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Future climate change may bring local benefits or penalties to surface air pollution, resulting from changing temperature, precipitation, and transport patterns, as well as changes in climate-sensitive natural precursor emissions. Here, we estimate the climate penalties and benefits at the end of this century with regard to surface ozone and fine particulate matter (PM ${}_{2.5}$; excluding dust and smoke) using a one-way offline coupling between a general circulation model and a global 3-D chemical-transport model. We archive meteorology for the present day (2005 to 2014) and end of this century (2090 to 2099) for seven future scenarios developed for Phase 6 of the Coupled Model Intercomparison Project. The model isolates the impact of forecasted anthropogenic precursor emission changes versus that of climate-only driven changes on surface ozone and PM ${}_{2.5}$for scenarios ranging from extreme mitigation to extreme warming. We then relate these changes to impacts on human mortality and crop production. We find ozone penalties over nearly all land areas with increasing warming. We find net benefits due to climate-driven changes in PM ${}_{2.5}$in the Northern Extratropics, but net penalties in the Tropics and Southern Hemisphere, where most population growth is forecast for the coming century. 

Abstract. Despite significant precursor emission reductions in theUS over recent decades, atmospheric nitrate deposition remains an importantterrestrial stressor. Here, we utilized statistical air mass back trajectoryanalysis and nitrogen stable isotope deltas (δ(15N)) toinvestigate atmospheric nitrate spatiotemporal trends in the northeastern USfrom samples collected at three US EPA Clean Air Status and Trends Network(CASTNET) sites from December 2016–2018. For the considered sites, similarseasonal patterns in nitric acid (HNO3) and particulate nitrate(pNO3) concentrations were observed with spatial differences attributedto nitrogen oxide (NOx) emission densities in source contributingregions that were typically ≤ 1000 km. Significant spatiotemporalδ(15N) variabilities in HNO3 and pNO3 were observedwith higher values during winter relative to summer, like previous reportsfrom CASTNET samples collected in the early 2000s for our study region. Inthe early 2000s, δ(15N) of atmospheric nitrate in the northeastUS had been suggested to be driven by NOx emissions; however, we didnot find significant spatiotemporal changes in the modeled NOxemissions by sector and fuel type or δ(15N, NOx) for thesource regions of the CASTNET sites. Instead, the seasonal and spatialdifferences in the observed δ(15N) of atmospheric nitrate weredriven by nitrate formation pathways (i.e., homogeneous reactions ofNO2 oxidation via hydroxyl radical or heterogeneous reactions ofdinitrogen pentoxide on wetted aerosol surfaces) and their associatedδ(15N) fractionation. Under the field conditions of lowNOx relative to O3 concentrations and when δ(15N,NOx) emission sources do not have significant variability, wedemonstrate that δ(15N) of atmospheric nitrate can be a robusttracer for diagnosing nitrate formation. 

Abstract. The northeastern US represents a mostly urban corridorimpacted by high population and fossil fuel combustion emission density.This has led to historically degraded air quality and acid rain that hasbeen a focus of regulatory-driven emissions reductions. Detailing thechemistry of atmospheric nitrate formation is critical for improving themodel representation of atmospheric chemistry and air quality. The oxygenisotopic compositions of atmospheric nitrate are useful indicators intracking nitrate formation pathways. Here, we measured oxygen isotope deltas(Δ(17O) and δ(18O)) for nitric acid (HNO3)and particulate nitrate (pNO3) from three US EPA Clean AirStatus and Trends Network (CASTNET) sites in the northeastern US fromDecember 2016 to 2018. The Δ(17O, HNO3) and δ(18O, HNO3) values ranged from 12.9 ‰ to 30.9 ‰ and from 46.9 ‰ to 82.1 ‰, and the Δ(17O, pNO3) and δ(18O, pNO3) ranged from 16.6 ‰ to 33.7 ‰ and from 43.6 ‰ to 85.3 ‰, respectively. There was distinct seasonality ofδ(18O) and Δ(17O), with higher values observedduring winter compared to during summer, suggesting a shift in O3 to HOxradical chemistry, as expected. Unexpectedly, there was a statisticaldifference in Δ(17O) between HNO3 and pNO3, withhigher values observed for pNO3 (27.1 ± 3.8) ‰relative to HNO3 (22.7 ± 3.6) ‰, andsignificant differences in the relationship between δ(18O) andΔ(17O). This difference suggests atmospheric nitratephase-dependent oxidation chemistry that is not predicted in models. Basedon the output from GEOS-Chem and both the δ(18O) and Δ(17O) observations, we quantify the production pathways of atmosphericnitrate. The model significantly overestimated the heterogeneousN2O5 hydrolysis production for both HNO3 and pNO3, afinding consistent with observed seasonal changes in δ(18O) andΔ(17O) of HNO3 and pNO3, though large uncertaintiesremain in the quantitative transfer of δ(18O) from majoratmospheric oxidants. This comparison provides important insight into therole of oxidation chemistry in reconciling a commonly observed positive biasfor modeled atmospheric nitrate concentrations in the northeastern US. 

