( 1 and Fig. 1(a)). These trends are primarily due to stricter air quality emission controls that candidate species for studying hemispheric gradients and long-term changes. 57We analyzed ten years of NMHC data collected at 44 remote global sampling sites from NOAA's 58 Global Greenhouse Gas Reference Network (GGGRN). We also include data from in-situ moni-59 toring at Summit, Greenland 8 , at Hohenpeissenberg (HPB) in Southern Germany 9 , Jungfraujoch resolved in-situ record from HPB has its minimum in 2009 ( Fig. 1 (e)), in agreement with the JFJ 78 FTIR column observations ( Fig. 1(c)). Focusing on the most recent five years (2009.5 -2014.5) 79 we find variable results in the observed rate of change; however, a consistent picture emerges 80 that shows the largest increases at NH sites (Fig. 3). Of 33 NH sites, 7 exhibit ethane growth 81 rates > 50 pmol mol -1 yr -1 , and 10 sites exhibit growth rates between 25-50 pmol -1 yr -1 (Table S1). one from JFJ ( Fig. 1(c)) 12 , and the other one from Lauder, New Zealand ( Fig. 1(d) emission increases outside of NA that currently cannot be well defined due to the sparsity of 170 observations in those regions (for instance in the middle-East, Africa, and Asia).

Abstract. During the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) campaign from 21 July to 3 August 2016, field experiments on leaf-level trace gas exchange of nitric oxide (NO), nitrogen dioxide (NO2), and ozone (O3) were conducted for the first time on the native American tree species Pinus strobus (eastern white pine), Acer rubrum (red maple), Populus grandidentata (bigtooth aspen), and Quercus rubra (red oak) in a temperate hardwood forest in Michigan, USA. We measured the leaf-level trace gas exchange rates and investigated the existence of an NO2 compensation point, hypothesized based on a comparison of a previously observed average diurnal cycle of NOx (NO2+NO) concentrations with that simulated using a multi-layer canopy exchange model. Known amounts of trace gases were introduced into a tree branch enclosure and a paired blank reference enclosure. The trace gas concentrations before and after the enclosures were measured, as well as the enclosed leaf area (single-sided) and gas flow rate to obtain the trace gas fluxes with respect to leaf surface. There was no detectable NO uptake for all tree types. The foliar NO2 and O3 uptake largely followed a diurnal cycle, correlating with that of the leaf stomatal conductance. NO2 and O3 fluxes were driven by their concentration gradient from ambient to leaf internal space. The NO2 loss rate at the leaf surface, equivalently the foliar NO2 deposition velocity toward the leaf surface, ranged from 0 to 3.6 mm s−1 for bigtooth aspen and from 0 to 0.76 mm s−1 for red oak, both of which are ∼90 % of the expected values based on the stomatal conductance of water. The deposition velocities for red maple and white pine ranged from 0.3 to 1.6 and from 0.01 to 1.1 mm s−1, respectively, and were lower than predicted from the stomatal conductance, implying a mesophyll resistance to the uptake. Additionally, for white pine, the extrapolated velocity at zero stomatal conductance was 0.4±0.08 mm s−1, indicating a non-stomatal uptake pathway. The NO2 compensation point was ≤60 ppt for all four tree species and indistinguishable from zero at the 95 % confidence level. This agrees with recent reports for several European and California tree species but contradicts some earlier experimental results where the compensation points were found to be on the order of 1 ppb or higher. Given that the sampled tree types represent 80 %–90 % of the total leaf area at this site, these results negate the previously hypothesized important role of a leaf-scale NO2 compensation point. Consequently, to reconcile these findings, further detailed comparisons between the observed and simulated in- and above-canopy NOx concentrations and the leaf- and canopy-scale NOx fluxes, using the multi-layer canopy exchange model with consideration of the leaf-scale NOx deposition velocities as well as stomatal conductances reported here, are recommended.

