Global deposition of total reactive nitrogen oxides from 1996 to 2014 constrained with satellite observations of NO2 columns

Reactive nitrogen oxides (NOy) are a major constituent of the nitrogen deposited from the atmosphere, but observational constraints on their deposition are limited by poor or nonexistent measurement coverage in many parts of the world. Here we apply NO2 observations from multiple satellite instruments (GOME, SCIAMACHY, and GOME-2) to constrain the global deposition of NOy over the last two decades. We accomplish this by producing top-down estimates of NOx emissions from inverse modeling of satellite NO2 columns over 1996–2014, and including these emissions in the GEOS-Chem chemical transport model to simulate chemistry, transport, and deposition of NOy. Our estimates of long-term mean wet nitrate (NO3−) deposition are highly consistent with available measurements in North America, Europe, and East Asia combined (r = 0.83, normalized mean bias = −7 %, N = 136). Likewise, our calculated trends in wet NO3− deposition are largely consistent with the measurements, with 129 of the 136 gridded model-data pairs sharing overlapping 95 % confidence intervals. We find that global mean NOy deposition over 1996–2014 is 56.0 Tg N yr−1, with a minimum in 2006 of 50.5 Tg N and a maximum in 2012 of 60.8 Tg N. Regional trends are large, with opposing signs in different parts of the world. Over 1996 to 2014, NOy deposition decreased by up to 60 % in eastern North America, doubled in regions of East Asia, and declined by 20 % in parts of Western Europe. About 40 % of the global NOy deposition occurs over oceans, with deposition to the North Atlantic Ocean declining and deposition to the northwestern Pacific Ocean increasing. Using the residual between NOx emissions and NOy deposition over specific land regions, we investigate how NOx export via atmospheric transport has changed over the last two decades. Net export from the continental United States decreased substantially, from 2.9 Tg N yr−1 in 1996 to 1.5 Tg N yr−1 in 2014. On the other hand, export from China more than tripled between 1996 and 2011 (from 1.0 Tg N yr−1 to 3.5 Tg N yr−1), before a striking decline to 2.5 Tg N yr−1 by 2014. We find that declines in NOx export from some Western European countries have counteracted increases in emissions from neighbouring countries to the east. A sensitivity study indicates that simulated NOy deposition is robust to uncertainties in NH3 emissions with a few exceptions. Our novel long-term study provides timely context on the rapid redistribution of atmospheric nitrogen transport and subsequent deposition to ecosystems around the world.https://www.atmos-chem-phys-discuss.net/acp-2016-1100/acp-2016-1100.pdfhttps://www.atmos-chem-phys-discuss.net/acp-2016-1100/acp-2016-1100.pdfhttps://www.atmos-chem-phys-discuss.net/acp-2016-1100/acp-2016-1100.pdfPublished versionPublished versio

Global deposition of total reactive nitrogen oxides from 1996 to 2014 constrained with satellite observations of NO2 columns

