ALBERT

Description: The prospective future adoption of molecular hydrogen (H2) to power the road transportation sector could greatly improve tropospheric air quality but also raises the question of whether the adoption would have adverse effects on the stratospheric ozone. The possibility of undesirable impacts must be fully evaluated to guide future policy decisions. Here we evaluate the possible impact of a future (2050) H2-based road transportation sector on stratospheric composition and chemistry, especially on the stratospheric ozone, with the MOZART (Model for OZone And Related chemical Tracers) model. Since future growth is highly uncertain, we evaluate the impact of two world evolution scenarios, one based on an IPCC (Intergovernmental Panel on Climate Change) high-emitting scenario (A1FI) and the other on an IPCC low-emitting scenario (B1), as well as two technological options: H2 fuel cells and H2 internal combustion engines. We assume a H2 leakage rate of 2.5% and a complete market penetration of H2 vehicles in 2050. The model simulations show that a H2-based road transportation sector would reduce stratospheric ozone concentrations as a result of perturbed catalytic ozone destruction cycles. The magnitude of the impact depends on which growth scenario evolves and which H2 technology option is applied. For the evolution growth scenario, stratospheric ozone decreases more in the H2 fuel cell scenarios than in the H2 internal combustion engine scenarios because of the NOx emissions in the latter case. If the same technological option is applied, the impact is larger in the A1FI emission scenario. The largest impact, a 0.54% decrease in annual average global mean stratospheric column ozone, is found with a H2 fuel cell type road transportation sector in the A1FI scenario; whereas the smallest impact, a 0.04% increase in stratospheric ozone, is found with applications of H2 internal combustion engine vehicles in the B1 scenario. The impacts of the other two scenarios fall between the above two boundary scenarios. However, the magnitude of these changes is much smaller than the increases in 2050 stratospheric ozone projected, as stratospheric ozone is expected to recover due to the limits in ozone depleting substance emissions imposed in the Montreal Protocol.

Description: Vehicles burning fossil fuel emit a number of substances that change the composition and chemistry of the atmosphere, and contribute to global air and water pollution and climate change. For example, nitrogen oxides and volatile organic compounds (VOCs) emitted as byproducts of fossil fuel combustion are key precursors to ground-level ozone and aerosol formation. In addition, on-road vehicles are major CO2 emitters. In order to tackle these problems, molecular hydrogen (H2) has been proposed as an energy carrier to substitute for fossil fuels in the future. However, before implementing any such strategy it is crucial to evaluate its potential impacts on air quality and climate. Here, we evaluate the impact of a future (2050) H2-based road transportation sector on tropospheric chemistry and air quality for several possible growth and technology adoption scenarios. The growth scenarios are based on the high and low emissions Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios, A1FI and B1, respectively. The technological adoption scenarios include H2 fuel cell and H2 internal combustion engine options. The impacts are evaluated with the Community Atmospheric Model Chemistry global chemistry transport model (CAM-Chem). Higher resolution simulations focusing on the contiguous United States are also carried out with the Community Multiscale Air Quality Modeling System (CMAQ) regional chemistry transport model. For all scenarios future air quality improves with the adoption of a H2-based road transportation sector; however, the magnitude and type of improvement depend on the scenario. Model results show that the adoption of H2 fuel cells would decrease tropospheric burdens of ozone (7%), CO (14%), NOx (16%), soot (17%), sulfate aerosol (4%), and ammonium nitrate aerosol (12%) in the A1FI scenario, and would decrease those of ozone (5%), CO (4%), NOx (11%), soot (7%), sulfate aerosol (4%), and ammonium nitrate aerosol (9%) in the B1 scenario. The adoption of H2 internal combustion engines would decrease tropospheric burdens of ozone (1%), CO (18%), soot (17%), and sulfate aerosol (3%) in the A1FI scenario, and would decrease those of ozone (1%), CO (7%), soot (7%), and sulfate aerosol (3%) in the B1 scenario. In the future, people residing in the contiguous United States could expect to experience significantly fewer days of elevated levels of pollution if a H2 fuel cell road transportation sector were to be adopted. Health benefits of transitioning to a H2 economy for citizens in developing nations, like China and India, will be much more dramatic, particularly in megacities with severe, intensifying air-quality problems.

Description: The continuing increase in demand for commercial aviation transport raises questions about the effects of resulting emissions on the environment. The purpose of this study is to investigate, using a global chemistry transport model, to what extent aviation emissions outside the boundary layer influence air quality in the boundary layer. The large-scale effects of current levels of aircraft emissions were studied through comparison of multiple simulations allowing for the separated effects of aviation emissions occurring in the low, middle and upper troposphere. We show that emissions near cruise altitudes (9–11 km in altitude) rather than emissions during landing and take-off are responsible for most of the total odd-nitrogen (NOy), ozone (O3) and aerosol perturbations near the ground with a noticeable seasonal difference. Overall, the perturbations of these species are smaller than 1 ppb even in winter when the perturbations are greater than in summer. Based on the widely used air quality standards and uncertainty of state-of-the-art models, we conclude that aviation-induced perturbations have a negligible effect on air quality even in areas with heavy air traffic. Aviation emissions lead to a less than 1% aerosol enhancement in the boundary layer due to a slight increase in ammonium nitrate (NH4NO3) during cold seasons and a statistically insignificant aerosol perturbation in summer. In addition, statistical analysis using probability density functions, Hellinger distance, and p value indicate that aviation emissions outside the boundary layer do not affect the occurrence of extremely high aerosol concentrations in the boundary layer. An additional sensitivity simulation assuming the doubling of surface ammonia emissions demonstrates that the aviation induced aerosol increase near the ground is highly dependent on background ammonia concentrations whose current range of uncertainty is large.

