Atmospheric nitrogen oxide and nitrogen dioxide (NO + NO, together termed as NO ) estimates from annual photochemical simulations for years 2002-2016 are compared to surface network measurements of NO and total gas-phase-oxidized reactive nitrogen (NO ) to evaluate the Community Multiscale Air Quality (CMAQ) modeling system performance by U.S. region, season, and time of day. In addition, aircraft measurements from 2011 Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality are used to evaluate how emissions, chemical mechanism, and measurement uncertainty each contribute to the overall model performance. We show distinct seasonal and time-of-day patterns in NO performance. Summertime NO is overpredicted with bimodal peaks in bias during early morning and evening hours and persisting overnight. The summertime morning NO bias dropped from between 28% and 57% for earlier years (2002-2012) to between -2% and 7% for later years (2013-2016). Summer daytime NO tends to be unbiased or underpredicted. In winter, the evening NO overpredictions remain, but NO is unbiased or underpredicted overnight, in the morning, and during the day. NO overpredictions are most pronounced in the Midwestern and Southern United States with Western regions having more of a tendency toward model underpredictions of NO . Modeled NO performance has improved substantially over time, reflecting updates to the emission inputs and the CMAQ air quality model. Model performance improvements are largest for years simulated with CMAQv5.1 or later and for emission inventory years 2014 and later, coinciding with reduced onroad NO emissions from vehicles with newer emission control technologies and improved treatment of chemistry, deposition, and vertical mixing in CMAQ. Our findings suggest that emissions temporalization of specific mobile source sectors have a small impact on model performance, while chemistry updates improve predictions of NO but do not improve summertime NO bias in the Baltimore/DC area. Sensitivity runs performed for different locations across the country suggest that the improvement in summer NO performance can be attributed to updates in vertical mixing incorporated in CMAQv5.1.

The Korea-United States Air Quality (KORUS-AQ) field study was conducted during May-June 2016. The effort was jointly sponsored by the National Institute of Environmental Research of South Korea and the National Aeronautics and Space Administration of the United States. KORUS-AQ offered an unprecedented, multi-perspective view of air quality conditions in South Korea by employing observations from three aircraft, an extensive ground-based network, and three ships along with an array of air quality forecast models. Information gathered during the study is contributing to an improved understanding of the factors controlling air quality in South Korea. The study also provided a valuable test bed for future air quality-observing strategies involving geostationary satellite instruments being launched by both countries to examine air quality throughout the day over Asia and North America. This article presents details on the KORUS-AQ observational assets, study execution, data products, and air quality conditions observed during the study. High-level findings from companion papers in this special issue are also summarized and discussed in relation to the factors controlling fine particle and ozone pollution, current emissions and source apportionment, and expectations for the role of satellite observations in the future. Resulting policy recommendations and advice regarding plans going forward are summarized. These results provide an important update to early feedback previously provided in a Rapid Science Synthesis Report produced for South Korean policy makers in 2017 and form the basis for the Final Science Synthesis Report delivered in 2020.

The growing awareness of climate change/global warming, and continuing concerns regarding stratospheric ozone depletion, will require continued measurements and standards for many compounds, in particular halocarbons that are linked to these issues. In order to track atmospheric mole fractions and assess the impact of policy on emission rates, it is necessary to demonstrate measurement equivalence at the highest levels of accuracy for assigned values of standards. Precise measurements of these species aid in determining small changes in their atmospheric abundance. A common source of standards/scales and/or well-documented agreement of different scales used to calibrate the measurement instrumentation are key to understanding many sets of data reported by researchers. This report describes the results of a comparison study among National Metrology Institutes and atmospheric research laboratories for the chlorofluorocarbons (CFCs) dichlorodifluoromethane (CFC-12), trichlorofluoromethane (CFC-11), and 1,1,2-trichlorotrifluoroethane (CFC-113); the hydrochlorofluorocarbons (HCFCs) chlorodifluoromethane (HCFC-22) and 1-chloro-1,1-difluoroethane (HCFC-142b); and the hydrofluorocarbon (HFC) 1,1,1,2-tetrafluoroethane (HFC-134a), all in a dried whole air sample. The objective of this study is to compare calibration standards/scales and the measurement capabilities of the participants for these halocarbons at trace atmospheric levels. The results of this study show agreement among four independent calibration scales to better than 2.5% in almost all cases, with many of the reported agreements being better than 1.0%.

