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Abstract. We implement the GEOS-Chem chemistry module as a chemical mechanism in version 2 of the Community Earth System Model (CESM). Our implementation allowsthe state-of-the-science GEOS-Chem chemistry module to be used with identical emissions, meteorology, and climate feedbacks as the CAM-chemchemistry module within CESM. We use coupling interfaces to allow GEOS-Chem to operate almost unchanged within CESM. Aerosols are converted at eachtime step between the GEOS-Chem bulk representation and the size-resolved representation of CESM's Modal Aerosol Model (MAM4). Land-type informationneeded for dry-deposition calculations in GEOS-Chem is communicated through a coupler, allowing online land–atmosphere interactions. Wet scavengingin GEOS-Chem is replaced with the Neu and Prather scheme, and a common emissions approach is developed for both CAM-chem and GEOS-Chem in CESM. We compare how GEOS-Chem embedded in CESM (C-GC) compares to the existing CAM-chem chemistry option (C-CC) when used to simulate atmosphericchemistry in 2016, with identical meteorology and emissions. We compare the atmospheric composition and deposition tendencies between the twosimulations and evaluate the residual differences between C-GC and its use as a stand-alone chemistry transport model in the GEOS-Chem HighPerformance configuration (S-GC). We find that stratospheric ozone agrees well between the three models, with differences of less than 10 % inthe core of themore »ozone layer, but that ozone in the troposphere is generally lower in C-GC than in either C-CC or S-GC. This is likely due to greatertropospheric concentrations of bromine, although other factors such as water vapor may contribute to lesser or greater extents depending on theregion. This difference in tropospheric ozone is not uniform, with tropospheric ozone in C-GC being 30 % lower in the Southern Hemisphere whencompared with S-GC but within 10 % in the Northern Hemisphere. This suggests differences in the effects of anthropogenic emissions. Aerosolconcentrations in C-GC agree with those in S-GC at low altitudes in the tropics but are over 100 % greater in the upper troposphere due todifferences in the representation of convective scavenging. We also find that water vapor concentrations vary substantially between the stand-aloneand CESM-implemented version of GEOS-Chem, as the simulated hydrological cycle in CESM diverges from that represented in the source NASA Modern-Era Retrospective analysis for Research and Applications (Version 2; MERRA-2)reanalysis meteorology which is used directly in the GEOS-Chem chemistrytransport model (CTM). Our implementation of GEOS-Chem as a chemistry option in CESM (including full chemistry–climate feedback) is publicly available and is beingconsidered for inclusion in the CESM main code repository. This work is a significant step in the MUlti-Scale Infrastructure for Chemistry andAerosols (MUSICA) project, enabling two communities of atmospheric researchers (CESM and GEOS-Chem) to share expertise through a common modelingframework, thereby accelerating progress in atmospheric science.« less

Model intercomparison studies often report a large spread in simulation results, but quantifying the causes of these differences is hindered by the fact that several processes contribute to the model spread simultaneously. Here we use the Multi‐Scale Infrastructure for Chemistry and Aerosols (MUSICA) version 0 to investigate the model resolution dependencies of simulated chemical species, with a focus on the differences between global uniform grid and regional refinement grid simulations with the same modeling framework. We construct two global (ne30 [∼112 km] and ne60 [∼56 km]) and two regional refinement grids over Korea (ne30x8 [∼14 km] and ne30x16 [∼7 km]). The grid resolution can change chemical concentrations by an order of magnitude in the boundary layer, and the importance increases as the species' reactivity increases (e.g., up to 50% and 1,000% changes for ethane and xylenes, respectively). The diurnal cycle of oxidants (OH, O₃, and NO₃) also varies with the grid resolution, which leads to different oxidation pathways of volatile organic compounds (e.g., the fraction of monoterpenes reacting with NO₃in Seoul around midnight is 90% for ne30, but 65% for ne30x16). The models with high‐resolution grids usually do a better job at reproducing aircraft observations during the KORUS‐AQ campaign, but not always, implyingmore »compensating errors in the coarse grid simulations. For example, ozone is better reproduced by the coarse grid due to the artificial mixing of NO_x. When developing new chemical mechanisms and evaluating models over urban areas, the uncertainties associated with model resolution should be considered.

