Erin E. McDuffie

Influence of Oil and Gas Emissions on Summertime Ozone in the Colorado Northern Front Range

Tropospheric O3 has been decreasing across much of the eastern U.S. but has remained steady or even increased in some western regions. Recent increases in VOC and NOx emissions associated with the production of oil and natural gas (O&NG) may contribute to this trend in some areas. The Northern Front Range of Colorado has regularly exceeded O3 air quality standards during summertime in recent years. This region has VOC emissions from a rapidly developing O&NG basin and low concentrations of biogenic VOC in close proximity to urban-Denver NOx emissions. Here VOC OH reactivity (OHR), O3 production efficiency (OPE), and an observationally constrained box model are used to quantify the influence of O&NG emissions on regional summertime O3 production. Analyses are based on measurements acquired over two summers at a central location within the Northern Front Range that lies between major regional O&NG and urban emission sectors. Observational analyses suggest that mixing obscures any OPE differences in air primarily influenced by O&NG or urban emission sector. The box model confirms relatively modest OPE differences that are within the uncertainties of the field observations. Box model results also indicate that maximum O3 at the measurement location is sensitive to changes in NOx mixing ratio but also responsive to O&NG VOC reductions. Combined, these analyses show that O&NG alkanes contribute over 80% to the observed carbon mixing ratio, roughly 50% to the regional VOC OHR, and approximately 20% to regional photochemical O3 production.

Observations of acyl peroxy nitrates during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ)

We report on measurements of acyl peroxy nitrates (APNs) obtained from two ground sites and the NSF/NCAR C-130 aircraft during the 2014 Front Range Air Pollution and Photochemistry Experiment (FRAPPE). The relative abundance of the APNs observed at the Boulder Atmospheric Observatory (BAO) indicates that anthropogenic emissions of volatile organic compounds (VOCs) are the dominant drivers of photochemistry during days with the most elevated PAN. Reduced major axis regression between PPN and PAN observed at BAO and from the C-130 produced a slope of 0.21 (R2 = 0.92). Periods of lower PPN/PAN ratios (~0.10) were associated with cleaner background air characterized by lower ammonia and formic acid abundances. The abundance of MPAN relative to PAN only exceeded 0.05 at BAO when PAN mixing ratios were < 300 pptv, implying low influence of isoprene oxidation during periods with substantial local PAN production. We show an example of a day (19 July) where high O3 was not accompanied by enhanced local PAN production. The contribution of biogenic VOCs to local O3 production on the other days in July with elevated O3 (22, 23, 28 and 29 July 2014) was small; evidence is provided in the high abundance of PPN to PAN (slopes between 0.18 – 0.26). The PAN chemistry observed from surface and aircraft platforms during FRAPPE implies that anthropogenic VOCs played a dominant role in PAN production during periods with the most O3, and that the relative importance of biogenic hydrocarbon chemistry decreased with increasing O3 production during FRAPPE.

Wintertime overnight NOx removal in a southeastern United States coal‐fired power plant plume: A model for understanding winter NOx processing and its implications

The authors would like to thank theNSF-NCAR Research Aircraft Facility staff. Data are available from NCAR at http://data.eol.ucar.edu/master_list/?project=WINTER. The model algorithm used was developed in IGOR Pro and is available at https://esrl.noaa.gov/csd/groups/csd7/measurements/2015win-ter/pubs/. Funding for Fibiger was sup-ported by NSF award 1433358

Flight Deployment of a High‐Resolution Time‐of‐Flight Chemical Ionization Mass Spectrometer: Observations of Reactive Halogen and Nitrogen Oxide Species