Freshwater estuaries throughout the Great Lakes region receive stormwater runoff and riverine inputs from heavily urbanized population centers. While human and animal feces contained in this runoff are often the focus of source tracking investigations, non-fecal bacterial loads from soil, aerosols, urban infrastructure, and other sources are also transported to estuaries and lakes. We quantified and characterized this non-fecal urban microbial component using bacterial 16S rRNA gene sequences from sewage, stormwater, rivers, harbor/estuary, and the lake surrounding Milwaukee, WI, USA. Bacterial communities from each of these environments had a distinctive composition, but some community members were shared among environments. We used a statistical biomarker discovery tool to identify the components of the microbial community that were most strongly associated with stormwater and sewage to describe an "urban microbial signature," and measured the presence and relative abundance of these organisms in the rivers, estuary, and lake. This urban signature increased in magnitude in the estuary and harbor with increasing rainfall levels, and was more apparent in lake samples with closest proximity to the Milwaukee estuary. The dominant bacterial taxa in the urban signature were , and , which are organisms associated with pipe infrastructure and soil and not typically found in pelagic freshwater environments. These taxa were highly abundant in stormwater and sewage, but sewage also contained a high abundance of and that appeared in lower abundance in stormwater outfalls and in trace amounts in aquatic environments. Urban signature organisms comprised 1.7% of estuary and harbor communities under baseflow conditions, 3.5% after rain, and >10% after a combined sewer overflow. With predicted increases in urbanization across the Great Lakes, further alteration of freshwater communities is likely to occur with potential long term impacts on the function of estuarine and nearshore ecosystems.

A recent paradigm shift from purely biophysical towards social-ecological assessment of watersheds has been proposed to understand, monitor, and manipulate the myriad interactions between human well-being and the ecosystem services that watersheds provide. However, large-scale, quantitative studies in this endeavour remain limited. We utilised two newly developed 'big-data' sets-the Index of Watershed Integrity (IWI) and the Human Well-Being Index (HWBI)-to explore the social-ecological condition of watersheds throughout the conterminous U.S., and identified environmental and socio-economic influences on watershed integrity and human well-being. Mean county IWI was highly associated with ecoregion, industry-dependence, and state, in a spatially-explicit regression model (R = 0.77, < 0.001), whereas HWBI was not (R = 0.31, < 0.001). HWBI is likely influenced by factors not explored here, such as governance structure and formal and informal organisations and institutions. 'Win-win' situations in which both IWI and HWBI were above the 75 percentile were observed in much of Utah, Colorado, and New Hampshire, and lessons from governance that has resulted in desirable outcomes might be learnt from here. Eastern Kentucky and West Virginia, along with large parts of the desert southwest, had intact watersheds but low HWBI, representing areas worthy of further investigation of how ecosystem services might be utilised to improve well-being. The Temperate Prairies and Central USA Plains had widespread areas of low IWI but high HWBI, likely a result of historic exploitation of watershed resources to improve well-being, particularly in farming-dependent counties. The lower Mississippi Valley had low IWI and HWBI, which is likely related to historical (temporal) and upstream (spatial) impacts on both watershed integrity and well-being. The results emphasise the importance of considering spatial and temporal trade-offs when utilising the ecosystem services provided by watersheds to improve human well-being.