Reactive nitrogen oxides (NOy) are a major constituent of the nitrogen deposited from the atmosphere, but observational constraints on their deposition are limited by poor or nonexistent measurement coverage in many parts of the world. Here we apply NO2 observations from multiple satellite instruments (GOME, SCIAMACHY, and GOME-2) to constrain the global deposition of NOy over the last 2 decades. We accomplish this by producing top-down estimates of NOx emissions from inverse modeling of satellite NO2 columns over 1996–2014, and including these emissions in the GEOS-Chem chemical transport model to simulate chemistry, transport, and deposition of NOy. Our estimates of long-term mean wet nitrate (NO3−) deposition are highly consistent with available measurements in North America, Europe, and East Asia combined (r = 0.83, normalized mean bias  = −7%, N = 136). Likewise, our calculated trends in wet NO3− deposition are largely consistent with the measurements, with 129 of the 136 gridded model–data pairs sharing overlapping 95% confidence intervals. We find that global mean NOy deposition over 1996–2014 is 56.0TgNyr−1, with a minimum in 2006 of 50.5TgN and a maximum in 2012 of 60.8TgN. Regional trends are large, with opposing signs in different parts of the world. Over 1996 to 2014, NOy deposition decreased by up to 60% in eastern North America, doubled in regions of East Asia, and declined by 20% in parts of western Europe. About 40% of the global NOy deposition occurs over oceans, with deposition to the North Atlantic Ocean declining and deposition to the northwestern Pacific Ocean increasing. Using the residual between NOx emissions and NOy deposition over specific land regions, we investigate how NOx export via atmospheric transport has changed over the last 2 decades. Net export from the continental United States decreased substantially, from 2.9TgNyr−1 in 1996 to 1.5TgNyr−1 in 2014. Export from China more than tripled between 1996 and 2011 (from 1.0 to 3.5TgNyr−1), before a striking decline to 2.5TgNyr−1 by 2014. We find that declines in NOx export from some western European countries have counteracted increases in emissions from neighboring countries to the east. A sensitivity study indicates that simulated NOy deposition is robust to uncertainties in NH3 emissions with a few exceptions. Our novel long-term study provides timely context on the rapid redistribution of atmospheric nitrogen transport and subsequent deposition to ecosystems around the world.This work was supported by NSERC and Environment and Climate Change Canada. We acknowledge the free use of tropospheric NO2 column data from the GOME, SCIAMACHY, and GOME-2 sensors from www.temis.nl. We further acknowledge the NADP, CAPMoN, EMEP, and EANET regional monitoring networks as well as the World Data Centre for Precipitation Chemistry for access to wet deposition data. (NSERC; Environment and Climate Change Canada)https://www.atmos-chem-phys.net/17/10071/2017/acp-17-10071-2017.pdfhttps://www.atmos-chem-phys.net/17/10071/2017/acp-17-10071-2017.pdfhttps://www.atmos-chem-phys.net/17/10071/2017/acp-17-10071-2017.pdfPublished versionPublished versio

Size-resolved aerosol fluxes above a temperate broadleaf forest

Aerosol fluxes were measured by eddy-correlation for 8 weeks of the summer and fall of 2011 above a temperate broadleaf forest in central Ontario, Canada. These size-resolved measurements apply to particles with optical diameters between 50 and 500 nm and are the first ones reported above a temperate deciduous forest. The particle spectrometer was located on top of the flux tower in order to reduce signal dampening in the tube and thus maximize measurement efficiency. The 8-week data set extends into autumn, capturing leaf senescence and loss, offering a rare opportunity to investigate the influence of leaf area index on particle transfer. A distinct pattern of emission and deposition that depends on the particle size is highlighted: while the smallest particles (dp  100 nm) are preferentially deposited (62% of the time). For the size bins with detection efficiency above 50% (68–292 nm), the median transfer velocity for each bin varies between +1.34 and −2.69 mm s−1 and is equal to −0.21 mm s−1 for the total particle count. The occurrence of the upward fluxes shows a marked diurnal pattern. Possible explanations for these upward fluxes are proposed. The measurements, and their comparison with an existing model, highlight some of the key drivers of the particle transfer onto a broadleaf forest: particle size, friction velocity, leaf area index and atmospheric stability.We are grateful to the Haliburton forest staff and owner for their support, as well as Ting Zheng and Jing Ming Chen (Dept of Geography, Univ. of Toronto) for sharing the TRAC instrument LAI data. The UHSAS and SMPS instruments were contributed by the Canadian Aerosol Research Network, funded by the Canada Foundation for Innovation. (Canada Foundation for Innovation)First author draf

Stratosphere-troposphere separation of nitrogen dioxide columns from the TEMPO geostationary satellite instrument