We describe the University of Washington airborne high-resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS) and evaluate its performance aboard the NCAR-NSF C-130 aircraft during the recent Wintertime INvestigation of Transport, Emissions and Reactivity (WINTER) experiment in February–March of 2015. New features include (i) a computer-controlled dynamic pinhole that maintains constant mass flow-rate into the instrument independent of altitude changes to minimize variations in instrument response times; (ii) continuous addition of low flow-rate humidified ultrahigh purity nitrogen to minimize the difference in water vapor pressure, hence instrument sensitivity, between ambient and background determinations; (iii) deployment of a calibration source continuously generating isotopically labeled dinitrogen pentoxide (N2O5) for in-flight delivery; and (iv) frequent instrument background determinations to account for memory effects resulting from the interaction between sticky compounds and instrument surface following encounters with concentrated air parcels. The resulting improvements to precision and accuracy, along with the simultaneous acquisition of these species and the full set of their isotopologues, allow for more reliable identification, source attribution, and budget accounting, for example, by speciating the individual constituents of nocturnal reactive nitrogen oxides (NOz = ClNO2 + 2 × N2O5 + HNO3 + etc.). We report on an expanded set of species quantified using iodide-adduct ionization such as sulfur dioxide (SO2), hydrogen chloride (HCl), and other inorganic reactive halogen species including hypochlorous acid, nitryl chloride, chlorine, nitryl bromide, bromine, and bromine chloride (HOCl, ClNO2, Cl2, BrNO2, Br2, and BrCl, respectively).

Heterogeneous N2O5 Uptake During Winter: Aircraft Measurements During the 2015 WINTER Campaign and Critical Evaluation of Current Parameterizations

Nocturnal dinitrogen pentoxide (N2O5) heterogeneous chemistry impacts regional air quality and the distribution and lifetime of tropospheric oxidants. Formed from the oxidation of nitrogen oxides, N2O5 is heterogeneously lost to aerosol with a highly variable reaction probability, γ(N2O5), dependent on aerosol composition and ambient conditions. Reaction products include soluble nitrate (HNO3 or NO3 ) and nitryl chloride (ClNO2). We report the first-ever derivations of γ(N2O5) from ambient wintertime aircraft measurements in the critically important nocturnal residual boundary layer. Box modeling of the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign over the eastern United States derived 2,876 individual γ(N2O5) values with a median value of 0.0143 and range of 2 × 10 5 to 0.1751. WINTER γ(N2O5) values exhibited the strongest correlation with aerosol water content, but weak correlations with other variables, such as aerosol nitrate and organics, suggesting a complex, nonlinear dependence on multiple factors, or an additional dependence on a nonobserved factor. This factor may be related to aerosol phase, morphology (i.e., core shell), or mixing state, none of which are commonly measured during aircraft field studies. Despite general agreement with previous laboratory observations, comparison of WINTER data with 14 literature parameterizations (used to predict γ(N2O5) in chemical transport models) confirms that none of the current methods reproduce the full range of γ(N2O5) values. Nine reproduce the WINTER median within a factor of 2. Presented here is the first field-based, empirical parameterization of γ(N2O5), fit to WINTER data, based on the functional form of previous parameterizations.

Tall Tower Vertical Profiles and Diurnal Trends of Ammonia in the Colorado Front Range

Ammonia (NH3) mixing ratios were measured between the surface and 280 m above ground level from a moveable carriage at the Boulder Atmospheric Observatory (BAO) tower in summer 2014 as part of the Front Range Air Pollution and Photochemistry Experiment (FRAPPE). The campaign median mixing ratio was 3.3 ppb, ranging from below detection limits to 192 ppb. Median vertical profiles show an overall increase in NH3 mixing ratios towards the surface of 6.7 ppb (89 %) during the day, and 3.9 ppb (141%) at night. In contrast to the overall increasing trend, some individual profiles show decreasing NH3 in the lowest 10 m. This suggests that the local surface is capable of acting as either a source or a sink, depending on the relative amounts of NH3 at the surface, and in advected air parcels. We further use this dataset to investigate the variation in diurnal patterns of NH3 as a function of height above the surface. At higher altitudes (100 ± 5 and 280 ± 5 m), NH3 mixing ratios reach a gradual maximum during the day between 9:00 and 16:00 local time, likely driven by changes in source region. At lower altitudes (10 ± 5 m), the daytime maximum begins earlier at about 7:00 local time, followed by a sharper increase at 9:00 local time. At this height we also observe a peak in NH3 mixing ratios during the night, likely driven by the trapping of emitted NH3 within the shallower nocturnal boundary layer.