The objective of the Indianapolis Flux Experiment (INFLUX) is to develop, evaluate and improve methods for measuring greenhouse gas (GHG) emissions from cities. INFLUX's scientific objectives are to quantify CO and CH emission rates at 1 km resolution with a 10% or better accuracy and precision, to determine whole-city emissions with similar skill, and to achieve high (weekly or finer) temporal resolution at both spatial resolutions. The experiment employs atmospheric GHG measurements from both towers and aircraft, atmospheric transport observations and models, and activity-based inventory products to quantify urban GHG emissions. Multiple, independent methods for estimating urban emissions are a central facet of our experimental design. INFLUX was initiated in 2010 and measurements and analyses are ongoing. To date we have quantified urban atmospheric GHG enhancements using aircraft and towers with measurements collected over multiple years, and have estimated whole-city CO and CH emissions using aircraft and tower GHG measurements, and inventory methods. Significant differences exist across methods; these differences have not yet been resolved; research to reduce uncertainties and reconcile these differences is underway. Sectorally- and spatially-resolved flux estimates, and detection of changes of fluxes over time, are also active research topics. Major challenges include developing methods for distinguishing anthropogenic from biogenic CO fluxes, improving our ability to interpret atmospheric GHG measurements close to urban GHG sources and across a broader range of atmospheric stability conditions, and quantifying uncertainties in inventory data products. INFLUX data and tools are intended to serve as an open resource and test bed for future investigations. Well-documented, public archival of data and methods is under development in support of this objective.

We evaluated the impacts of domestic and agricultural rainwater harvesting (RWH) systems in three watersheds within the Albemarle-Pamlico river basin (southeastern U.S.) using life cycle assessment (LCA) and life cycle cost assessment. Life cycle impact assessment (LCIA) categories included energy demand, fossil fuel, metals, ozone depletion, global warming, acidification, smog, blue and green water use, ecotoxicity, eutrophication, and human health effects. Building upon previous LCAs of near-optimal domestic and agricultural RWH systems in the region, we scaled functional unit LCIA scores for adoption rates of 25%, 50%, 75%, and 100% and compared these to conventional municipal water and well water systems. In addition to investigating watershed-scale impacts of RWH adoption, which few studies have addressed, potential life cycle cost savings due to reduced cumulative energy demand were scaled in each watershed for a more comprehensive analysis. The importance of managing the holistic water balance, including blue water (surface/ground water), green water (rainwater) use, and annual precipitation and their relationship to RWH are also addressed. RWH contributes to water resource sustainability by offsetting surface and ground water consumption and by reducing environmental and human health impacts compared to conventional sources. A watershed-wide RWH adoption rate of 25% has a number of ecological and human health benefits including blue water use reduction ranging from 2-39 Mm, cumulative energy savings of 12-210 TJ, and reduced global warming potential of 600-10,100 Mg CO eq. Potential maximum lifetime energy cost savings were estimated at $5M and $24M corresponding to domestic RWH in Greens Mill and agricultural RWH in Back Creek watersheds.

Quantifying greenhouse gas (GHG) emissions from cities is a key challenge towards effective emissions management. An inversion analysis from the INdianapolis FLUX experiment (INFLUX) project, as the first of its kind, has achieved a top-down emission estimate for a single city using CO data collected by the dense tower network deployed across the city. However, city-level emission data, used as emissions, are also a key component in the atmospheric inversion framework. Currently, fine-grained emission inventories (EIs) able to resolve GHG city emissions at high spatial resolution, are only available for few major cities across the globe. Following the INFLUX inversion case with a global 1×1 km ODIAC fossil fuel CO emission dataset, we further improved the ODIAC emission field and examined its utility as a prior for the city scale inversion. We disaggregated the 1×1 km ODIAC non-point source emissions using geospatial datasets such as the global road network data and satellite-data driven surface imperviousness data to a 30×30 m resolution. We assessed the impact of the improved emission field on the inversion result, relative to priors in previous studies (Hestia and ODIAC). The posterior total emission estimate (5.1 MtC/yr) remains statistically similar to the previous estimate with ODIAC (5.3 MtC/yr). However, the distribution of the flux corrections was very close to those of Hestia inversion and the model-observation mismatches were significantly reduced both in forward and inverse runs, even without hourly temporal changes in emissions. EIs reported by cities often do not have estimates of spatial extents. Thus, emission disaggregation is a required step when verifying those reported emissions using atmospheric models. Our approach offers gridded emission estimates for global cities that could serves as a prior for inversion, even without locally reported EIs in a systematic way to support city-level Measuring, Reporting and Verification (MRV) practice implementation.