Separating the stratospheric and tropospheric contributions in satellite retrievals of atmospheric NO2 column abundance is a crucial step in the interpretation and application of the satellite observations. A variety of stratosphere–troposphere separation algorithms have been developed for sun-synchronous instruments in low Earth orbit (LEO) that benefit from global coverage, including broad clean regions with negligible tropospheric NO2 compared to stratospheric NO2. These global sun-synchronous algorithms need to be evaluated and refined for forthcoming geostationary instruments focused on continental regions, which lack this global context and require hourly estimates of the stratospheric column. Here we develop and assess a spatial filtering algorithm for the upcoming TEMPO geostationary instrument that will target North America. Developments include using independent satellite observations to identify likely locations of tropospheric enhancements, using independent LEO observations for spatial context, consideration of diurnally varying partial fields of regard, and a filter based on stratospheric to tropospheric air mass factor ratios. We test the algorithm with LEO observations from the OMI instrument with an afternoon overpass, and from the GOME-2 instrument with a morning overpass. We compare our TEMPO field of regard algorithm against an identical global algorithm to investigate the penalty resulting from the limited spatial coverage in geostationary orbit, and find excellent agreement in the estimated mean daily tropospheric NO2 column densities (R2=0.999, slope=1.009 for July and R2=0.998, slope=0.999 for January). The algorithm performs well even when only small parts of the continent are observed by TEMPO. The algorithm is challenged the most by east coast morning retrievals in the wintertime (e.g., R2=0.995, slope=1.038 at 14:00 UTC). We find independent global LEO observations (corrected for time of day) provide important context near the field-of-regard edges. We also test the performance of the TEMPO algorithm without these supporting global observations. Most of the continent is unaffected (R2=0.924 and slope=0.973 for July and R2=0.996 and slope=1.008 for January), with 90 % of the pixels having differences of less than ±0.2×1015 molecules cm−2 between the TEMPO tropospheric NO2 column density and the global algorithm. For near-real-time retrieval, even a climatological estimate of the stratospheric NO2 surrounding the field of regard would improve this agreement. In general, the additional penalty of a limited field of regard from TEMPO introduces no more error than normally expected in most global stratosphere–troposphere separation algorithms. Overall, we conclude that hourly near-real-time stratosphere–troposphere separation for the retrieval of NO2 tropospheric column densities by the TEMPO geostationary instrument is both feasible and robust, regardless of the diurnally varying limited field of regard.The authors are grateful to Kelly Chance, Xiong Liu, John Houck, Peter Zoogman, and other members of the TEMPO trace gas retrieval team for their input in preparation of this paper. Work at Dalhousie University was supported by Environment and Climate Change Canada. The authors also gratefully acknowledge the free use of TEMIS NO2 data from the GOME-2 sensor provided by http://www.temis.nl, last access: 12 November 2018, and the NASA Standard Product NO2 data from OMI provided by https://disc.gsfc.nasa.gov/datasets/OMNO2_V003/summary, last access: 9 November 2018. (Environment and Climate Change Canada)https://www.atmos-meas-tech.net/11/6271/2018/Published versio

Biases in Long-term NO2 Averages Inferred from Satellite Observations Due to Cloud Selection Criteria

Retrievals of atmospheric trace gas column densities from space are compromised by the presence of clouds, requiring most studies to exclude observations with significant cloud fractions in the instrument's field of view. Using NO2 observations at three ground stations representing urban, suburban, and rural environments, and tropospheric vertical column densities measured by the Ozone Monitoring Instrument (OMI) over each site, we show that the observations from space represent monthly averaged ground-level pollutant conditions well (R=0.86) under relatively cloud-free conditions. However, by analyzing the ground-level data and applying the OMI cloud fraction as a filter, we show there is a significant bias in long-term averaged NO2 as a result of removing the data during cloudy conditions. For the ground-based sites considered in this study, excluding observations on days when OMI-derived cloud fractions were greater than 0.2 causes 12:00-14:00 mean summer mixing ratios to be underestimated by 12%+/-6%, 20%+/-7%, and 40%+/-10% on average (+/-1 standard deviation) at the urban, suburban, and rural sites respectively. This bias was investigated in particular at the rural site, a region where pollutant transport is the main source of NO2, and where longterm observations of NOy were also available. Evidence of changing photochemical conditions and a correlation between clear skies and the transport of cleaner air masses play key roles in explaining the bias. The magnitude of a bias is expected to vary from site to site depending on meteorology and proximity to NOx sources, and decreases when longer averaging times of ground station data (e.g. 24-h) are used for the comparison

Land cover change impacts on atmospheric chemistry: simulating projected large-scale tree mortality in the United States

