Short-term methane emissions from 2 dairy farms in California estimated by different measurement techniques and US Environmental Protection Agency inventory methodology: A case study.

Reported estimates of CH4 emissions from ruminants and manure management are up to 2 times higher in atmospheric top-down calculations than in bottom-up (BU) inventories. We explored this discrepancy by estimating CH4 emissions of 2 dairy facilities in California with US Environmental Protection Agency (US EPA) methodology, which is used for BU inventories, and 3 independent measurement techniques: (1) open-path measurements with inverse dispersion modeling (hereafter open-path), (2) vehicle measurements with tracer flux ratio method, and (3) aircraft measurements with the closed-path method. All 3 techniques were used to estimate whole-facility CH4 emissions during 3 to 6 d per farm in the summer of 2016. In addition, open-path was used to estimate whole-facility CH4 emissions over 13 to 14 d per farm in the winter of 2017. Our objectives were to (1) compare whole-facility CH4 measurements utilizing the different measurement techniques, (2) compare whole-facility CH4 measurements to US EPA inventory methodology estimates, and (3) compare CH4 emissions between 2 dairies. Whole-facility CH4 estimates were similar among measurement techniques. No seasonality was detected for CH4 emissions from animal housing, but CH4 emissions from liquid manure storage were 3 to 6 times greater during the summer than during the winter measurement periods. The findings confirm previous studies showing that whole-facility CH4 emissions need to be measured throughout the year to estimate and evaluate annual inventories. Open-path measurements for liquid manure storage emissions were similar to monthly US EPA estimates during the summer, but not during the winter measurement periods. However, the numerical difference was relatively small considering yearly emission estimates. Manure CH4 emissions contributed 69 to 79% and 26 to 47% of whole-facility CH4 emissions during the summer and winter measurement periods, respectively. Methane yields from animal housing were similar between farms (on average 20.9 g of CH4/kg of dry matter intake), but CH4 emissions normalized by volatile solids (VS) loading from liquid manure storage (g of CH4 per day/kg of VS produced by all cattle per day) at 1 dairy were 1.7 and 3.5 times greater than at the other during the summer (234 vs. 137 g of CH4/kg of VS) and winter measurement periods (78 vs. 22 g of CH4/kg of VS), respectively. We attributed much of this difference to the proportion of manure stored in liquid (anaerobic) form, and suggest that manure management practices that reduce the amount of manure solids stored in liquid form could significantly reduce dairy CH4 emissions.

New Approaches to Measuring Sticky Molecules: Improvement of Instrumental Response Times Using Active Passivation.

A novel method has been developed to improve sampling system response times for nominally "sticky" molecules such as HNO3 and NH3. The method reported here makes use of active, continuous passivation, where the instrument interfaces are continuously exposed to 0.01-1 ppm of fluorinated acidic or basic surfactants. To reduce HNO3 response times, perfluoroheptanoic acid and perfluorobutanesulfonic acid vapors are evaluated as passivation species. 1H,1H-perfluorooctylamine is used to improve NH3 response times. The resulting time responses using the perfluoroalkanoic acids are on the order of 0.4-0.7 s for a 75% quantitative recovery of HNO3, and 1-5 s for 90% recovery. Similar response time improvements are seen in detection of NH3 using perfluorooctylamine (<1 s for a 75% recovery, ∼ 2 s for 90% recovery). This generally applicable methodology significantly improves the capability of eddy covariance flux and real-time plume-based measurements of highly polar molecules that have historically been hampered by slow response times due to adsorption on sampling system surfaces. The utility of this approach is demonstrated by field measurements of HNO3 eddy covariance fluxes in a central U.S. prairie.

Vehicle emissions of radical precursors and related species observed in the 2009 SHARP campaign.

UNLABELLED: The 2009 Study of Houston Atmospheric Radical Precursors (SHARP) field campaign had several components that yielded information on the primary vehicular emissions of formaldehyde (HCHO) and nitrous acid (HONO), in addition to many other species. Analysis of HONO measurements at the Moody Tower site in Houston, TX, yielded emission ratios of HONO to the vehicle exhaust tracer species NOx and CO of 14 pptv/ppbv and 2.3 pptv/ppbv, somewhat smaller than recently published results from the Galleria site, although evidence is presented that the Moody Tower values should be upper limits to the true ratios of directly emitted HONO, and are consistent with ratios used in current standard emissions models. Several other Moody Tower emission ratios are presented, in particular a value for HCHO/CO of 2.4 pptv/ppbv. Considering only estimates of random errors, this would be significantly lower than a previous value, though the small sample size and possible systematic differences should be taken into account. Emission factors for CO, NOx, and HCHO, as well as various volatile organic compounds (VOCs), were derived from mobile laboratory measurements both in the Washburn Tunnel and in on-road exhaust plume observations. These two sets of results and others reported in the literature all agree well, and are substantially larger than the CO, NOx, and HCHO emission factors derived from the emission ratios reported from the Galleria site. IMPLICATIONS: Emission factors for the species measured in the various components of the 2009 SHARP campaign in Houston, TX, including HCHO, HONO, CO, CO2, nitrogen oxides, and VOCs, are needed to support regional air quality monitoring. Components of the SHARP campaign measured these species in several different ways, each with their own potential for systematic errors and differences in vehicle fleets sampled. Comparisons between data sets suggest that differences in sampling place and time may result in quite different emission factors, while also showing that different vehicle mixes can yield surprisingly similar emission factors.

Atmospheric measurements of the physical evolution of aircraft exhaust plumes.