Abstract. This paper describes version 2.0 of the Global Change and Air Pollution (GCAP 2.0) model framework, a one-way offline coupling between version E2.1 of the NASA Goddard Institute for Space Studies (GISS) general circulation model (GCM) and the GEOS-Chem global 3-D chemical-transport model (CTM). Meteorology for driving GEOS-Chem has been archived from the E2.1 contributions to phase 6 of the Coupled Model Intercomparison Project (CMIP6) for the pre-industrial era and the recent past. In addition, meteorology is available for the near future and end of the century for seven future scenarios ranging from extreme mitigation to extreme warming. Emissions and boundary conditions have been prepared for input to GEOS-Chem that are consistent with the CMIP6 experimental design. The model meteorology, emissions, transport, and chemistry are evaluated in the recent past and found to be largely consistent with GEOS-Chem driven by the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) product and with observational constraints. 

Methane is a powerful greenhouse gas and a key player in atmospheric chemistry. Important uncertainties remain in the global atmospheric methane budget, with natural geologic emissions being one of the particularly uncertain terms. In recent bottom-up studies, geologic emissions have been estimated to comprise up to 10% of the global budget (40–60 Teragrams of methane per year, Tg CH4 yr–1). In contrast, top-down constraints from 14C of methane in preindustrial air extracted from ice cores indicate that the geologic methane source is approximately an order of magnitude lower. Recent bottom-up inventories propose microseepage (diffuse low-level flux of methane through soils over large areas) as the largest single component of the geologic methane flux. In this study, we present new measurements of methane microseepage from the Appalachian Basin (Western New York State) and compare these with prior microseepage measurements from other regions and with predicted values from the most recent bottom-up inventory. Our results show lower microseepage values than most prior data sets and indicate that positive microseepage fluxes in this region are not as widespread as previously assumed. A statistical analysis of our results indicates that mean microseepage flux in this region has very likely been overestimated by the bottom-up inventory, even though our measurements more likely than not underestimate the true mean flux. However, this is a small data set from a single region and as such cannot be used to evaluate the validity of the microseepage emissions inventory as a whole. Instead, the results demonstrate the need for a more extensive network of direct geologic emission measurements in support of improved bottom-up inventories. 

Abstract. Important uncertainties remain in our understanding of the spatial andtemporal variability of atmospheric hydroxyl radical concentration ([OH]).Carbon-14-containing carbon monoxide (14CO) is a useful tracer that canhelp in the characterization of [OH] variability. Prior measurements ofatmospheric 14CO concentration ([14CO] are limited in both theirspatial and temporal extent, partly due to the very large air sample volumes that have been required for measurements (500–1000 L at standardtemperature and pressure, L STP) and the difficulty and expense associatedwith the collection, shipment, and processing of such samples. Here wepresent a new method that reduces the air sample volume requirement to≈90 L STP while allowing for [14CO] measurement uncertainties that are on par with or better than prior work (≈3 % or better, 1σ). The method also for the first time includes accurate characterization of the overall procedural [14CO] blank associated with individual samples, which is a key improvement over prior atmospheric 14CO work. The method was used to make measurements of [14CO] at the NOAA Mauna Loa Observatory, Hawaii, USA, between November 2017 and November 2018. The measurements show the expected [14CO] seasonal cycle (lowest in summer)and are in good agreement with prior [14CO] results from anotherlow-latitude site in the Northern Hemisphere. The lowest overall [14CO]uncertainties (2.1 %, 1σ) are achieved for samples that aredirectly accompanied by procedural blanks and whose mass is increased to≈50 µgC (micrograms of carbon) prior to the 14Cmeasurement via dilution with a high-CO 14C-depleted gas. 