Pharmaceuticals and personal care products, including antimicrobials, can be found at trace levels in treated wastewater effluent. Impacts of chemical contaminants on coastal aquatic microbial community structure and pathogen abundance are unknown despite the potential for selection through antimicrobial resistance. In particular, , a marine bacterial genus that includes several human pathogens, displays resistance to the ubiquitous antimicrobial compound triclosan. Here we demonstrated through use of natural seawater microcosms that triclosan (at a concentration of ~5 ppm) can induce a significant growth response (68-1,700 fold increases) in comparison with no treatment controls for three distinct coastal ecosystems: Looe Key Reef (Florida Keys National Marine Sanctuary), Doctors Arm Canal (Big Pine Key, FL), and Clam Bank Landing (North Inlet Estuary, Georgetown, SC). Additionally, microbial community analysis by 16 S rRNA gene sequencing for Looe Key Reef showed distinct changes in microbial community structure with exposure to 5 ppm triclosan, with increases observed in the relative abundance of Vibrionaceae (17-fold), Pseudoalteromonadaceae (65-fold), Alteromonadaceae (108-fold), Colwelliaceae (430-fold), and Oceanospirillaceae (1,494-fold). While the triclosan doses tested were above concentrations typically observed in coastal surface waters, results identify bacterial families that are potentially resistant to triclosan and/or adapted to use triclosan as a carbon source. The results further suggest the potential for selection of in coastal environments, especially sediments, where triclosan may accumulate at high levels.

Assessment of spatial and temporal variation in the impacts of ozone on human health, vegetation, and climate requires appropriate metrics. A key component of the is the consistent calculation of these metrics at thousands of monitoring sites globally. Investigating temporal trends in these metrics required that the same statistical methods be applied across these ozone monitoring sites. The nonparametric Mann-Kendall test (for significant trends) and the Theil-Sen estimator (for estimating the magnitude of trend) were selected to provide robust methods across all sites. This paper provides the scientific underpinnings necessary to better understand the implications of and rationale for selecting a specific TOAR metric for assessing spatial and temporal variation in ozone for a particular impact. The rationale and underlying research evidence that influence the derivation of specific metrics are given. The form of 25 metrics (4 for model-measurement comparison, 5 for characterization of ozone in the free troposphere, 11 for human health impacts, and 5 for vegetation impacts) are described. Finally, this study categorizes health and vegetation exposure metrics based on the extent to which they are determined only by the highest hourly ozone levels, or by a wider range of values. The magnitude of the metrics is influenced by both the distribution of hourly average ozone concentrations at a site location, and the extent to which a particular metric is determined by relatively low, moderate, and high hourly ozone levels. Hence, for the same ozone time series, changes in the distribution of ozone concentrations can result in different changes in the magnitude and direction of trends for different metrics. Thus, dissimilar conclusions about the effect of changes in the drivers of ozone variability (e.g., precursor emissions) on health and vegetation exposure can result from the selection of different metrics.

Ozone (O) is a key air pollutant that is produced from precursor emissions and has adverse impacts on human health and ecosystems. In the U.S., the Clean Air Act (CAA) regulates O levels to protect public health and welfare, but unraveling the origins of surface O is complicated by the presence of contributions from multiple sources including background sources like stratospheric transport, wildfies, biogenic precursors, and international anthropogenic pollution, in addition to U.S. anthropogenic sources. In this report, we consider more than 100 published studies and assess current knowledge on the spatial and temporal distribution, trends, and sources of background O over the continental U.S., and evaluate how it inflattainment of the air quality standards. We conclude that spring and summer seasonal mean U.S. background O (USB O), or O formed from natural sources plus anthropogenic sources in countries outside the U.S., is greatest at high elevation locations in the western U.S., with monthly mean maximum daily 8-hour average (MDA8) mole fractions approaching 50 parts per billion (ppb) and annual 4 highest MDA8s exceeding 60 ppb, at some locations. At lower elevation sites, e.g., along the West and East Coasts, seasonal mean MDA8 USB O is in the range of 20-40 ppb, with generally smaller contributions on the highest O days. The uncertainty in U.S. background O is around ±10 ppb for seasonal mean values and higher for individual days. Noncontrollable O sources, such as stratospheric intrusions or precursors from wildfires, can make significant contributions to O on some days, but it is challenging to quantify accurately these contributions. We recommend enhanced routine observations, focused fi studies, process-oriented modeling studies, and greater emphasis on the complex photochemistry in smoke plumes as key steps to reduce the uncertainty associated with background O in the U.S.