Sources and Secondary Production of Organic Aerosols in the Northeastern United States during WINTER

Most intensive field studies investigating aerosols have been conducted in summer, and thus, wintertime aerosol sources and chemistry are comparatively poorly understood. An aerosol mass spectrometer was flown on the National Science Foundation/National Center for Atmospheric Research C-130 during the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) 2015 campaign in the northeast United States. The fraction of boundary layer submicron aerosol that was organic aerosol (OA) was about a factor of 2 smaller than during a 2011 summertime study in a similar region. However, the OA measured inWINTERwas almost as oxidized as OAmeasured in several other studies in warmermonths of the year. Fifty-eight percent of the OA was oxygenated (secondary), and 42% was primary (POA). Biomass burning OA (likely from residential heating) was ubiquitous and accounted for 33% of the OA mass. Using nonvolatile POA, one of two default secondary OA (SOA) formulations in GEOS-Chem (v10-01) shows very large underpredictions of SOA and O/C (5×) and overprediction of POA (2×). We strongly recommend against using that formulation in future studies. Semivolatile POA, an alternative default in GEOS-Chem, or a simplified parameterization (SIMPLE) were closer to the observations, although still with substantial differences. A case study of urban outflow from metropolitan New York City showed a consistent amount and normalized rate of added OA mass (due to SOA formation) compared to summer studies, although proceeding more slowly due to lower OH concentrations. A boxmodel and SIMPLE perform similarly forWINTER as for Los Angeles, with an underprediction at ages <6 hr, suggesting that fast chemistry might be missing from the models.

Top‐Down Estimates of NOx and CO Emissions From Washington, D.C.‐Baltimore During the WINTER Campaign

Airborne mass balance experiments were conducted around the Washington, D.C.-Baltimore area using research aircraft from Purdue University and the University of Maryland to quantify emissions of nitrogen oxides (NOx = NO + NO2) and carbon monoxide (CO). The airborne mass balance experiments supported the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign, an intensive airborne study of anthropogenic emissions along the Northeastern United States in February–March 2015, and the Fluxes of Atmospheric Greenhouse Gases in Maryland project which seeks to provide best estimates of anthropogenic emissions from the Washington, D.C.-Baltimore area. Top-down emission rates of NOx and CO estimated from the mass balance flights are compared with the Environmental Protection Agency’s 2011 and 2014 National Emissions Inventory (NEI-11 and NEI-14). Inventory and observation-derived NOx emission rates are consistent within the measurement uncertainty. Observed CO emission rates are a factor of 2 lower than reported by the NEI. The NEI’s accuracy has been evaluated for decades by studies of anthropogenic emissions, yet despite continuous inventory updates, observation-inventory discrepancies persist. WINTER NOx/CO2 enhancement ratios are consistent with inventories, but WINTER CO/NOx and CO/CO2 enhancement ratios are lower than those reported by other urban summertime studies, suggesting a strong influence of CO seasonal trends and/or nationwide CO reductions. There is a need for reliable observation-based criterion pollutant emission rate measurements independent of the NEI. Such determinations could be supplied by the community’s reporting of sector-specific criteria pollutant/CO2 enhancement ratios and subsequent multiplication with currently available and forthcoming high-resolution CO2 inventories.

Tropospheric sources and sinks of gas-phase acids in the Colorado Front Range

We measured organic and inorganic gas-phase acids in the Front Range of Colorado to better understand their tropospheric sources and sinks using a high-resolution time-of-flight chemical ionization mass spectrometer. Measurements were conducted from 4 to 13 August 2014 at the Boulder Atmospheric Observatory during the Front Range Air Pollution and Photochemistry Éxperiment. Diurnal increases in mixing ratios are consistent with photochemical sources of HNO3, HNCO, formic, propionic, butyric, valeric, and pyruvic acid. Vertical profiles taken on the 300 m tower demonstrate net surface-level emissions of alkanoic acids, but net surface deposition of HNO3 and pyruvic acid. The surface-level alkanoic acid source persists through both day and night, and is thus not solely photochemical. Reactions between O3 and organic surfaces may contribute to the surface-level alkanoic acid source. Nearby traffic emissions and agricultural activity are a primary source of propionic, butyric, and valeric acids, and likely contribute photochemical precursors to HNO3 and HNCO. The combined diel and vertical profiles of the alkanoic acids and HNCO are inconsistent with dry deposition and photochemical losses being the only sinks, suggesting additional loss mechanisms.