Aerodyne Research, Inc. drove ground-based transects in a mobile laboratory equipped with highly precise Aerodyne tunable infrared laser direct absorption spectrometers measuring a variety of species . A LI-COR non-dispersive infrared instrument measured CO2 and H2O. Meteorological and positional data were collected at all tracer release sites and on the vehicle, using multiple AIRMAR 200WX WeatherStation® instruments and a Hemisphere V103 GPS Compass. To minimize drift and maintain accurate baseline values on the TILDAS instruments in the minAML, a valve sequence enabled overblowing of the inlet with ultra-zero air every 15 min for 45 s . Scientific Aviation equipped an aircraft with a Picarro G2301-f cavity ringdown spectrometer , TILDAS Vaisala HMP60 humidity and temperature probe, and Hemisphere VS330 GPS Compass used for positioning and calculating wind velocity . Since SA had a TILDAS on board measuring C2H6 during these times, black plastic pots it was possible to treat these flights as a tracer release experiment similar to that performed with the ground-based equipment. A full description of the equipment used during this project can be found in the Supplement of Arndt et al. .
During this study, the aircraft flew low and close to the sites, at an average distance of ∼ 900 m and an altitude of ∼ 325 m. Each site had a combination of spread out point source emitters and large open area sources . SA conducted 11 flights over 6 d, usually flying twice a day, in the late morning and midafternoon. Flights typically lasted 1–2 h for a given farm, flying in spirals looping around the perimeter of the animal housing and manure management areas. ARI measured for 3 d at Dairy 1 and 5 d at Dairy 2. The mobile lab drove at several different times of day for each site, trying to capture any diurnal effect, but always overlapped with the aircraft at least once a day.Tracer gases, ethane and acetylene, were released from ground-based tripods at a variety of locations on the dairy farms with the intention of co-locating with known emission sources . Tracers were used to distinguish and quantify sources by positioning them within each respective emission area. Often, each tracer was released at a single point from each major source, typically the liquid manure management and animal housing areas . For this study, only the position and release rate of C2H6 is relevant. Release rates of C2H6 ranged from 10 to 40 slpm throughout the project .
A schematic of tracer release being performed at a dairy farm is shown in Fig. 1. Detailed descriptions of the tracer flux ratio technique used during this work can be found in Arndt et al. or more generally in Roscioli et al. . In summary, tracer gas released close to a source produces a plume that experiences the local wind dynamics and meteorological conditions akin to the nearby emission of interest, thereby proving a representation of those emissions. A plume is considered to be a co-located enhancement above ambient concentrations of CH4 and tracer gas. Active tracer release overlapped with on-site flight transects for approximately 11 h during this week-long project. Exact timing of the overlap between the release of C2H6 and sampling periods by the aircraft is shown in Table 1. Ethane was selected over other gases due to the lack of potential interference with nearby sources and its long atmospheric lifetime. At one of the two sites, C2H6 from a small well pad could be observed on the ground at close distances. This interference was characterized and eliminated using its measured C2H6 : CH4 ratio in combination with wind direction and farm layout.Analysis of tracer flux data involves comparing slopes or areas of enhancements between tracer gas and site CH4 emissions. The molar enhancement ratio, scaled by the amount of tracer gas released, determines a CH4 emission rate for the specific plume encounter. Area analysis compares integrated plumes of CH4 and C2H6, particularly necessary during close transects when plumes do not temporally or spatially co-align. Both analysis methods were performed on this dataset and are discussed in further detail in Sect. 3.2.
Due to the speed of the aircraft , observations of plume emissions were brief. On average, identified plumes lasted 12 s , not including a significant amount of time collected before and after enhancements to ensure accuracy of baseline calculations during analysis. Prior to analysis, all data had appropriate calibration factors applied, correcting minor deviations in flow rate by mass flow controllers and instrument performance for specific species. Instrument calibrations occurred in the field at several times during this campaign using mixed-gas standards diluted with ultra-zero air. Distance between tracer release locations and aircraft position was determined using basic trigonometry. Uncertainties for emission rate estimates are determined as 95 % confidence intervals. Plumes observed by the aircraft were included in the analysis after meeting certain criteria. Requirements included tracer gas flowing on-site for more than 10 min prior to observation, correlated plumes of CH4 and C2H6 based on high coefficient of determination from a least-squares fit , and positive enhancements above baseline for CH4 and C2H6. After meeting these standards, each plume was viewed and additional conditions were manually considered: wind direction and speed , duration of the enhancement, validity of the linear regression fits, quality of calculated baseline for integration purposes, location of the aircraft relative to the sources, and correlation between CH4 and other species indicating interferences or source allocation.During each flight, identifiable plumes of CH4 were observed regularly, approximately every 1–2 min. Figure 4 depicts repeated measurements of CH4 emissions representative of the whole farm, revealing characteristics about emission sources at each site. Viewed from the south, manure and animal housing areas at Dairy 1 line up together, whereas at Dairy 2 the anaerobic lagoon and settling cells are offset from the housing areas. While these observations largely depend on wind direction and distance from the source, some features gave insight into where emissions came from on-site. Broad emissions can be readily attributed to the large collection of point source emitters milling around barns and open lots . Sharp peaks and broad plateaus indicate an encounter with out gassing by a large area source . Gaussian shapes appear to be an amalgamation of both major sources mixed downwind. Temporal and spatial differences exist between the aircraft measurements used in this dataset and the ground-based measurements collected as part of the initial study at each dairy farm showing characteristics of emitted methane plumes as observed by the aircraft downwind to the south. Each time trace depicts the high rate of repetition in the flown transects around each site. 2018). Measurements by the minAML occurred during the day and night at a variety of distances from each site . The aircraft had good coverage during the middle of the day, plastic pot black with flights in the late morning and early afternoon performing frequently repeated transects around each site . The ground-based tracer release experiment observed very low plume enhancements in the hot midday conditions due to low winds and strong vertical mixing while the aircraft saw good signal, but it had no issue collecting nighttime measurements when the aircraft did not operate. Tracer flux ratio methodology thrives with strong winds and downwind road access perpendicular to the dominant wind direction. Close placement of tracer gas to a point source and distant measurements by the mobile lab allow time and space for the tracer to co-disperse with emission gas and merge together in the measured plume.
