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The Whole Atmosphere Community Climate Model version 6 (WACCM6) is a major update of the whole atmosphere modeling capability in the Community Earth System Model (CESM), featuring enhanced physical, chemical and aerosol parameterizations. This work describes WACCM6 and some of the important features of the model. WACCM6 can reproduce many modes of variability and trends in the middle atmosphere, including the quasi‐biennial oscillation, stratospheric sudden warmings, and the evolution of Southern Hemisphere springtime ozone depletion over the twentieth century. WACCM6 can also reproduce the climate and temperature trends of the 20th century throughout the atmospheric column. The representation of the climate has improved in WACCM6, relative to WACCM4. In addition, there are improvements in high‐latitude climate variability at the surface and sea ice extent in WACCM6 over the lower top version of the model (CAM6) that comes from the extended vertical domain and expanded aerosol chemistry in WACCM6, highlighting the importance of the stratosphere and tropospheric chemistry for high‐latitude climate variability. This manuscript describes the Whole Atmosphere Community Climate Model Version 6 (WACCM6), a chemistry and climate model which extends up to 140 km in the upper atmosphere. WACCM6 reproduces many important features of the climate system, and the addition of detailed chemistry and the higher than normal model top produces slightly improved simulations of the Arctic region. WACCM6 is a major upgrade to previous versionsWACCM6 can reproduce many modes of variability and trends in the middle atmosphereWACCM6 provides improvements in high‐latitude climate variability at the surface and sea ice extent over a low top model

Global multiconstituent concentration and emission fields obtained from the assimilation of the satellite retrievals of ozone, CO, NO2, HNO3, and SO2from the Ozone Monitoring Instrument (OMI), Global Ozone Monitoring Experiment 2, Measurements of Pollution in the Troposphere, Microwave Limb Sounder, and Atmospheric Infrared Sounder (AIRS)/OMI are used to understand the processes controlling air pollution during the Korea‐United States Air Quality (KORUS‐AQ) campaign. Estimated emissions in South Korea were 0.42 Tg N for NOxand 1.1 Tg CO for CO, which were 40% and 83% higher, respectively, than the a priori bottom‐up inventories, and increased mean ozone concentration by up to 7.5 ± 1.6 ppbv. The observed boundary layer ozone exceeded 90 ppbv over Seoul under stagnant phases, whereas it was approximately 60 ppbv during dynamical conditions given equivalent emissions. Chemical reanalysis showed that mean ozone concentration was persistently higher over Seoul (75.10 ± 7.6 ppbv) than the broader KORUS‐AQ domain (70.5 ± 9.2 ppbv) at 700 hPa. Large bias reductions (>75%) in the free tropospheric OH show that multiple‐species assimilation is critical for balanced tropospheric chemistry analysis and emissions. The assimilation performance was dependent on the particular phase. While the evaluation of data assimilation fields shows an improved agreement with aircraft measurements in ozone (to less than 5 ppbv biases), CO, NO2, SO2, PAN, and OH profiles, lower tropospheric ozone analysis error was largest at stagnant conditions, whereas the model errors were mostly removed by data assimilation under dynamic weather conditions. Assimilation of new AIRS/OMI ozone profiles allowed for additional error reductions, especially under dynamic weather conditions. Our results show the important balance of dynamics and emissions both on pollution and the chemical assimilation system performance. Global multi‐constituent concentration and emission fields obtained from the assimilation of the satellite retrievals are used to understand the processes controlling air pollution during the Korea U.S.‐Air Quality (KORUS‐AQ) campaign. Our results show the important balance of dynamics and emissions both on pollution and the chemical assimilation system performance. Multiconstituent data assimilation during KORUS‐AQ showed that emissions in South Korea were 0.42 Tg N for NOxand 1.1 Tg CO for COThese emissions were 40% and 83% higher, respectively, than the a priori bottom‐up inventories and increased ozone by up to 7.5 ± 1.6 ppbvMean ozone concentration was persistently higher over Seoul (75.1 ± 7.6 ppbv) than the broader KORUS‐AQ domain (70.5 ± 9.2 ppbv) at 700 hPa

In the Southern Hemisphere and tropics, the main contribution to carbon monoxide (CO) variability is from fire emissions, which are connected to climate through the availability, type, and dryness of fuel. Here we assess the data‐driven relationships between CO and climate, aiming to predict atmospheric loading during fire seasons. Observations of total column CO from the Measurements Of Pollution In The Troposphere satellite instrument are used to build a record of monthly anomalies between 2001 and 2016, focusing on seven biomass burning regions of the Southern Hemisphere and tropics. With the exception of 2015, the range of absolute variability in CO is similar between regions. We model CO anomalies in each of the regions using climate indices for the climate modes: El Niño–Southern Oscillation, Indian Ocean Dipole, Tropical South Atlantic, and Antarctic Oscillation. Stepwise forward and backward variable selection is used to choose from statistical regression models that use combinations of climate indices, at lag times between 1 and 8 months relative to CO anomalies. The Bayesian information criterion selects models with the best predictive power. We find that all climate mode indices are required to model CO in each region, generally explaining over 50% of the variability and over 70% for tropical regions. First‐order interaction terms of the climate modes are necessary, producing greatly improved explanation of CO variability over single terms. Predictive capability is assessed for the Maritime Southeast Asia and the predicted peak CO anomaly in 2015 is within 20% of the measurements. The long record of satellite‐measured CO from MOPITT enables study of interannual variability (IAV)ENSO climate mode index is important, but we find that other modes are also required to predict CO IAVInteraction of climate modes is necessary to explain CO variability

