ALBERT

Beschreibung: Emissions of methane (CH 4 ) and volatile organic compounds (VOCs) from oil and gas production may have large impacts on air quality and climate change. Methane and VOCs were measured over the Haynesville and Marcellus shale gas plays onboard the NCAR C-130 and NOAA WP-3D research aircraft in June-July of 2013. We used an eddy covariance technique to measure in situ fluxes of CH 4 and benzene from both C-130 flights with high-resolution data (10 Hz) and WP-3D flights with low-resolution data (1 Hz). Correlation (R=0.65) between CH 4 and benzene fluxes was observed when flying over shale gas operations and the enhancement ratio of fluxes was consistent with the corresponding concentration observations. Fluxes calculated by the eddy covariance method show agreement with a mass balance approach within their combined uncertainties. In general, CH 4 fluxes in the shale gas regions follow a lognormal distribution, with some deviations for relatively large fluxes (〉10 µg m −2 s −1 ). Statistical analysis of the fluxes shows that a small number of facilities (i.e. ~10%) are responsible for up to ~40% of the total CH 4 emissions in the two regions. We show that the airborne eddy covariance method can also be applied in some circumstances when meteorological conditions do not favor application of the mass balance method. We suggest that the airborne eddy covariance method is a reliable alternative and complementary analysis method to estimate emissions from oil and gas extraction.

Beschreibung: Tropospheric O 3 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 NO x 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 O 3 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 NO x emissions. Here, VOC OH reactivity (OHR), O 3 production efficiency (OPE), and an observationally constrained box model are used to quantify the influence of O&NG emissions on regional summertime O 3 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 emissions sectors. The box model confirms relatively modest OPE differences that are within the uncertainties of the field observations. Box model results also indicate that maximum O 3 at the measurement location is sensitive to changes in NO x 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 O 3 production.

Beschreibung: We use a 0-D photochemical box model and a 3-D global chemistry-climate model, combined with observations from the NOAA Southeast Nexus (SENEX) aircraft campaign, to understand the sources and sinks of glyoxal over the Southeast United States. Box model simulations suggest a large difference in glyoxal production among three isoprene oxidation mechanisms (AM3ST, AM3B, and MCM v3.3.1). These mechanisms are then implemented into a 3-D global chemistry-climate model. Comparison with field observations shows that the average vertical profile of glyoxal is best reproduced by AM3ST with an effective reactive uptake coefficient γ glyx of 2 × 10 -3 , and AM3B without heterogeneous loss of glyoxal. The two mechanisms lead to 0-0.8 µg m -3 secondary organic aerosol (SOA) from glyoxal in the boundary layer of the Southeast U.S. in summer. We consider this to be the lower limit for the contribution of glyoxal to SOA, as other sources of glyoxal other than isoprene are not included in our model. In addition, we find that AM3B shows better agreement on both formaldehyde and the correlation between glyoxal and formaldehyde ( R GF  = [GLYX]/[HCHO]), resulting from the suppression of δ-isoprene peroxy radicals (δ-ISOPO 2 ). We also find that MCM v3.3.1 may underestimate glyoxal production from isoprene oxidation, in part due to an underestimated yield from the reaction of IEPOX peroxy radicals (IEPOXOO) with HO 2 . Our work highlights that the gas-phase production of glyoxal represents a large uncertainty in quantifying its contribution to SOA.