Description: The interaction between atmospheric chemistry and ozone (O3) in the upper troposphere–lower stratosphere (UTLS) presents a major uncertainty in understanding the effects of aviation on climate. In this study, two configurations of the atmospheric model from the Community Earth System Model (CESM), Community Atmosphere Model with Chemistry, Version 4 (CAM4) and Version 5 (CAM5), are used to evaluate the effects of aircraft nitrogen oxide (NOx = NO + NO2) emissions on ozone and the background chemistry in the UTLS. CAM4 and CAM5 simulations were both performed with extensive tropospheric and stratospheric chemistry including 133 species and 330 photochemical reactions. CAM5 includes direct and indirect aerosol effects on clouds using a modal aerosol module (MAM), whereby CAM4 uses a bulk aerosol module, which can only simulate the direct effect. To examine the accuracy of the aviation NOx-induced ozone distribution in the two models, results from the CAM5 and CAM4 simulations are compared to ozonesonde data. Aviation NOx emissions for 2006 were obtained from the AEDT (Aviation Environmental Design Tool) global commercial aircraft emissions inventory. Differences between simulated O3 concentrations and ozonesonde measurements averaged at representative levels in the troposphere and different regions are 13% in CAM5 and 18% in CAM4. Results show a localized increase in aviation-induced O3 concentrations at aviation cruise altitudes that stretches from 40° N to the North Pole. The results indicate a greater and more disperse production of aviation NOx-induced ozone in CAM5, with the annual tropospheric mean O3 perturbation of 1.2 ppb (2.4%) for CAM5 and 1.0 ppb (1.9%) for CAM4. The annual mean O3 perturbation peaks at about 8.2 ppb (6.4%) and 8.8 ppb (5.2%) in CAM5 and CAM4, respectively. Aviation emissions also result in increased hydroxyl radical (OH) concentrations and methane (CH4) loss rates, reducing the tropospheric methane lifetime in CAM5 and CAM4 by 1.69 and 1.40%, respectively. Aviation NOx emissions are associated with an instantaneous change in global mean short-term O3 radiative forcing (RF) of 40.3 and 36.5 mWm−2 in CAM5 and CAM4, respectively.

Description: The prospective future adoption of hydrogen to power the road transportation sector could greatly improve tropospheric air quality but also raises the question whether the adoption would have adverse effects on stratospheric ozone. The possibility of these undesirable impacts must be fully evaluated to guide future policy decisions. Here we evaluate the possible impact of a future (2050) H2-based road transportation sector on stratospheric composition and chemistry, especially on stratospheric ozone, with the MOZART chemical transport model. Since future growth is highly uncertain we evaluate the impact for two world evolution scenarios, one based on a high emitting scenario (IPCC A1FI) and the other on a low emitting scenario (IPCC B1), as well as two technological options: H2 fuel cells and H2 internal combustion engines. We assume a H2 leakage rate of 2.5% and a complete market penetration of H2 vehicles in 2050. The model simulations show that a H2-based road transportation sector would reduce stratospheric ozone concentrations as a result of perturbed catalytic ozone destruction cycles. The magnitude of the impact depends on which growth scenario the world evolves and which H2 technology option is applied. For the same world evolution scenario, stratospheric ozone decreases more in the H2 fuel cell scenarios than in the H2 internal combustion engine scenarios because of the NOx emissions in the latter case. If the same technological option is applied, the impact is larger in the A1FI emission scenario. The largest impact, a 0.54% decrease in annual average global mean stratospheric column ozone, is found with a H2 fuel cell type road transportation sector in the A1FI scenario; whereas the smallest impact, a 0.04% increase in stratospheric ozone, is found with applications of H2 internal combustion engine vehicles in the B1 scenario. The impacts of the other two scenarios fall between the above two bounding scenarios. However, the magnitude of these changes is much smaller than the increases in 2050 stratospheric ozone expected as stratospheric ozone recovers due to the limits in ozone depleting substance emissions imposed in the Montreal Protocol.

Description: The continuing increase in demand for commercial aviation transport raises questions about the effects of resulting emissions on the environment. The purpose of this study is to investigate, using a global chemistry transport model, to what extent aviation emissions outside the boundary layer influence air quality in the boundary layer. The effects of current levels of aircraft emissions were studied through comparison of multiple simulations allowing for the separated effects of aviation emissions occurring in the low, middle and upper troposphere. We show that emissions near cruise altitudes rather than emissions during landing and take-off are responsible for most of the total odd-nitrogen (NOy), ozone (O3) and aerosol perturbations near the ground with a noticeable seasonal difference. Overall, the perturbations of these species are smaller than 1 ppb even in winter when the perturbations are greater than in summer. Based on the widely used air quality standards and uncertainty of state-of-the-art models, we conclude that aviation-induced perturbations have a negligible effect on air quality even in areas with heavy air traffic. Aviation emissions lead to a less than 1% aerosol enhancement in the boundary layer due to a slight increase in ammonium nitrate (NH4NO3) during cold seasons and a statistically insignificant aerosol perturbation in summer. In addition, statistical analysis using probability density functions, Hellinger distance, and p-value indicate that aviation emissions outside the boundary layer do not affect the occurrence of extremely high aerosol concentrations in the boundary layer. An additional sensitivity simulation assuming the doubling of surface ammonia emissions demonstrates that the aviation induced aerosol increase near the ground is highly dependent on background ammonia concentrations whose current range of uncertainty is large.