Drawing from a series of field measurement activities including the Alternative Aviation Fuels Experiments (AAFEX1 and AAFEX2), we present experimental measurements of particle number, size, and composition-resolved mass that describe the physical and chemical evolution of aircraft exhaust plumes on the time scale of 5 s to 2-3 min. As the plume ages, the particle number emission index initially increases by a factor of 10-50, due to gas-to-particle formation of a nucleation/growth mode, and then begins to fall with increased aging. Increasing the fuel sulfur content causes the initial increase to occur more rapidly. The contribution of the nucleation/growth mode to the overall particle number density is most pronounced at idle power and decreases with increasing engine power. Increasing fuel sulfur content, but not fuel aromatic content causes the nucleation/growth mode to dominate the particle number emissions at higher powers than for a fuel with "normal" sulfur and aromatic content. Particle size measurements indicate that the observed particle number emissions trends are due to continuing gas-to-particle conversion and coagulation growth of the nucleation/growth mode particles, processes which simultaneously increase particle mass and reduce particle number density. Measurements of nucleation/growth mode mass are consistent with the interpretation of particle number and size data and suggest that engine exit plane measurements may underestimate the total particle mass by as much as a factor of between 5 and 10.

Adaptation of a proton transfer reaction mass spectrometer instrument to employ NO+ as reagent ion for the detection of 1,3-butadiene in the ambient atmosphere.

A proton transfer reaction mass spectrometer (PTR-MS) instrument was adapted to employ NO+ as a chemical reagent ion without any hardware changes by switching the reagent ion source gas from water vapor to dry air. Ionization of dry air within the hollow cathode ion source generates a very intense source of NO+ with only a minor impurity of NO2+. The intensities of the primary NO+ reagent ion and the unwanted impurity NO2+ are controllable and dependent on the operational conditions of the hollow cathode ion source. Ion source tuning parameters are described, which maintain an intense source of NO+ while keeping the impurity NO2+ signal to less than 2% of the total reagent ion intensity. This method is applied to the detection of 1,3-butadiene. NO+ reacts efficiently with 1,3-butadiene via a charge exchange reaction to produce only the molecular ion, which is detected at m/z 54. Detection sensitivities of the order of 45 pptv for a 1-s measurement of 1,3-butadiene are demonstrated. We present the first real-time on-line sub parts per billion measurement of 1,3-butadiene in the ambient atmosphere. The only likely interference is from 1,2-butadiene. Concurrent measurements of benzene are provided and suggest that the vehicular emissions are the predominant source of 1,3-butadiene in a suburban Boston area monitoring location.

Using observations and source specific model tracers to characterize pollutant transport during FRAPPÉ and DISCOVER-AQ.

Transport is a key parameter in air quality research and plays a dominant role in the Colorado Northern Front Range Metropolitan Area (NFRMA), where terrain induced flows and recirculation patterns can lead to vigorous mixing of different emission sources. To assess different transport processes and their connection to air quality in the NFRMA during the FRAPPÉ and DISCOVER-AQ campaigns in summer 2014, we use the Weather Research and Forecasting Model with inert tracers. Overall, the model represents well the measured winds and the inert tracers are in good agreement with observations of comparable trace gas concentrations. The model tracers support the analysis of surface wind and ozone measurements and allow for the analysis of transport patterns and interactions of emissions. A main focus of this study is on characterizing pollution transport from the NFRMA to the mountains by mountain-valley flows and the potential for recirculating pollution back into the NFRMA. One such event on 12 August 2014 was well captured by the aircraft and is studied in more detail. The model represents the flow conditions and demonstrates that during upslope events, frequently there is a separation of air masses that are heavily influenced by oil and gas emissions to the North and dominated by urban emissions to the South. This case study provides evidence that NFRMA pollution not only can impact the nearby Foothills and mountain areas to the East of the Continental Divide, but that pollution can "spill over" into the valleys to the West of the Continental Divide.

Short-term variation in near-highway air pollutant gradients on a winter morning.

Quantification of exposure to traffic-related air pollutants near highways is hampered by incomplete knowledge of the scales of temporal variation of pollutant gradients. The goal of this study was to characterize short-term temporal variation of vehicular pollutant gradients within 200-400 m of a major highway (>150 000 vehicles/d). Monitoring was done near Interstate 93 in Somerville (Massachusetts) from 06:00 to 11:00 on 16 January 2008 using a mobile monitoring platform equipped with instruments that measured ultrafine and fine particles (6-1000 nm, particle number concentration (PNC)); particle-phase (>30 nm) [Formula: see text], [Formula: see text], and organic compounds; volatile organic compounds (VOCs); and CO(2), NO, NO(2), and O(3). We observed rapid changes in pollutant gradients due to variations in highway traffic flow rate, wind speed, and surface boundary layer height. Before sunrise and peak traffic flow rates, downwind concentrations of particles, CO(2), NO, and NO(2) were highest within 100-250 m of the highway. After sunrise pollutant levels declined sharply (e.g., PNC and NO were more than halved) and the gradients became less pronounced as wind speed increased and the surface boundary layer rose allowing mixing with cleaner air aloft. The levels of aromatic VOCs and [Formula: see text], [Formula: see text] and organic aerosols were generally low throughout the morning, and their spatial and temporal variations were less pronounced compared to PNC and NO. O(3) levels increased throughout the morning due to mixing with O(3)-enriched air aloft and were generally lowest near the highway reflecting reaction with NO. There was little if any evolution in the size distribution of 6-225 nm particles with distance from the highway. These results suggest that to improve the accuracy of exposure estimates to near-highway pollutants, short-term (e.g., hourly) temporal variations in pollutant gradients must be measured to reflect changes in traffic patterns and local meteorology.