Land use and land cover changes impact climate and air quality by altering the exchange of trace gases between the Earth's surface and atmosphere. Large-scale tree mortality that is projected to occur across the United States as a result of insect and disease may therefore have unexplored consequences for tropospheric chemistry. We develop a land use module for the GEOS-Chem global chemical transport model to facilitate simulations involving changes to the land surface, and to improve consistency across land–atmosphere exchange processes. The model is used to test the impact of projected national-scale tree mortality risk through 2027 estimated by the 2012 USDA Forest Service National Insect and Disease Risk Assessment. Changes in biogenic emissions alone decrease monthly mean O₃ by up to 0.4 ppb, but reductions in deposition velocity compensate or exceed the effects of emissions yielding a net increase in O₃ of more than 1 ppb in some areas. The O₃ response to the projected change in emissions is affected by the ratio of baseline NO[subscript x]: VOC concentrations, suggesting that in addition to the degree of land cover change, tree mortality impacts depend on whether a region is NO[subscript x]-limited or NO[subscript x]-saturated. Consequently, air quality (as diagnosed by the number of days that 8 h average O₃ exceeds 70 ppb) improves in polluted environments where changes in emissions are more important than changes to dry deposition, but worsens in clean environments where changes to dry deposition are the more important term. The influence of changes in dry deposition demonstrated here underscores the need to evaluate treatments of this physical process in models. Biogenic secondary organic aerosol loadings are significantly affected across the US, decreasing by 5–10 % across many regions, and by more than 25 % locally. Tree mortality could therefore impact background aerosol loadings by between 0.5 and 2 µg m⁻³. Changes to reactive nitrogen oxide abundance and partitioning are also locally important. The regional effects simulated here are similar in magnitude to other scenarios that consider future biofuel cropping or natural succession, further demonstrating that biosphere–atmosphere exchange should be considered when predicting future air quality and climate. We point to important uncertainties and further development that should be addressed for a more robust understanding of land cover change feedbacks.National Science Foundation (U.S.) (Grant AGC-1238109

Ozone and nitrogen dioxide pollution in a coastal urban environment: the role of sea breezes, and implications of their representation for remote sensing of local air quality

We present an analysis of sea breeze conditions for the Boston region and examine their impact on the concentration of local air pollutants over the past decade. Sea breezes occur about one-third of the days during the summer and play an important role in the spatial distribution and temporal evolution of NO2 and O3 across the urban area. Mornings preceding a sea breeze are characterized by low horizontal wind speeds, low background O3, and an accumulation of local primary emissions. Air pollution is recirculated inland during sea breezes, frequently coinciding with the highest O3 measured at the urban center. We use "Ox" (= NO2 + O3) to account for temporary O3 suppression by NO and find large horizontal gradients (differences in Ox greater than 30 ppb across less than 15 km), which are not observed on otherwise westerly or easterly prevailing days. This implies a challenge in surface monitoring networks to adequately represent the spatial variability of secondary air pollution in coastal urban areas. We investigate satellite-based climatologies of tropospheric NO2, and find evidence of selection biases due to cloud conditions, but show that sea breeze days are well observed due to the fair weather conditions generally associated with these events. The fine scale of the sea breeze in Boston is not reliably represented by meteorological reanalyses products commonly used in chemical transport models required to provide inputs for the satellite-based retrievals. This implies a higher systematic error in the operational retrievals on sea breeze days compared to other days.17‐NIP17‐0030 - National Aeronautics and Space AdministrationPublished versio

Changes in the relative importance of biogenic isoprene and Soil NOx emissions on ozone concentrations in nonattainment areas of the United States

Reductions in anthropogenic emissions have drawn increasing attention to the role of the biosphere in O3 production chemistry in U.S. cities. We report the results of chemical transport model sensitivity simulations exploring the relative impacts of biogenic isoprene and soil nitrogen oxides (NOx) emissions on O3 and its temporal variability. We compare scenarios with high and low anthropogenic NOx emissions representing the reductions that have occurred in recent decades. As expected, summertime O3 concentrations become less sensitive to perturbations in biogenic isoprene emissions as anthropogenic NOx emissions decline. However, we demonstrate for the first time that across policy relevant O3 nonattainment areas of the United States, O3 becomes more sensitive to perturbations in soil NOx emissions than to identical perturbations in isoprene emissions. We show that interannual variability in soil NOx emissions may now have larger impacts on interannual O3 variability than isoprene emissions in many areas where the latter would have dominated in the recent past.Published versio