Despite modern sewer system infrastructure, the release of sewage from deteriorating pipes and sewer overflows is a major water pollution problem in US cities, particularly in coastal watersheds that are highly developed with large human populations. We quantified fecal pollution sources and loads entering Lake Michigan from a large watershed of mixed land use using host-associated indicators. Wastewater treatment plant influent had stable concentrations of human and human with geometric mean concentrations of 2.77 × 10 and 5.94 × 10 copy number (by quantitative PCR) per 100 ml, respectively. Human-associated indicator levels were four orders of magnitude higher than norovirus concentrations, suggesting that these human-associated bacteria could be sensitive indicators of pathogen risk. Norovirus concentrations in these same samples were used in calculations for quantitative microbial risk assessment. Assuming a typical recreational exposure to untreated sewage in water, concentrations of 7,800 copy number of human per 100 mL or 14,000 copy number of human per 100 mL corresponded to an illness risk of 0.03. These levels were exceeded in estuarine waters during storm events with greater than 5 cm of rainfall. Following overflows from combined sewer systems (which must accommodate both sewage and stormwater), concentrations were 10-fold higher than under rainfall conditions. Automated high frequency sampling allowed for loads of human-associated markers to be determined, which could then be related back to equivalent volumes of untreated sewage that were released. Evidence of sewage contamination decreased as ruminant-associated indicators increased approximately one day post-storm, demonstrating the delayed impact of upstream agricultural sources on the estuary. These results demonstrate that urban areas are a diffuse source of sewage contamination to urban waters and that storm-driven release of sewage, particularly when sewage overflows occur, creates a serious though transient human health risk.

Measurements of volatile organic compounds (VOCs) have been ongoing for decades to track growth rates and assist in curbing emissions of these compounds into the atmosphere. To accurately establish mole fraction trends and assess the role of these gas-phase compounds in atmospheric chemistry it is essential to have good calibration standards. A necessity and precursor to accurate VOC gas standards are the gas cylinders and the internal wall treatments that aid in maintaining the stability of the mixtures over long periods of time, measured in years. This paper will discuss the stability of VOC gas mixtures in different types of gas cylinders and internal wall treatments. Stability data will be given for 85 VOCs studied in gas mixtures by National Metrology Institutes and other agency laboratories. This evaluation of cylinder treatment materials is the outcome of an activity of the VOC Expert Group within the framework of the World Meteorological Organization (WMO) Global Atmospheric Watch (GAW) program.

As sources of reactive halogens, snowpacks in sea ice regions control the oxidative capacity of the Arctic atmosphere. However, measurements of snowpack halide concentrations remain sparse, particularly in the high Arctic, limiting our understanding of and ability to parameterize snowpack participation in tropospheric halogen chemistry. To address this gap, we measured concentrations of chloride, bromide, and sodium in snow samples collected during polar spring above remote multi-year sea ice (MYI) and first-year sea ice (FYI) north of Greenland and Alaska, as well as in the central Arctic, and compared these measurements to a larger dataset collected in the Alaskan coastal Arctic by Krnavek et al. (2012). Regardless of sea ice region, these surface snow samples generally featured lower salinities, compared to coastal snow. Surface snow in FYI regions was typically enriched in bromide and chloride compared to seawater, indicating snowpack deposition of bromine and chlorine-containing trace gases and an ability of the snowpack to participate further in bromine and chlorine activation processes. In contrast, surface snow in MYI regions was more often depleted in bromide, indicating it served as a source of bromine-containing trace gases to the atmosphere prior to sampling. Measurements at various snow depths indicate that the deposition of sea salt aerosols and halogen-containing trace gases to the snowpack surface played a larger role in determining surface snow halide concentrations compared to upward brine migration from sea ice. Calculated enrichment factors for bromide and chloride, relative to sodium, in the MYI snow samples suggests that MYI regions, in addition to FYI regions, have the potential to play an active role in Arctic boundary layer bromine and chlorine chemistry. The ability of MYI regions to participate in springtime atmospheric halogen chemistry should be considered in regional modeling of halogen activation and interpretation of satellite-based tropospheric bromine monoxide column measurements.

