ATMOS
Atmospheric and Climate Science Lab.
Atmospheric Chemistry Modelling And Future Projections
Atmospheric Chemistry Modelling And Future Projections
Atmospheric chemistry modelling is an essential scientific tool employed to comprehend the chemical composition of the Earth's atmosphere and its temporal variations. These models facilitate the examination of the sources, transformations, and ultimate disposition of pollutants and greenhouse gases by simulating interactions among diverse gases, aerosols, and external factors, including sunlight and human activities. Atmospheric models vary from basic box models to intricate three-dimensional global models, facilitating the examination of local air quality, climate change, and the long-range transport of atmospheric elements. These simulations enable scientists to forecast future atmospheric conditions, assess the effects of policy decisions, and enhance our comprehension of the Earth's climate system.
GEOS-Chem Chemical Transport Model
GEOS-Chem is a state-of-the-art global 3D chemical transport model (CTM) used to simulate atmospheric composition, with a focus on understanding the chemical and physical processes that affect air quality, climate, and biogeochemical cycles. Driven by meteorological data from NASA's Goddard Earth Observing System (GEOS), GEOS-Chem simulates the transport, chemical transformation, emissions, and deposition of trace gases and aerosols in the atmosphere.
The model is widely used in both research and policy assessment due to its detailed treatment of atmospheric chemistry and its modular, community-driven design. Applications of GEOS-Chem include studying tropospheric ozone formation, aerosol-climate interactions, the global carbon cycle, and the transport of pollutants across continents. It is also used in inverse modeling to estimate emissions based on observed atmospheric concentrations.
GEOS-Chem can operate in both global and nested high-resolution modes, allowing researchers to zoom in on specific regions while maintaining a global perspective. Its flexibility and comprehensive chemical mechanism make it one of the leading tools in atmospheric chemistry modelling today.
We have successfully deployed GEOS-Chem Classic version 14.4 along with the GCAP 2.0 framework on the Paramshakti high-performance computing (HPC) system at IIT Kharagpur and the Ginsburg Cluster at Columbia University in the City of New York.
Areas of Research:
1. Using GEOS-Chem to look at NOx-VOC-O3 sensitivity and in particular the impact of aerosol inhibited regimes due to reactive uptake and photochemistry
Using the GEOS-Chem chemical transport model to investigate NOₓ–VOC–O₃ sensitivity provides valuable insight into the nonlinear chemistry that governs ozone (O₃) formation in the troposphere. This relationship is particularly complex due to the interplay between nitrogen oxides (NOₓ), volatile organic compounds (VOCs), and aerosols, all of which influence photochemical reactions and radical budgets.
GEOS-Chem allows for detailed analysis of these interactions by simulating the spatial and temporal variability of emissions, transport, and chemical transformations. A key focus in recent studies is the role of aerosol-inhibited regimes, where particulate matter—especially those containing aqueous components—modifies the chemical environment through reactive uptake of key species such as HO₂, N₂O₅, and other intermediates. This uptake alters radical cycling and can suppress or enhance O₃ production depending on local conditions.
Additionally, aerosols affect photolysis rates by scattering and absorbing solar radiation, which in turn modifies the rate of photochemical reactions central to ozone formation. GEOS-Chem’s integrated aerosol chemistry and radiative feedback make it particularly suited for capturing these subtleties. By applying sensitivity simulations and tagged tracers within GEOS-Chem, we can quantify how ozone production responds to changes in precursor emissions under varying aerosol loads, and identify whether regions are NOₓ- or VOC-limited or fall into a transitional regime. This is especially relevant in polluted urban environments and during haze episodes, where elevated aerosol concentrations can significantly shift chemical regimes.
Understanding these effects is critical for designing effective air quality policies, as traditional control strategies targeting NOₓ or VOCs may have unintended consequences if aerosol-driven suppression or enhancement of ozone formation is not accounted for. Ultimately, GEOS-Chem provides a robust platform for advancing our understanding of the coupled impacts of gas-phase and aerosol chemistry on ozone sensitivity in the atmosphere.
2. Modelling surface level formaldehyde distribution and its effect on public health
Surface-level formaldehyde (HCHO) is a key atmospheric compound with significant implications for both air quality and human health. It is formed primarily through the oxidation of volatile organic compounds (VOCs), both anthropogenic and biogenic, and is also directly emitted from combustion processes. Using atmospheric chemistry models like GEOS-Chem, researchers can simulate the distribution and temporal variability of surface-level formaldehyde, providing critical insights into its sources, transport, and chemical transformations.
These models integrate emissions inventories, meteorological data, and detailed chemical mechanisms to predict HCHO concentrations at high spatial and temporal resolutions. Formaldehyde is not only a marker for VOC oxidation and a proxy for assessing ozone production potential, but it is also a known toxic air pollutant and classified human carcinogen. Prolonged exposure, especially in urban and industrial regions, is associated with respiratory issues, eye and skin irritation, and increased risks of cancer. Modelling formaldehyde concentrations allows for the identification of population hotspots with elevated exposure levels and supports the assessment of associated public health risks. Coupling chemical transport model outputs with epidemiological data and health impact functions enables quantitative estimates of disease burden attributable to HCHO exposure.