Beschreibung: Methane (CH 4 ) is the primary component of natural gas and has a larger global warming potential than CO 2 . Recent top-down studies based on observations showed CH 4 emissions in California's South Coast Air Basin (SoCAB) were greater than those expected from population-apportioned bottom-up state inventories. In this study, we quantify CH 4 emissions with an advanced mesoscale inverse modeling system at a resolution of 8 km × 8 km, using aircraft measurements in the SoCAB during the 2010 CalNex campaign to constrain the inversion. To simulate atmospheric transport, we use the FLEXPART-WRF Lagrangian particle dispersion model driven by three configurations of the Weather Research and Forecasting (WRF) mesoscale model. We determine surface fluxes of CH 4 using a Bayesian least squares method in a 4-dimensional inversion. Simulated CH 4 concentrations with the posterior emission inventory achieve much better correlations with the measurements (R 2 =0.7) than using the prior inventory (US EPA's National Emission Inventory 2005, R 2 =0.5). The emission estimates for CH 4 in the posterior, 46.3 ± 9.2 Mg CH 4 /hr, is consistent with published observation-based estimates. Changes in the spatial distribution of CH 4 emissions in the SoCAB between the prior and posterior inventories are discussed. Missing or underestimated emissions from dairies, the oil/gas system, and landfills in the SoCAB seem to explain the differences between the prior and posterior inventories. We estimate that dairies contributed 5.9 ± 1.7 Mg CH 4 /hr and the two sectors of oil and gas industry (production and downstream) and landfills together contributed 39.6 ± 8.1 Mg CH 4 /hr in the SoCAB.

Beschreibung: Nighttime chemistry in the troposphere is closely tied to the dinitrogen pentoxide (N 2 O 5 ) budget, but high uncertainties remain regarding the model representation of the heterogeneous hydrolysis of N 2 O 5 on aerosol particles. In this study we used the community model WRF-Chem to simulate a three-day period during the California Nexus (CalNex) Campaign in 2010. We extended WRF-Chem to include the heterogeneous hydrolysis of N 2 O 5 and contrasted the impact of different published parameterizations of N 2 O 5 heterogeneous hydrolysis on the spatial distribution of uptake coefficients and the resulting N 2 O 5 concentrations. For all the cases, modeled N 2 O 5 uptake coefficients showed strong spatial variability, with higher values in the nocturnal boundary layer compared to the residual layer, especially in environments with high relative humidities, such as over the ocean and along the coast. The best agreement of modeled and observed uptake coefficients was obtained using the parameterization by Davis et al. [2008] combined with the treatment of organic coating by Riemer et al. [2009]. For this case the temporal evolution of lower boundary layer N 2 O 5 mixing ratios was reproduced well, and the predictions of surface mixing ratios of ozone and NO x were improved. However, the model still overpredicted the uptake coefficients in the residual layer and consequently underpredicted N 2 O 5 concentrations in the residual layer. This study also highlights that environments with low relative humidities pose a challenge for aerosol thermodynamic models in calculating aerosol water uptake, and this impacts N 2 O 5 heterogeneous hydrolysis parameterizations.

Beschreibung: ABSTRACT California's ambient ozone concentrations have two principal contributions: U.S. background ozone and enhancements produced from anthropogenic precursor emissions; only the latter effectively respond to California emission controls. From 1980-2015 ozone has been monitored in eight air basins in Southern California. The temporal evolution of the largest measured concentrations, i.e. those that define the ozone design value (ODV) upon which the National Ambient Air Quality Standard (NAAQS) is based, is described very well by an exponential decrease on top of a positive offset. We identify this offset as the ODV due to the U.S. background ozone (i.e., the concentration that would be present if U.S. anthropogenic precursor emissions were reduced to zero), and is estimated to be 62.0 ± 1.9 ppb in six of the basins. California's emission control efforts have reduced the anthropogenic ozone enhancements by a factor of ~5 since 1980. However, assuming that the current rate of exponential decrease is maintained and that U.S. background ODV remains constant, projections of the past decrease suggests that ~35 years of additional emission control efforts will be required to reach the new NAAQS of 70 ppb in the Los Angeles area. The growing predominance of U.S. background ozone contributions has shifted the maximum ozone concentrations in all air basins from later to earlier in the summer. Comparisons indicate that currently accepted model estimates of U.S. background ozone concentrations in southern California are somewhat underestimated; thus reducing ozone in this region to the 2015 NAAQS may be more difficult than currently expected.