To understand the characteristics of air quality in the Seoul Metropolitan Area, intensive measurements were conducted under the Korea-United States Air Quality (KORUS-AQ) campaign. Trace gases such as O, NO, NO, SO, CO, and volatile organic compounds (VOCs), photochemical byproducts such as HO and HCHO, aerosol species, and meteorological variables including planetary boundary layer height were simultaneously measured at Olympic Park in Seoul. During the measurement period, high O episodes that exceeded the 90 ppbv hourly maximum occurred on 14 days under four distinct synoptic meteorological conditions. Furthermore, local circulation such as land-sea breeze and diurnal evolution of the boundary layer were crucial in determining the concentrations of precursor gases, including NO and VOC as well as O. During such episodes, the nighttime NO and VOC and daytime UV levels were higher compared to non-episode days. The overall precursor levels and photochemical activity were represented fairly well by variations in the HCHO, which peaked in the morning during the high O episodes. This study revealed that toluene was the most abundant VOC in Seoul, and its concentration increased greatly with NO due to the large local influence under stagnant conditions. When O was highly elevated concurrently with PM under dominant westerlies, NO and VOCs were relatively lower and CO was noticeably higher than in other episodes. Additionally, the O production efficiency was the highest due to a low NO with the highest NO/NO ratio among the four episodes. When westerlies were dominant in transport-south episode, the nighttime concentration of O remained as high as 40~50 ppbv due to the minimum level of NO titration. Overall, the Seoul Metropolitan Area is at NO-saturated and VOC-limited conditions, which was diagnosed by indicator species and VOC/NO ratios.

The Korea - United States Air Quality Study (May - June 2016) deployed instrumented aircraft and ground-based measurements to elucidate causes of poor air quality related to high ozone and aerosol concentrations in South Korea. This work synthesizes data pertaining to aerosols (specifically, particulate matter with aerodynamic diameters <2.5 micrometers, PM) and conditions leading to violations of South Korean air quality standards (24-hr mean PM < 35 μg m). PM variability from AirKorea monitors across South Korea is evaluated. Detailed data from the Seoul vicinity are used to interpret factors that contribute to elevated PM. The interplay between meteorology and surface aerosols, contrasting synoptic-scale behavior vs. local influences, is presented. Transboundary transport from upwind sources, vertical mixing and containment of aerosols, and local production of secondary aerosols are discussed. Two meteorological periods are probed for drivers of elevated PM. Clear, dry conditions, with limited transport (Stagnant period), promoted photochemical production of secondary organic aerosol from locally emitted precursors. Cloudy humid conditions fostered rapid heterogeneous secondary inorganic aerosol production from local and transported emissions (Transport/Haze period), likely driven by a positive feedback mechanism where water uptake by aerosols increased gas-to-particle partitioning that increased water uptake. Further, clouds reduced solar insolation, suppressing mixing, exacerbating PM accumulation in a shallow boundary layer. The combination of factors contributing to enhanced PM is challenging to model, complicating quantification of contributions to PM from local versus upwind precursors and production. We recommend co-locating additional continuous measurements at a few AirKorea sites across South Korea to help resolve this and other outstanding questions: carbon monoxide/carbon dioxide (transboundary transport tracer), boundary layer height (surface PM mixing depth), and aerosol composition with aerosol liquid water (meteorologically-dependent secondary production). These data would aid future research to refine emissions targets to further improve South Korean PM air quality.