Characterization of the insulin sensitivity of ghrelin receptor KO mice using glycemic clamps

<p>Abstract</p> <p>Background</p> <p>We and others have demonstrated previously that ghrelin receptor (<it>GhrR</it>) knock out (KO) mice fed a high fat diet (HFD) have increased insulin sensitivity and metabolic flexibility relative to WT littermates. A striking feature of the HFD-fed <it>GhrR </it>KO mouse is the dramatic decrease in hepatic steatosis. To characterize further the underlying mechanisms of glucose homeostasis in <it>GhrR </it>KO mice, we conducted both hyperglycemic (HG) and hyperinsulinemic-euglycemic (HI-E) clamps. Additionally, we investigated tissue glucose uptake and specifically examined liver insulin sensitivity.</p> <p>Results</p> <p>Consistent with glucose tolerance-test data, in HG clamp experiments, <it>GhrR </it>KO mice showed a reduction in glucose-stimulated insulin release relative to WT littermates. Nevertheless, a robust 1^stphase insulin secretion was still achieved, indicating that a healthy β-cell response is maintained. Additionally, <it>GhrR </it>KO mice demonstrated both a significantly increased glucose infusion rate and significantly reduced insulin requirement for maintenance of the HG clamp, consistent with their relative insulin sensitivity. In HI-E clamps, both LFD-fed and HFD-fed <it>GhrR </it>KO mice showed higher peripheral insulin sensitivity relative to WT littermates as indicated by a significant increase in insulin-stimulated glucose disposal (Rd), and decreased hepatic glucose production (HGP). HFD-fed <it>GhrR </it>KO mice showed a marked increase in peripheral tissue glucose uptake in a variety of tissues, including skeletal muscle, brown adipose tissue and white adipose tissue. <it>GhrR </it>KO mice fed a HFD also showed a modest, but significant decrease in conversion of pyruvate to glucose, as would be anticipated if these mice displayed increased liver insulin sensitivity. Additionally, the levels of UCP2 and UCP1 were reduced in the liver and BAT, respectively, in <it>GhrR </it>KO mice relative to WT mice.</p> <p>Conclusions</p> <p>These results indicate that improved glucose homeostasis of <it>GhrR </it>KO mice is characterized by robust improvements of glucose disposal in both normal and metabolically challenged states, relative to WT controls. <it>GhrR </it>KO mice have an intact 1^stphase insulin response but require significantly less insulin for glucose disposal. Our experiments reveal that the insulin sensitivity of <it>GhrR </it>KO mice is due to both BW independent and dependent factors. We also provide several lines of evidence that a key feature of the <it>GhrR </it>KO mouse is maintenance of hepatic insulin sensitivity during metabolic challenge.</p

Linking soil microbial community structure to potential carbon mineralization: A continental scale assessment of reduced tillage

Potential carbon mineralization (Cmin) is a commonly used indicator of soil health, with greater Cmin values interpreted as healthier soil. While Cmin values are typically greater in agricultural soils managed with minimal physical disturbance, the mechanisms driving the increases remain poorly understood. This study assessed bacterial and archaeal community structure and potential microbial drivers of Cmin in soils maintained under various degrees of physical disturbance. Potential carbon mineralization, 16S rRNA sequences, and soil characterization data were collected as part of the North American Project to Evaluate Soil Health Measurements (NAPESHM). Results showed that type of cropping system, intensity of physical disturbance, and soil pH influenced microbial sensitivity to physical disturbance. Furthermore, 28% of amplicon sequence variants (ASVs), which were important in modeling Cmin, were enriched under soils managed with minimal physical disturbance. Sequences identified as enriched under minimal disturbance and important for modeling Cmin, were linked to organisms which could produce extracellular polymeric substances and contained metabolic strategies suited for tolerating environmental stressors. Understanding how physical disturbance shapes microbial communities across climates and inherent soil properties and drives changes in Cmin provides the context necessary to evaluate management impacts on standardized measures of soil microbial activity