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Abstract. Emissions are a central component of atmosphericchemistry models. The Harmonized Emissions Component (HEMCO) is a softwarecomponent for computing emissions from a user-selected ensemble of emissioninventories and algorithms. It allows users to re-grid, combine, overwrite,subset, and scale emissions from different inventories through aconfiguration file and with no change to the model source code. Theconfiguration file also maps emissions to model species with appropriateunits. HEMCO can operate in offline stand-alone mode, but more importantlyit provides an online facility for models to compute emissions at runtime.HEMCO complies with the Earth System Modeling Framework (ESMF) forportability across models. We present a new version here, HEMCO 3.0, thatfeatures an improved three-layer architecture to facilitate implementationinto any atmospheric model and improved capability for calculatingemissions at any model resolution including multiscale and unstructuredgrids. The three-layer architecture of HEMCO 3.0 includes (1) the Data InputLayer that reads the configuration file and accesses the HEMCO library ofemission inventories and other environmental data, (2) the HEMCO Core thatcomputes emissions on the user-selected HEMCO grid, and (3) the ModelInterface Layer that re-grids (if needed) and serves the data to theatmospheric model and also serves model data to the HEMCO Core forcomputing emissions dependent on model state (such as from dust or vegetation). Themore »HEMCO Core is common to the implementation in all models, whilethe Data Input Layer and the Model Interface Layer are adaptable to themodel environment. Default versions of the Data Input Layer and ModelInterface Layer enable straightforward implementation of HEMCO in any simplemodel architecture, and options are available to disable features such asre-gridding that may be done by independent couplers in more complexarchitectures. The HEMCO library of emission inventories and algorithms iscontinuously enriched through user contributions so that new inventoriescan be immediately shared across models. HEMCO can also serve as a generaldata broker for models to process input data not only for emissions but forany gridded environmental datasets. We describe existing implementations ofHEMCO 3.0 in (1) the GEOS-Chem “Classic” chemical transport model withshared-memory infrastructure, (2) the high-performance GEOS-Chem (GCHP)model with distributed-memory architecture, (3) the NASA GEOS Earth SystemModel (GEOS ESM), (4) the Weather Research and Forecasting model withGEOS-Chem (WRF-GC), (5) the Community Earth System Model Version 2 (CESM2),and (6) the NOAA Global Ensemble Forecast System – Aerosols(GEFS-Aerosols), as well as the planned implementation in the NOAA Unified ForecastSystem (UFS). Implementation of HEMCO in CESM2 contributes to theMulti-Scale Infrastructure for Chemistry and Aerosols (MUSICA) by providinga common emissions infrastructure to support different simulations ofatmospheric chemistry across scales.« less

Abstract. Secondary organic aerosol (SOA) is a dominant contributor of fine particulate matter in the atmosphere, but the complexity of SOA formation chemistry hinders the accurate representation of SOA in models. Volatility-based SOA parameterizations have been adopted in many recent chemistry modeling studies and have shown a reasonable performance compared to observations. However, assumptions made in these empirical parameterizations can lead to substantial errors when applied to future climatic conditions as they do not include the mechanistic understanding of processes but are rather fitted to laboratory studies of SOA formation. This is particularly the case for SOA derived from isoprene epoxydiols (IEPOX SOA), for which we have a higher level of understanding of the fundamental processes than is currently parameterized in most models. We predict future SOA concentrations using an explicit mechanism and compare the predictions with the empirical parameterization based on the volatility basis set (VBS) approach. We then use the Community Earth System Model 2 (CESM2.1.0) with detailed isoprene chemistry and reactive uptake processes for the middle and end of the 21st century under four Shared Socioeconomic Pathways (SSPs): SSP1–2.6, SSP2–4.5, SSP3–7.0, and SSP5–8.5. With the explicit chemical mechanism, we find that IEPOX SOA is predicted to increasemore »on average under all future SSP scenarios but with some variability in the results depending on regions and the scenario chosen. Isoprene emissions are the main driver of IEPOX SOA changes in the future climate, but the IEPOX SOA yield from isoprene emissions also changes by up to 50 % depending on the SSP scenario, in particular due to different sulfur emissions. We conduct sensitivity simulations with and without CO2 inhibition of isoprene emissions that is highly uncertain, which results in factor of 2 differences in the predicted IEPOX SOA global burden, especially for the high-CO2 scenarios (SSP3–7.0 and SSP5–8.5). Aerosol pH also plays a critical role in the IEPOX SOA formation rate, requiring accurate calculation of aerosol pH in chemistry models. On the other hand, isoprene SOA calculated with the VBS scheme predicts a nearly constant SOA yield from isoprene emissions across all SSP scenarios; as a result, it mostly follows isoprene emissions regardless of region and scenario. This is because the VBS scheme does not consider heterogeneous chemistry; in other words, there is no dependency on aerosol properties. The discrepancy between the explicit mechanism and VBS parameterization in this study is likely to occur for other SOA components as well, which may also have dependencies that cannot be captured by VBS parameterizations. This study highlights the need for more explicit chemistry or for parameterizations that capture the dependence on key physicochemical drivers when predicting SOA concentrations for climate studies.« less