During this field campaign, the aircraft flew close to the site measuring emissions in a calm wind and saw an abundance of signal due to strong surface heating. These conditions proved favorable for the aircraft and mass balance calculations but stretch the possible application of the tracer release method. Even so, the attempt to perform a tracer release experiment observed from an aircraft proved largely successful and provided direct insight as to how these measurements relate to the ground based observations. Due to the sensitivity of the C2H6 instrument on the aircraft, it was readily apparent when the tracer gas was present and intermingling with the farm emissions. Figure 5 visualizes the initiation of tracer release at Dairy 2 and the time it takes for tracer gas to disperse on-site. Prior to releasing any tracer gas, the concentration of C2H6 shows a relatively steady baseline. After initiating the release of tracer gas at 20 slpm, it took approximately 20 min before the aircraft begins to detect it initially and another 15 min before the plume characteristics were stabilized. We suspect this was due to the prevailing conditions of weak horizontal winds and strong but varying vertical mixing at the site. The aircraft ascended above the emission plume for 10–20 min after the release began, taking it out of plume detection range, which may have lengthened the time it took to first detect tracer gas. Based on the average wind direction and horizontal speed from 10:39 PDT to 11:00 PDT , we could expect to begin seeing tracer gas after ∼ 6 min at a distance of 1.6 km . Instead, we saw the first spike around 11 min after beginning release. For the plumes reported in this dataset, there is no observed dependence of emission rate with sampling altitude. In Fig. 6, CH4 emissions are plotted versus aircraft altitude. Emissions between 0 and 6500 kg d−1 appear to be randomly distributed between 100 and 600 m at each site . Two outliers show higher emission rates at low altitudes, unmatched at higher altitudes. Above 650 m are three other points scattered across a wide range of emissions . These outliers occurred when the aircraft flew close to the site at an angle that put the lagoon between the aircraft and the tracer release point. The impact of measuring a source closer than the tracer is a potential overestimation of the emission due to differences in dispersion . Increasing emissions with decreasing height, in some cases, could be attributed to the influence of a strongly lofted lagoon signal at a site. Lower flights could then cause the aircraft to encounter a larger proportion of the manure-related emissions instead of the ideal case: a well-mixed plume representative of the entire site.Swirling and calm winds shifted emissions around each site at various times over multiple days. When selecting valid plumes, proximity of the aircraft during an enhancement to a single source introduces a dilemma. Varying distances between the tracer gas release point and presumed source could affect the determined emission rate, due to imperfect codispersion. For example, using a tracer plume located 500 m away to represent a source 300 m away would be problematic. When measuring at greater distances with better resolution , it is often trivial to identify when the tracer inadequately represents the emission. Flying several times faster than the driven transect provided notable repeatability but made spatial understanding of the site difficult with respect to emission sources. Direct estimates of liquid manure emissions proved unrealistic at both dairies due to sparse number of CH4 plumes with sufficient tracer representation, despite favorable wind direction and aircraft position. A few plumes of acceptable data quality were identified as being related to liquid manure emissions at Dairy 2 , but estimates were significantly higher than reported in Arndt et al. at 4893±1331 kg CH4 d −1 . Due to concerns that the tracer release location was not close enough to the liquid manure source to be representative, especially due to non-ideal transect geometry and limited horizontal wind, these data are not reported in Table 2. Relative apportionment of CH4 between sources showed manure-associated plumes leading the fractional contribution at Dairy 1 and Dairy 2 . This was an expected finding based on US EPA methodology estimates for this month at Dairy 2 . Given the temporal nature of manure emissions, as reported by Leytem et al. , it should be reinforced that these results only represent a short period of time in a single season. Despite the difficulty of collecting or identifying many distinct manure associated plumes via measurements taken from this aircraft, the general apportionment of source emissions appears to remain evident. Clear hot measurement days could have stimulated anaerobic activity in manure lagoons and caused greater release of gases , while strong thermal convection lofted concentrated and unmixed plumes.