Understanding changes in the burden and growth rate of atmospheric methane (CH4) has been the focus of several recent studies but still lacks scientific consensus. Here we investigate the role of decreasing anthropogenic carbon monoxide (CO) emissions since 2002 on hydroxyl radical (OH) sinks and tropospheric CH4loss. We quantify this impact by contrasting two model simulations for 2002–2013: (1) a Measurement of the Pollution in the Troposphere (MOPITT) CO reanalysis and (2) a Control‐Run without CO assimilation. These simulations are performed with the Community Atmosphere Model with Chemistry of the Community Earth System Model fully coupled chemistry climate model with prescribed CH4surface concentrations. The assimilation of MOPITT observations constrains the global CO burden, which significantly decreased over this period by ~20%. We find that this decrease results to (a) increase in CO chemical production, (b) higher CH4oxidation by OH, and (c) ~8% shorter CH4lifetime. We elucidate this coupling by a surrogate mechanism for CO‐OH‐CH4that is quantified from the full chemistry simulations. Decreases in tropospheric CO from 2002 to 2012 indicate an increase in CH4chemical lossThere is a positive trend in the chemical production of CO from CH4in the tropicsWe infer a reduced growth rate in CH4burden due to decreases in anthropogenic CO emissions

Black carbon (BC) concentrations observed in 22 snowpits sampled in the northwest sector of the Greenland ice sheet in April 2014 have allowed us to identify a strong and widespread BC aerosol deposition event, which was dated to have accumulated in the pits from two snow storms between 27 July and 2 August 2013. This event comprises a significant portion (57% on average across all pits) of total BC deposition over 10 months (July 2013 to April 2014). Here we link this deposition event to forest fires burning in Canada during summer 2013 using modeling and remote sensing tools. Aerosols were detected by both the Cloud‐Aerosol Lidar with Orthogonal Polarization (on board CALIPSO) and Moderate Resolution Imaging Spectroradiometer (Aqua) instruments during transport between Canada and Greenland. We use high‐resolution regional chemical transport modeling (WRF‐Chem) combined with high‐resolution fire emissions (FINNv1.5) to study aerosol emissions, transport, and deposition during this event. The model captures the timing of the BC deposition event and shows that fires in Canada were the main source of deposited BC. However, the model underpredicts BC deposition compared to measurements at all sites by a factor of 2–100. Underprediction of modeled BC deposition originates from uncertainties in fire emissions and model treatment of wet removal of aerosols. Improvements in model descriptions of precipitation scavenging and emissions from wildfires are needed to correctly predict deposition, which is critical for determining the climate impacts of aerosols that originate from fires. An event in late July/early August deposited 57% of black carbon that accumulated in northwest Greenland from July 2013 to April 2014Satellite observations and modeling indicate that the origin of this event is emissions from forest fires burning in CanadaChemical transport modeling predicts the event at the right time but underpredicts black carbon deposition compared to observations

We incorporate the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) module in the Community Earth System Model version 2 (CESM2) Community Atmosphere Model Version 6 with interactive chemistry (CAM6‐chem), and couple it with the four mode version of the Modal Aerosol Module (MAM4). The MOSAIC module is used to simulate the thermodynamics of the gas‐aerosol mass exchange, with a special focus on simulating nitrate aerosol. By comparing against ground and satellite observations, we found that the MOSAIC/MAM4 scheme performs reasonably well in simulating spatiotemporal distributions of aerosols, including nitrate aerosol. We conducted a series of model experiments with and without nitrate aerosols, and examined the radiative effect (RE) associated with nitrate aerosols in 1975, 2000, and 2010, and accessed the radiative forcing (RF) of nitrate aerosols between the present day and pre‐industrial periods. Comparing with the nitrate aerosol RE, we predicted relatively small RF of anthropogenic nitrate aerosol from aerosol‐radiation interactions (RFari: −0.014 W m−2) and large RF from aerosol‐cloud interactions (RFaci: −0.219 W m−2). Regional signatures of nitrate RE/RF are noticeable and important: for instance, very small changes in REariin Europe and USA, but 2.8–3 times increases in REariin India and China from 1975 to 2010, while REaci/RFaciin China is a warming effect due to the competing effect between sulfate and nitrate aerosols as cloud condensation nuclei. We incorporated the Model for Simulating Aerosol Interactions and Chemistry aerosol module in Community Earth System Model version 2 (Community Atmosphere Model Version 6 with interactive chemistry) modelWe found small radiative forcing due to nitrate‐radiation interactions but large radiative forcing due to nitrate‐cloud interactionsWe found rapid increases in nitrate aerosol REari in China and India since 1975 and positive nitrate aerosol REaci in China after 2000 We incorporated the Model for Simulating Aerosol Interactions and Chemistry aerosol module in Community Earth System Model version 2 (Community Atmosphere Model Version 6 with interactive chemistry) model We found small radiative forcing due to nitrate‐radiation interactions but large radiative forcing due to nitrate‐cloud interactions We found rapid increases in nitrate aerosol REari in China and India since 1975 and positive nitrate aerosol REaci in China after 2000