Beschreibung: Wildfires emit significant amounts of pollutants that degrade air quality. Plumes from three wildfires in the western U.S. were measured from aircraft during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC 4 RS) and the Biomass Burning Observation Project (BBOP), both in summer 2013. This study reports an extensive set of emission factors (EFs) for over 80 gases and 5 components of submicron particulate matter (PM 1 ) from these temperate wildfires. These include rarely, or never before, measured oxygenated volatile organic compounds and multifunctional organic nitrates. The observed EFs are compared with previous measurements of temperate wildfires, boreal forest fires, and temperate prescribed fires. The wildfires emitted high amounts of PM 1 (with organic aerosol (OA) dominating the mass) with an average EF that is more than 2 times the EFs for prescribed fires. The measured EFs were used to estimate the annual wildfire emissions of carbon monoxide, nitrogen oxides, total nonmethane organic compounds, and PM 1 from 11 western U.S. states. The estimated gas emissions are generally comparable with the 2011 National Emissions Inventory (NEI). However, our PM 1 emission estimate (1530 ± 570 Gg yr −1 ) is over 3 times that of the NEI PM 2.5 estimate and is also higher than the PM 2.5 emitted from all other sources in these states in the NEI. This study indicates that the source of OA from biomass burning in the western states is significantly underestimated. In addition, our results indicate that prescribed burning may be an effective method to reduce fine particle emissions.

Beschreibung: We quantify methane (CH 4 ) emissions in California's San Joaquin Valley (SJV) using four days of aircraft measurements from a field campaign during May-June 2010 together with a Bayesian inversion method and a mass-balance approach. For the inversion estimates, we use the FLEXible PARTicle dispersion model (FLEXPART) to establish the source-receptor relationship between sampled atmospheric concentrations and surface fluxes. Our prior CH 4 emissions estimates are from the California Greenhouse Gas Emissions Measurements (CALGEM) inventory. We use three meteorological configurations to drive FLEXPART and subsequently construct three inversions to analyze the final optimized estimates and their uncertainty (one standard deviation) . We conduct May and June inversions independently, and derive similar total CH 4 emissions estimates for the SJV: 135 ± 28 Mg/hr in May and 135 ± 19 Mg/hr in June. The inversion result is 1.7 times higher than the prior estimate from CALGEM. We also use an independent mass-balance approach to estimate CH 4 emissions in the northern SJV for one flight when meteorological conditions allowed. The mass-balance estimate provides a confirmation of our inversion results, and these two independent estimates of the total CH 4 emissions in the SJV are consistent with previous studies. In this study, we provide optimized CH 4 emissions estimates at 0.1° horizontal resolution. Using independent spatial information on major CH 4 sources, we estimate that livestock contribute 75–77% and oil/gas production contributes 15–18% of the total CH 4 emissions in the SJV. Livestock explain most of the discrepancies between the prior and the optimized emissions from our inversion.

Beschreibung: Stratosphere-to-troposphere transport (STT) and biomass burning (BB) are two important natural sources for tropospheric ozone, that can result in elevated ozone and air-quality episode events. High-resolution observations of multiple related species are critical for complex ozone source attribution. In this article, we present an analysis of coinciding ground-based and airborne observations, including ozone lidar, ozonesonde, high spectral resolution lidar (HSRL), and multiple airborne in situ measurements, made on 28 and 29 June 2013 during the Southeast Nexus (SENEX) field campaign. The ozone lidar and HSRL reveal detailed ozone and aerosol structures as well as the temporal evolution associated with a cold front passage. The observations also captured two enhanced (+30 ppbv) ozone layers in the free troposphere (FT), which were determined from this study to be caused by a mixture of BB and stratospheric sources. The mechanism for this STT is tropopause folding associated with a cutoff upper-level low pressure system according to the analysis of its potential vorticity structure. The depth of the tropopause fold appears to be shallow for this case compared to events observed in other seasons, however, the impact on lower tropospheric ozone was clearly observed. This event suggests that strong STT may occur in the southeast United States during the summer and can potentially impact lower troposphere during these times. Statistical analysis of the airborne observations of trace gases suggests a coincident influence of BB transport in the FT impacting the vertical structure of ozone during this case study.