Abstract. Secondary organic aerosol derived from isopreneepoxydiols (IEPOX-SOA) is thought to contribute the dominant fraction oftotal isoprene SOA, but the current volatility-based lumped SOAparameterizations are not appropriate to represent the reactive uptake ofIEPOX onto acidified aerosols. A full explicit modeling of this chemistryis however computationally expensive owing to the many species and reactionstracked, which makes it difficult to include it in chemistry–climate modelsfor long-term studies. Here we present three simplified parameterizations(version 1.0) for IEPOX-SOA simulation, based on an approximateanalytical/fitting solution of the IEPOX-SOA yield and formation timescale.The yield and timescale can then be directly calculated using the globalmodel fields of oxidants, NO, aerosol pH and other key properties, and drydeposition rates. The advantage of the proposed parameterizations is thatthey do not require the simulation of the intermediates while retaining thekey physicochemical dependencies. We have implemented the newparameterizations into the GEOS-Chem v11-02-rc chemical transport model,which has two empirical treatments for isoprene SOA (the volatility-basis-set, VBS, approach and a fixed 3 % yield parameterization), and comparedall of them to the case with detailed fully explicit chemistry. The bestparameterization (PAR3) captures the global tropospheric burden of IEPOX-SOAand its spatiotemporal distribution (R²=0.94) vs. thosesimulated by the full chemistry, while being more computationally efficient(∼5 times faster),more »and accurately captures the response tochanges in NO_x and SO₂ emissions. On the other hand, the constant3 % yield that is now the default in GEOS-Chem deviates strongly (R²=0.66), as does the VBS (R²=0.47, 49 % underestimation), withneither parameterization capturing the response to emission changes. Withthe advent of new mass spectrometry instrumentation, many detailed SOAmechanisms are being developed, which will challenge global and especiallyclimate models with their computational cost. The methods developed in thisstudy can be applied to other SOA pathways, which can allow includingaccurate SOA simulations in climate and global modeling studies in thefuture.

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ABSTRACT To explore the various couplings across space and time and between ecosystems in a consistent manner, atmospheric modeling is moving away from the fractured limited-scale modeling strategy of the past toward a unification of the range of scales inherent in the Earth system. This paper describes the forward-looking Multi-Scale Infrastructure for Chemistry and Aerosols (MUSICA), which is intended to become the next-generation community infrastructure for research involving atmospheric chemistry and aerosols. MUSICA will be developed collaboratively by the National Center for Atmospheric Research (NCAR) and university and government researchers, with the goal of serving the international research and applications communities. The capability of unifying various spatiotemporal scales, coupling to other Earth system components, and process-level modularization will allow advances in both fundamental and applied research in atmospheric composition, air quality, and climate and is also envisioned to become a platform that addresses the needs of policy makers and stakeholders.