Methane and nonmethane volatile organic compounds (VOCs) were monitored near Boulder in the Northern Colorado Front Range to investigate their spatial distribution and sources as a part of the Front Range Air Pollution and Photochemistry Experiment (FRAPPE) and the Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) campaign, in summer 2014. A particular emphasis was the study of the contribution of emissions from oil and natural gas (O&NG) operations on the regional air quality. One network extended along an elevation gradient from the City of Boulder (elevation ≈1,600 m) to the University of Colorado Mountain Research Station (≈2900 m) on the eastern slopes of the Rocky Mountains. Light alkane petroleum hydrocarbons had the highest mole fraction of the VOCs that could be analyzed with the applied techniques. The longer lived VOCs ethane and propane decreased with increasing elevation, suggesting that Boulder and the surrounding plains were a source of these anthropogenic compounds. VOC diurnal time series showed a few events with elevated mole fractions at the mountain sites, which were likely the result of the upslope transport of plumes with elevated VOCs from the plains. Within the other site network, which extended into suburban East Boulder County (EBC), VOCs were monitored at 5 sites increasingly close to O&NG development in the Denver Julesburg Basin. Mean mole fractions and variability of primarily O&NG-associated VOCs (ethane, propane, butane isomers) increased by a factor of 2.4–5.2 with closer proximity to the O&NG producing region. Median mole fractions of C2–C5 n-alkanes and of imuch-butane at the EBC sites were higher than those previously reported from 28 larger urban areas in the United States. Among the VOCs that could be quantified with the gas chromatography methods, VOCs most clearly associated to O&NG-related emissions (C2–C5 alkanes) accounted for 52%–79% of the VOC hydroxyl radical reactivity (OHR). The horizontal gradient in OHR of the considered VOCs, with ≈3 times higher values at the furthest eastern sites, points toward higher chemical reactivity and ozone production potential from these ozone precursors in the eastern area of the county than within the City of Boulder.

During the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) campaign from July 21 to August 3, 2016, field experiments of leaf-level trace gas exchange of nitric oxide (NO), nitrogen dioxide (NO2), and ozone (O3) were conducted for the first time on the native American tree species Pinus strobus (eastern white pine), 10Acer rubrum (red maple), Populus grandidentata (bigtooth aspen), and Quercus rubra (red oak) in a temperate hardwood forest in Michigan, USA. We measured the leaf-level trace gas exchange rates and investigated the existence of an NO2 compensation point of 1 ppb, hypothesized based on a comparison of a previously observed average diurnal cycle of NOx (NO2 + NO) concentrations with that simulated using a multi-layer canopy exchange model. Known amounts of trace gases were introduced into a tree branch enclosure and a paired blank reference enclosure. The trace gas concentrations before and after 15 the enclosures were measured, as well as the enclosed leaf area (single-sided) and gas flow rate to obtain the trace gas fluxes with respect to leaf surface. There was no detectable NO uptake for all tree types. The foliar NO2 and O3 uptake largely followed a diurnal cycle, correlating with that of the leaf stomatal conductance. NO2 and O3 fluxes were driven by their concentration gradient from ambient to leaf internal space. The NO2 loss rate at leaf surface, equivalently, the foliar NO2 deposition velocity toward the leaf surface, ranged from 0-3.6 mm s -1 for bigtooth aspen, and 0-0.76 mm s -1 for red oak, both 20 of which are ~90% of the expected values based on the stomatal conductance of water. The deposition velocity for red maple and white pine ranged from 0.3-1.6 mm s -1 and from 0.01-1.1 mm s -1 , respectively, and were lower than predicted from the stomatal conductance, implying a mesophyll resistance to the uptake. Additionally, for white pine, the extrapolated velocity at zero stomatal conductance was 0.4 ± 0.08 mm s -1 , indicating a non-stomatal uptake pathway. The NO2 compensation point was ≤60 ppt for all four tree species and indistinguishable from zero at the 95% confidence level. This agrees with recent 25 reports for several European and California tree species but contradicts some earlier experimental results where the compensation points were found to be on the order of 1 ppb or higher. Given that the sampled tree types represent 80-90% of the total leaf area at this site, these results negate the previously hypothesized important role of a leaf-scale NO2 compensation point. Consequently, to reconcile these findings, further detailed comparisons between the observed and the simulated in-and above-canopy NOx concentrations, and the leaf-and canopy-scale NOx fluxes, using the multi-layer canopy exchange model 30The reactive nitrogen species nitric oxide (NO) and nitrogen dioxide (NO2) are key components in tropospheric oxidation chemistry, affecting air quality by triggering the production of ground-level ozone, secondary organic aerosol, and acid r...

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