important environmental impacts. Global Eulerian chemical transport models (CTMs) fail to preserve these plumes due to fast numerical dissipation. We examine the causes of this dissipation and how it can be cured. Goddard Earth Observing System (GEOS-5) meteorological data at 0.25°  ×  0.3125° horizontal resolution and  ∼  0.5 km vertical resolution in the free troposphere are used to drive a worldwide ensemble of GEOS-Chem CTM plumes at resolutions from 0.25°  ×  0.3125° to 4°  ×  5°, in both 2-D (horizontal) and 3-D. Two-dimensional simulations enable examination of the sensitivity of numerical dissipation to grid resolution. We show that plume decay is driven by flow divergence and shear, filamenting the plumes until GEOS-Chem's high-order advection scheme cannot resolve gradients and fast numerical diffusion ensues. This divergence can be measured by the Lyapunov exponent (λ) of the flow. Dissipation of plumes is much faster at extratropical latitudes than in the tropics and this can be explained by stronger divergence. The plume decay constant (α) is linearly related to λ, and increasing grid resolution provides only modest benefits toward plume preservation. Three-dimensional simulations show near-complete dissipation of plumes within a few days, independent of horizontal grid resolution and even in the tropics. This is because vertical grid resolution is inadequate in all cases to properly resolve plume gradients. We suggest that finer vertical grid resolution in the free troposphere is essential for models to resolve intercontinental plumes, while current horizontal resolution in these models (∼  1°) is sufficient.

We present an updated mechanism for tropospheric halogen (Cl + Br + I) chemistry in the GEOS-Chem global atmospheric chemical transport model and apply it to investigate halogen radical cycling and implications for tropospheric oxidants. Improved representation of HOBr heterogeneous chemistry and its pH dependence in our simulation leads to less efficient recycling and mobilization of bromine radicals and enables the model to include mechanistic sea salt aerosol debromination without generating excessive BrO. The resulting global mean tropospheric BrO mixing ratio is 0.19 ppt (parts per trillion), lower than previous versions of GEOS-Chem. Model BrO shows variable consistency and biases in comparison to surface and aircraft observations in marine air, which are often near or below the detection limit. The model underestimates the daytime measurements of Cl2 and BrCl from the ATom aircraft campaign over the Pacific and Atlantic, which if correct would imply a very large missing primary source of chlorine radicals. Model IO is highest in the marine boundary layer and uniform in the free troposphere, with a global mean tropospheric mixing ratio of 0.08 ppt, and shows consistency with surface and aircraft observations. The modeled global mean tropospheric concentration of Cl atoms is 630 cm−3, contributing 0.8 % of the global oxidation of methane, 14 % of ethane, 8 % of propane, and 7 % of higher alkanes. Halogen chemistry decreases the global tropospheric burden of ozone by 11 %, NOx by 6 %, and OH by 4 %. Most of the ozone decrease is driven by iodine-catalyzed loss. The resulting GEOS-Chem ozone simulation is unbiased in the Southern Hemisphere but too low in the Northern Hemisphere.

We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O3) concentrations, except in polar regions where the model now underestimates O3 concentrations. Halogen chemistry reduces the global tropospheric O3 burden by 18.6 %, with the O3 lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28  ×  106 molecules cm−3 are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH4 lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH4 by Cl is small (∼  2 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼  15–27 %). Oxidation of VOCs by Br is smaller, representing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde.

Proposed contrail avoidance schemes rely on being able to robustly predict which contrails cause the most climate warming. However, it has not yet been shown that different contrail models agree sufficiently to support the targeting of individual contrails by climate impact. To address this, we compare the most widespread contrail model, the Contrail Cirrus Prediction (CoCiP), to a higher-fidelity contrail model, the Aircraft Plume Chemistry, Emissions, and Microphysics Model (APCEMM), under parameterized meteorological conditions. The results show that the time-integrated total extinction (a proxy for climate impact) in APCEMM is 3.8 times that in CoCiP and that the models have opposite sensitivities of their time-integrated total extinction to relative humidity. We argue that these differences are due to the differing representations of the distribution of ice particles in space and in size across the contrails. The use of a monodisperse ice particle size distribution in a Gaussian plume means that CoCiP models the contrail exclusively as an accelerating, falling mass. The use of a spatially gridded and size-resolved aerosol scheme allows APCEMM to represent the separation of the precipitation plume from the contrail core, hence modeling behavior beyond the initial phase in which the contrail grows unconstrained. This behavior is consistent with prior large-eddy simulation studies, and it accounts for 92 % of the aggregate APCEMM time-integrated total extinction across all simulations. This suggests that models lacking a size-resolved aerosol scheme may underestimate the time-integrated total extinction. While a strategy avoiding a given proportion of persistent contrails in an unbiased way is still expected to yield a proportional reduction in the time-integrated total extinction, implementing strategies using contrail models to select the specific contrails to avoid may lead to fewer reductions in the time-integrated total extinction, primarily due to the current level of disagreement between models. We therefore recommend more research to establish confidence in model predictions at later contrail ages.