The El Niño‐driven fire season in Indonesia in September–October 2015 produced the largest fire emissions on record since NASA's EOS satellites started making observations of tropospheric pollutants from space. In this study, measurements of carbon monoxide (CO) from the Measurement of Pollution in the Troposphere (MOPITT) on Terra and the Microwave Limb Sounder are used to characterize the anomalously high CO emitted during the 2015 Indonesian fire season transported into the tropical upper troposphere and stratosphere. The satellite measurements indicate that CO emitted from wildfires was transported into the upper troposphere with time lags up to ∼2 months and continued to be transported into the stratosphere, which resulted in higher concentrations of CO extending up to ∼20 hPa by the end of 2016. Hydrogen cyanide (HCN) measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) confirms that anomalously high HCN emitted from the same wildfires was also transported into the tropical stratosphere and persisted throughout 2017. Simulations of CO from the Community Atmosphere Model with Chemistry (CAM‐chem) show a significant increase in CO concentrations in the troposphere in October 2015. However, comparisons between CAM‐chem and MOPITT CO suggest that the model underestimates the amount of CO even with doubled emissionsmore »of CO in October 2015. Both the satellite measurements and the model simulations show that the pollution emitted from the wildfires over Indonesia was transported to and persisted in the tropical stratosphere much longer than the previous El‐Niño driven fire events due to unprecedented amount of the fire emissions.

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Reactive chlorine and bromine species emitted from snow and aerosols can significantly alter the oxidative capacity of the polar boundary layer. However, halogen production mechanisms from snow remain highly uncertain, making it difficult for most models to include descriptions of halogen snow emissions and to understand the impact on atmospheric chemistry. We investigate the influence of Arctic halogen emissions from snow on boundary layer oxidation processes using a one‐dimensional atmospheric chemistry and transport model (PACT‐1D). To understand the combined impact of snow emissions and boundary layer dynamics on atmospheric chemistry, we model Cl₂and Br₂primary emissions from snow and include heterogeneous recycling of halogens on both snow and aerosols. We focus on a 2‐day case study from the 2009 Ocean‐Atmosphere‐Sea Ice‐Snowpack campaign at Utqiaġvik, Alaska. The model reproduces both the diurnal cycle and high quantity of Cl₂observed, along with the measured concentrations of Br₂, BrO, and HOBr. Due to the combined effects of emissions, recycling, vertical mixing, and atmospheric chemistry, reactive chlorine is typically confined to the lowest 15 m of the atmosphere, while bromine can impact chemistry up to and above the surface inversion height. Upon including halogen emissions and recycling, the concentration of HO_x(HO_x = OH + HO₂) at the surface increases bymore »as much as a factor of 30 at mid‐day. The change in HO_xdue to halogen chemistry, as well as chlorine atoms derived from snow emissions, significantly reduce volatile organic compound lifetimes within a shallow layer near the surface.

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An advanced aerosol treatment, with a focus on semivolatile nitrate formation, is introduced into the Community Atmosphere Model version 5 with interactive chemistry (CAM5‐chem) by coupling the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) with the 7‐mode Modal Aerosol Module (MAM7). An important feature of MOSAIC is dynamic partitioning of all condensable gases to the different fine and coarse mode aerosols, as governed by mode‐resolved thermodynamics and heterogeneous chemical reactions. Applied in the free‐running mode from 1995 to 2005 with prescribed historical climatological conditions, the model simulates global distributions of sulfate, nitrate, and ammonium in good agreement with observations and previous studies. Inclusion of nitrate resulted in ∼10% higher global average accumulation mode number concentrations, indicating enhanced growth of Aitken mode aerosols from nitrate formation. While the simulated accumulation mode nitrate burdens are high over the anthropogenic source regions, the sea‐salt and dust modes respectively constitute about 74% and 17% of the annual global average nitrate burden. Regional clear‐sky shortwave radiative cooling of up to −5 W m⁻²due to nitrate is seen, with a much smaller global average cooling of −0.05 W m⁻². Significant enhancements in regional cloud condensation nuclei (at 0.1% supersaturation) and cloud droplet number concentrations are also attributed tomore »nitrate, causing an additional global average shortwave cooling of −0.8 W m⁻². Taking into consideration of changes in both longwave and shortwave radiation under all‐sky conditions, the net change in the top of the atmosphere radiative fluxes induced by including nitrate aerosol is −0.7 W m⁻².

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