Measurements of hydrogen sulfide (H 2S) using PTR-MS: calibration, humidity dependence, inter-comparison and results from field studies in an oil and gas production region
Natural gas production is associated with emissions of several trace gases, some of them classified as air toxics. While volatile organic compounds (VOCs) have received much attention, hydrogen sulfide (H 2S) can also be of concern due to the known health impacts of exposure to this hazardous air pollutant. Here, we present quantitative, fast time-response measurements of H 2S using proton-transfer-reaction mass-spectrometry (PTR-MS) instruments. An ultra-light-weight PTR-MS (ULW-PTR-MS) in a mobile laboratory was operated for measurements of VOCs and H 2S in a gas and oil field during the Uintah Basin Winter Ozone Study (UBWOS) 2012 campaign. Measurements of VOCs and H 2S by a PTR-MS were also made at the Horse Pool ground site in the Uintah Basin during UBWOS 2013. The H 2S measurement by PTR-MS is strongly humidity dependent because the proton affinity of H 2S is only slightly higher than that of water. The H 2S sensitivity of PTR-MS ranged between 0.6–1.4 ncps ppbv −1 during UBWOS 2013. We compare the humidity dependence determined in the laboratory with in-field calibrations and determine the H 2S mixing ratios for the mobile and ground measurements. The PTR-MS measurements at Horse Pool are evaluated by comparison with simultaneous H 2S measurements using a PTR time-of-flight MS (PTR-ToF-MS) and a Picarro cavity ring down spectroscopy (CRDS) instrument for H 2S / CH 4. On average 0.6 ± 0.3 ppbv H 2S was present at Horse Pool during UBWOS 2013. The correlation between H 2S and methane enhancements suggests that the source of H 2S is associated with oil and gas extraction in the basin. Significant H 2S mixing ratios of up to 9 ppmv downwind of storage tanks were observed during the mobile measurements. This study suggests that H 2S emissions associated with oil and gas production can lead to short-term high levels close to point sources, and elevated background levels away from those sources. In addition, our work has demonstrated that PTR-MS can make reliable measurements of H 2S at levels below 1 ppbv.