Satellite-based retrievals of tropospheric NO2 columns are widely used to infer NOx (≡ NO + NO2) emissions. These retrievals rely on model information for the vertical distribution of NO2. The free tropospheric background above 2 km is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NOx also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC4RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating NOx in the free troposphere and attribute it to sources. NO2 measurements during the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) and Deep Convective Clouds and Chemistry (DC3) campaigns over the southeastern U.S. in summer show increasing concentrations in the upper troposphere above 10 km, which are not replicated by the GEOS-Chem, although the model is consistent with the NO measurements. Using concurrent NO, NO2, and ozone observations from a DC3 flight in a thunderstorm outflow, we show that the NO2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO2 reservoirs such as peroxynitric acid (HNO4) and methyl peroxy nitrate (MPN). We find that NO2 concentrations calculated from the NO measurements and NO–NO2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO2 profiles throughout the troposphere for SEAC4RS and DC3 but overestimates NO2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NOx chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the Atmospheric Tomography Mission (ATom) campaigns over the Pacific and Atlantic oceans, indicating a missing NOx source over the oceans. We find that we can account for this missing source by including in the model the photolysis of particulate nitrate on sea salt aerosols at rates inferred from laboratory studies and field observations of nitrous acid (HONO) over the Atlantic. The median PSS-inferred tropospheric NO2 column density for the ATom campaign is 1.7 ± 0.44 × 1014 molec. cm-2, and the NO2 column density simulated by the four models is in the range of 1.4–2.4 × 1014 molec. cm-2, implying that the uncertainty from using modeled NO2 tropospheric columns over clean areas in the retrievals for stratosphere–troposphere separation is about 1 × 1014 molec. cm-2. We find from GEOS-Chem that lightning is the main primary NOx source in the free troposphere over the tropics and southern midlatitudes, but aircraft emissions dominate at northern midlatitudes in winter and in summer over the oceans. Particulate nitrate photolysis increases ozone concentrations by up to 5 ppbv (parts per billion by volume) in the free troposphere in the northern extratropics in the model, which would largely correct the low model bias relative to ozonesonde observations. Global tropospheric OH concentrations increase by 19 %. The contribution of the free tropospheric background to the tropospheric NO2 columns observed by satellites over the contiguous U.S. increases from 25 ± 11 % in winter to 65 ± 9 % in summer, according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NOx emissions from satellite NO2 column measurements.

As carbon-free fuel, ammonia has been proposed as an alternative fuel to facilitate maritime decarbonization. Deployment of ammonia-powered ships is proposed as soon as 2024. However, NO x , NH 3 and N 2 O from ammonia combustion could impact air quality and climate. In this study, we assess whether and under what conditions switching to ammonia fuel might affect climate and air quality. We use a bottom–up approach combining ammonia engine experiment results and ship track data to estimate global tailpipe NO x , NH 3 and N 2 O emissions from ammonia-powered ships with two possible engine technologies (NH 3 –H 2 (high NO x , low NH 3 emissions) vs pure NH 3 (low NO x , very high NH 3 emissions) combustion) under three emission regulation scenarios (with corresponding assumptions in emission control technologies), and simulate their air quality impacts using GEOS–Chem high performance global chemical transport model. We find that the tailpipe N 2 O emissions from ammonia-powered ships have climate impacts equivalent to 5.8% of current shipping CO 2 emissions. Globally, switching to NH 3 –H 2 engines avoids 16 900 mortalities from PM 2.5 and 16 200 mortalities from O 3 annually, while the unburnt NH 3 emissions (82.0 Tg NH 3 yr −1 ) from pure NH 3 engines could lead to 668 100 additional mortalities from PM 2.5 annually under current legislation. Requiring NH 3 scrubbing within current emission control areas leads to smaller improvements in PM 2.5 -related mortalities (22 100 avoided mortalities for NH 3 –H 2 and 623 900 additional mortalities for pure NH 3 annually), while extending both Tier III NO x standard and NH 3 scrubbing requirements globally leads to larger improvement in PM 2.5 -related mortalities associated with a switch to ammonia-powered ships (66 500 avoided mortalities for NH 3 –H 2 and 1200 additional mortalities for pure NH 3 annually). Our findings suggest that while switching to ammonia fuel would reduce tailpipe greenhouse gas emissions from shipping, stringent ammonia emission control is required to mitigate the potential adverse effects on air quality.