This approach helps policymakers evaluate the benefits of emission control strategies targeting VOCs and combustion sources. Furthermore, model simulations can be validated against satellite observations (e.g., from TROPOMI) and ground-based monitoring networks to improve accuracy and reliability. Ultimately, modelling the surface-level distribution of formaldehyde plays a crucial role in advancing air quality management, guiding regulatory frameworks, and protecting public health in both developed and developing regions.
3. Future projection using SSP-RCP scenarios and GCAP2.0 framework
Future projections of atmospheric composition and air quality are essential for understanding the long-term impacts of emissions, climate policies, and socio-economic development pathways. The GCAP 2.0 (Global Change and Air Pollution) model framework integrates emissions trajectories from the Shared Socioeconomic Pathways (SSPs) with the GEOS-Chem chemical transport model to simulate future air pollutant concentrations under different global development scenarios. SSPs represent a range of narratives combining demographic trends, economic growth, energy use, and climate policy ambitions, from sustainability-focused (e.g., SSP1-2.6) to fossil-fueled development (e.g., SSP5-8.5).
GCAP 2.0 uses harmonized emissions datasets from the CMIP6 database and applies them within GEOS-Chem to assess changes in key pollutants like ozone (O₃), PM₂.₅, and reactive nitrogen species over the 21st century. By accounting for projected shifts in emissions, land use, and climate-driven feedback such as temperature and humidity changes, GCAP 2.0 enables detailed exploration of how air quality and human exposure might evolve. These simulations provide insights into potential public health outcomes, particularly in rapidly developing regions where future emissions may increase significantly. Additionally, GCAP 2.0 facilitates evaluation of co-benefits from climate mitigation efforts, such as reduced air pollution-related mortality when transitioning to low-carbon energy systems.
The model's ability to capture regional chemical regimes and long-range transport also supports international assessments of transboundary air pollution. By applying SSP-specific policy interventions and technology adoption assumptions, GCAP 2.0 helps inform global and regional strategies for achieving sustainable development goals, ensuring clean air, and mitigating climate change impacts through evidence-based decision-making.
4. Polar Atmospheric Chemistry
Polar atmospheric chemical composition and chemistry plays a critical role in the dramatic seasonal depletion of stratospheric ozone, particularly over Antarctica and, to a lesser extent, the Arctic. This phenomenon is largely driven by reactions occurring on the surfaces of polar stratospheric clouds (PSCs), which form under extremely cold conditions in the lower stratosphere. These reactions convert inert chlorine reservoir species, such as HCl and ClONO₂, into reactive forms like Cl₂ and HOCl. Upon exposure to sunlight in early spring, these species photolyse to release chlorine radicals, which catalytically destroy ozone molecules. The GEOS-Chem model, when configured with stratospheric chemistry extensions, can simulate these processes by incorporating heterogeneous reaction mechanisms and PSC formation criteria based on temperature and nitric acid concentrations. By using meteorological fields from NASA's GEOS system, GEOS-Chem accurately captures the dynamics and temperature conditions necessary for PSC formation and the spatial extent of ozone depletion. It also tracks the redistribution of nitrogen and chlorine species due to sedimentation and denitrification processes, which are crucial for sustaining ozone loss over extended periods. GEOS-Chem's modular chemical mechanism allows for sensitivity studies that assess the role of various precursors and transport processes in controlling polar ozone chemistry. Simulations can also evaluate the recovery trajectory of the ozone layer under international regulations like the Montreal Protocol by projecting halogen loading trends. Furthermore, GEOS-Chem's capability to assimilate satellite data enables validation against observed ozone profiles and chlorine species distributions. Overall, GEOS-Chem provides a comprehensive framework for understanding and quantifying the mechanisms behind polar ozone depletion, offering valuable insights into stratospheric chemical-climate interactions and the effectiveness of global policy interventions.
References:
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Henze, D.K., Hakami, A. and Seinfeld, J.H., 2007. Development of the adjoint of GEOS-Chem. Atmospheric Chemistry and Physics, 7(9), pp.2413-2433.
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Vinken, G.C., Boersma, K.F., Jacob, D.J. and Meijer, E.W., 2011. Accounting for non-linear chemistry of ship plumes in the GEOS-Chem global chemistry transport model. Atmospheric Chemistry and Physics, 11(22), pp.11707-11722.
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Eastham, S.D., Long, M.S., Keller, C.A., Lundgren, E., Yantosca, R.M., Zhuang, J., Li, C., Lee, C.J., Yannetti, M., Auer, B.M. and Clune, T.L., 2018. GEOS-Chem High Performance (GCHP v11-02c): a next-generation implementation of the GEOS-Chem chemical transport model for massively parallel applications. Geoscientific Model Development, 11(7), pp.2941-2953.
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Murray, L.T., Leibensperger, E.M., Orbe, C., Mickley, L.J. and Sulprizio, M., 2021. GCAP 2.0: a global 3-D chemical-transport model framework for past, present, and future climate scenarios. Geoscientific Model Development Discussions, 2021, pp.1-52.
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Jacob, D.J., 2000. Heterogeneous chemistry and tropospheric ozone. Atmospheric environment, 34(12-14), pp.2131-2159.
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Seinfeld, J.H. and Pandis, S.N., 2016. Atmospheric chemistry and physics: from air pollution to climate change. John Wiley & Sons.