Development of a Monitoring System for Semicontinuous Measurements of Stable Carbon Isotope Ratios in Atmospheric Carbonaceous Aerosols: Optimized Methods and Application to Field Measurements.

Carbon content constitutes a major fraction of atmospheric particulate matter (PM) and directly influences the earth's climate and human health. The stable carbon isotope ratios (δ13C) can be used to track potential sources and atmospheric processes of carbonaceous aerosols. Previously, determination of δ13C was always conducted in offline carbonaceous aerosol samples. The poor time-resolution results cannot provide information regarding the temporal evolution of δ13C at a short-time scale. In this study, we developed a new system for online measurements of δ13C in atmospheric carbonaceous aerosols by combining a semicontinuous organic carbon/elemental carbon (OC/EC) analyzer and online cavity ring-down spectroscopy (CRDS) (OC/EC analyzer-CRDS). To provide better stability in the determination of δ13C, a carrier gas with CO2 (∼200 ppm) in "balance gas" was used, and Keeling analysis was employed to separate the δ13C signal of the sample from background CO2 gas. Our results showed that the accuracy and absolute precision of the δ13C measurements by the OC/EC analyzer-CRDS system were better than 0.1‰ and 0.5‰, respectively, for the samples containing carbon content more than 5 µg. Furthermore, we employed this system to monitor δ13C (δ13C-TC) in particulate total carbon (TC) with a time resolution of 2-4 h over Beijing in late summer and early autumn, 2019. During the sampling period, the TC concentrations varied from 0.1 to 12.0 µg m-3 with a mean value of 6.0 ± 2.4 µg m-3. The δ13C-TC ranged from -28.2 to -24.2‰ (mean value was -25.9 ± 0.9‰) without significant diurnal variations, suggesting similar contributing sources to TC. Comparing the δ13C signatures of different emissions, we found that liquid fuels and primary and secondary C3 plants were likely the dominant sources of particulate TC. Finally, we found that atmospheric heavy precipitation washed out the aged aerosols from the polluted air, resulting in significant depletion (∼2.4‰) of δ13C-TC in the atmosphere. This paper described a novel system for conducting online measurements of δ13C in atmospheric carbonaceous aerosols and provided us information to better understand the temporal evolution of emission sources and atmospheric processes of carbonaceous aerosols.

Short-term landfill methane emissions dependency on wind.

Short-term (2-10h) variations of whole-landfill methane emissions have been observed in recent field studies using the tracer dilution method for emissions measurement. To investigate the cause of these variations, the tracer dilution method is applied using 1-min emissions measurements at Sandtown Landfill (Delaware, USA) for a 2-h measurement period. An atmospheric dispersion model is developed for this field test site, which is the first application of such modeling to evaluate atmospheric effects on gas plume transport from landfills. The model is used to examine three possible causes of observed temporal emissions variability: temporal variability of surface wind speed affecting whole landfill emissions, spatial variability of emissions due to local wind speed variations, and misaligned tracer gas release and methane emissions locations. At this site, atmospheric modeling indicates that variation in tracer dilution method emissions measurements may be caused by whole-landfill emissions variation with wind speed. Field data collected over the time period of the atmospheric model simulations corroborate this result: methane emissions are correlated with wind speed on the landfill surface with R(2)=0.51 for data 2.5m above ground, or R(2)=0.55 using data 85m above ground, with emissions increasing by up to a factor of 2 for an approximately 30% increase in wind speed. Although the atmospheric modeling and field test are conducted at a single landfill, the results suggest that wind-induced emissions may affect tracer dilution method emissions measurements at other landfills.

Measuring emissions from oil and natural gas well pads using the mobile flux plane technique.

We present a study of methane emissions from oil and gas producing well pad facilities in the Barnett Shale region of Texas, measured using an innovative ground-based mobile flux plane (MFP) measurement system, as part of the Barnett Coordinated Campaign.1 Using only public roads, we measured the emissions from nearly 200 well pads over 2 weeks in October 2013. The population of measured well pads is split into well pads with detectable emissions (N = 115) and those with emissions below the detection limit of the MFP instrument (N = 67). For those well pads with nonzero emissions, the distribution was highly skewed, with a geometric mean of 0.63 kg/h, a geometric standard deviation of 4.2, and an arithmetic mean of 1.72 kg/h. Including the population of nonemitting well pads, we find that the arithmetic mean of the well pads sampled in this study is 1.1 kg/h. This distribution implies that 50% of the emissions is due to the 6.6% highest emitting well pads, and 80% of the emissions is from the 22% highest emitting well pads.

Quantifying methane emission from fugitive sources by combining tracer release and downwind measurements - a sensitivity analysis based on multiple field surveys.

Using a dual species methane/acetylene instrument based on cavity ring down spectroscopy (CRDS), the dynamic plume tracer dispersion method for quantifying the emission rate of methane was successfully tested in four measurement campaigns: (1) controlled methane and trace gas release with different trace gas configurations, (2) landfill with unknown emission source locations, (3) landfill with closely located emission sources, and (4) comparing with an Fourier transform infrared spectroscopy (FTIR) instrument using multiple trace gasses for source separation. The new real-time, high precision instrument can measure methane plumes more than 1.2 km away from small sources (about 5 kg h(-1)) in urban areas with a measurement frequency allowing plume crossing at normal driving speed. The method can be used for quantification of total methane emissions from diffuse area sources down to 1 kg per hour and can be used to quantify individual sources with the right choice of wind direction and road distance. The placement of the trace gas is important for obtaining correct quantification and uncertainty of up to 36% can be incurred when the trace gas is not co-located with the methane source. Measurements made at greater distances are less sensitive to errors in trace gas placement and model calculations showed an uncertainty of less than 5% in both urban and open-country for placing the trace gas 100 m from the source, when measurements were done more than 3 km away. Using the ratio of the integrated plume concentrations of tracer gas and methane gives the most reliable results for measurements at various distances to the source, compared to the ratio of the highest concentration in the plume, the direct concentration ratio and using a Gaussian plume model. Under suitable weather and road conditions, the CRDS system can quantify the emission from different sources located close to each other using only one kind of trace gas due to the high time resolution, while the FTIR system can measure multiple trace gasses but with a lower time resolution.

Applications of cavity ring-down spectroscopy to high precision isotope ratio measurement of 13C/12C in carbon dioxide.

Recent measurements of carbon isotopes in carbon dioxide using near-infrared, diode-laser-based cavity ring-down spectroscopy (CRDS) are presented. The CRDS system achieved good precision, often better than 0.2 per thousand, for 4% CO2 concentrations, and also achieved 0.15-0.25 per thousand precision in a 78 min measurement time with cryotrap-based pre-concentration of ambient CO2 concentrations (360 ppmv). These results were obtained with a CRDS system possessing a data rate of 40 ring-downs per second and a loss measurement of 4.0 x 10(-11) cm(-1) Hz(-1/2). Subsequently, the measurement time has been reduced to under 10 min. This standard of performance would enable a variety of high concentration (3-10%) isotopic measurements, such as medical human breath analysis or animal breath experiments. The extension of this ring-down to the 2 microm region would enable isotopic analysis at ambient concentrations, which, combined with the small size, robust design, and potential for frequent measurements at a remote site, make CRDS technology attractive for remote atmospheric measurement applications.