We present a quasi-continuous record of measured atmospheric opacity obtained at 278 GHz (1.1 mm wavelength) from McMurdo Station, Antarctica during austral spring seasons in 1986 and 1987. McMurdo Station, at 78°S, 166°W, is easier to access than the Amundsen-Scott (South Pole) Station, although representing a warmer, sea level site with substantially higher typical opacity: the present record may be of interest to those contemplating mm-wave astronomical or atmospheric observations within the Antarctic region. Observations were made over a 256 MHz bandpass in 1986, during the period August 30 to October 30. In 1987, a 512 MHz bandpass was used, and observations were made during the period September 4 to October 13. All data are reduced to represent opacity in the zenith direction, and measurements were taken approximately every 20 minutes, except during storms or other periods of high opacity. The periods covered represent transitions from the polar winter towards summer conditions, and thus represent neither the best nor the worst that this site has to offer.

Tropospheric ozone (O3) pollution is known to damage vegetation, reducing photosynthesis and stomatal conductance, resulting in modified plant transpiration to the atmosphere. We use an Earth system model to show that global transpiration response to near‐present‐day surface tropospheric ozone results in large‐scale global perturbations to net outgoing long‐wave and incoming shortwave radiation. Our results suggest that the radiative effect is dominated by a reduction in shortwave cloud forcing in polluted regions, in response to ozone‐induced reduction in land‐atmosphere moisture flux and atmospheric humidity. We simulate a statistically significant response of annual surface air temperature of up to ~ +1.5 K due to this ozone effect in vegetated regions subjected to ozone pollution. This mechanism is expected to further increase the net warming resulting from historic and future increases in tropospheric ozone. Ozone is a pollutant near the Earth's surface, where it is harmful to health and vegetation. Ozone is formed by chemical reactions in the atmosphere driven by the action of sunlight on emissions from fossil fuel combustion and other sources. Ozone harms vegetation by entering leaves through small pores on leaves called stomata. These stomata are also the route by which gases such as water vapor and CO2are naturally exchanged between plants and the atmosphere. Ozone damage to vegetation affects the efficiency with which gases pass through plant stomata, typically reducing both photosynthesis and stomatal conductance due to biochemical damage. This results in a change in the amount of water vapor that plants put into the atmosphere. In this study we use a computer model to estimate for the first time how this modification in plant water vapor source to the atmosphere changes climate. We show widespread surface warming and changes in clouds due to the impact of ozone on plants. This has important implications for policies aimed at limiting global and regional temperature increases in the presence of ozone pollution and provides evidence for an additional climate benefit to reducing ozone pollution. We make the first estimate of global radiation balance and surface temperature response to vegetation damage by ozoneRadiative changes are dominated by atmospheric moisture and cloud response to ozone‐induced changes in plant transpirationOzone‐vegetation‐hydrology interactions should be considered in future climate scenarios and simulations

We use a global model, TOMCAT, and an inverse model, INVICAT, to estimate emissions of ethane (C2H6) between 2008 and 2014 by assimilating surface flask observations. We find that baseline global emissions in 2008 need to increase by a factor of 2.04–2.11 in order for the model to capture C2H6observations, indicating large biases in current emission inventories. Most of this increase occurs over North America and Eurasia, with temperate North American emissions accounting for 23–26% of the total global emission increase and temperate Eurasian emissions accounting for 35–37%. Further to this, recent peer‐reviewed analysis of long‐term observational records shows an increase in C2H6in the Northern Hemisphere since ~2009. Our results indicate that annual global emissions of C2H6have increased at a rate of 0.27 ± 0.54–0.33 ± 0.44 Tg/yr2between 2008 and 2014. A statistically significant positive trend of 0.20 ± 0.11–0.24 ± 0.13 Tg/yr2(p≤ 0.01) is found in temperate North America, resulting in emissions that are 31–32% larger in 2014 than in 2008. Our results corroborate previous studies' conclusions that a rapid increase in oil and natural gas production in United States over this time period is likely a large driver of the change in emissions. Inverse modeling improves global simulations of ethane by increasing global annual 2008 ethane emissions from ~7 to ~15 Tg/yearThe largest negative biases in baseline 2008 emissions were found in North America and EurasiaA positive trend in North American emissions between 2008 and 2014 likely explains observed increases in NH ethane