Abstract The history of tropospheric O₃, an important atmospheric oxidant, is poorly constrained because of uncertainties in its historical budget and a dearth of independent records. Here, we estimate the mean tropospheric O₃burden during the Last Interglacial period (LIG; 115 to 130 thousand years ago) using a record of the clumped isotopic composition of O₂(i.e., Δ₃₆values) preserved in Antarctic ice. The measured LIG Δ₃₆value is 0.03 ± 0.02‰ (95% CI) higher than the late pre‐industrial Holocene (PI; 1,590–1,850 CE) value and corresponds to a modeled 9% reduction in LIG tropospheric O₃burden (95% CI: 3%–15%), caused in part by a substantial reduction in biomass burning emissions during the LIG relative to the PI. These results are consistent with the hypothesis that late‐Pleistocene megafaunal extinctions caused woody and grassy fuels to accumulate on land, leading to enhanced biomass burning in the preindustrial Holocene. 

Abstract Ice cores and other paleotemperature proxies, together with general circulation models, have provided information on past surface temperatures and the atmosphere's composition in different climates. Little is known, however, about past temperatures at high altitudes, which play a crucial role in Earth's radiative energy budget. Paleoclimate records at high‐altitude sites are sparse, and the few that are available show poor agreement with climate model predictions. These disagreements could be due to insufficient spatial coverage, spatiotemporal biases, or model physics; new records that can mitigate or avoid these uncertainties are needed. Here, we constrain the change in upper‐tropospheric temperature at the global scale during the Last Glacial Maximum (LGM) using the clumped‐isotope composition of molecular oxygen trapped in polar ice cores. Aided by global three‐dimensional chemical transport modeling, we exploit the intrinsic temperature sensitivity of the clumped‐isotope composition of atmospheric oxygen to infer that the upper troposphere (effective mean altitude 10–11 km) was 6–9°C cooler during the LGM than during the late preindustrial Holocene. A complementary energy balance approach supports a minor or negligible steepening of atmospheric lapse rates during the LGM, which is consistent with a range of climate model simulations. Proxy‐model disagreements with other high‐altitude records may stem from inaccuracies in regional hydroclimate simulation, possibly related to land‐atmosphere feedbacks. 

Abstract Tropospheric¹⁸O¹⁸O is an emerging proxy for past tropospheric ozone and free‐tropospheric temperatures. The basis of these applications is the idea that isotope‐exchange reactions in the atmosphere drive¹⁸O¹⁸O abundances toward isotopic equilibrium. However, previous work used an offline box‐model framework to explain the¹⁸O¹⁸O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the¹⁸O¹⁸O proxy, we developed a scheme to simulate atmospheric¹⁸O¹⁸O abundances (quantified as ∆₃₆values) online within the GEOS‐Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O₂isotopic equilibration scheme within GEOS‐Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆₃₆values are too high elsewhere. Investigations using a comprehensive model of the O‐O₂‐O₃isotopic photochemical system and proof‐of‐principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆₃₆values: the anomalously efficient titration of¹⁸O¹⁸O to form ozone. Incorporating these effects into the online ∆₃₆calculation scheme in GEOS‐Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of¹⁸O¹⁸O in O₂, the modeled change in the mean tropospheric ∆₃₆value since 1850 CE is only slightly altered; it is still quantitatively consistent with the ice‐core ∆₃₆record, implying that the tropospheric ozone burden increased less than 40% over the twentieth century.