Flight Deployment of a High‐Resolution Time‐of‐Flight Chemical Ionization Mass Spectrometer: Observations of Reactive Halogen and Nitrogen Oxide Species

Abstract

We describe the University of Washington airborne high‐resolution time‐of‐flight chemical ionization mass spectrometer (HRToF‐CIMS) and evaluate its performance aboard the NCAR‐NSF C‐130 aircraft during the recent Wintertime INvestigation of Transport, Emissions and Reactivity (WINTER) experiment in February–March of 2015. New features include (i) a computer‐controlled dynamic pinhole that maintains constant mass flow‐rate into the instrument independent of altitude changes to minimize variations in instrument response times; (ii) continuous addition of low flow‐rate humidified ultrahigh purity nitrogen to minimize the difference in water vapor pressure, hence instrument sensitivity, between ambient and background determinations; (iii) deployment of a calibration source continuously generating isotopically labeled dinitrogen pentoxide (15N2O5) for in‐flight delivery; and (iv) frequent instrument background determinations to account for memory effects resulting from the interaction between sticky compounds and instrument surface following encounters with concentrated air parcels. The resulting improvements to precision and accuracy, along with the simultaneous acquisition of these species and the full set of their isotopologues, allow for more reliable identification, source attribution, and budget accounting, for example, by speciating the individual constituents of nocturnal reactive nitrogen oxides (NOz = ClNO2 + 2 × N2O5 + HNO3 + etc.). We report on an expanded set of species quantified using iodide‐adduct ionization such as sulfur dioxide (SO2), hydrogen chloride (HCl), and other inorganic reactive halogen species including hypochlorous acid, nitryl chloride, chlorine, nitryl bromide, bromine, and bromine chloride (HOCl, ClNO2, Cl2, BrNO2, Br2, and BrCl, respectively).

Widespread Pollution From Secondary Sources of Organic Aerosols During Winter in the Northeastern United States

“…Figure shows that these SOA formation rates are not limited to concentrated urban plumes but are prevalent over the entire region. WINTER observations of oxidized VOCs (oVOCs) with the HR‐ToF chemical ionization mass spectrometer (Lee et al, ; HRToF) show significant amounts of secondary, more oxidized, low volatility, anthropogenic oVOCs, which are consistent with the high concentration of pollution SOA (Figure S2).…”

Wintertime Gas‐Particle Partitioning and Speciation of Inorganic Chlorine in the Lower Troposphere Over the Northeast United States and Coastal Ocean

“…Sustained, elevated HCl concentrations were observed in the range of 1,800 pptv offshore on several flights in areas that are not likely impacted by anthropogenic emission of HCl but rather secondary production of HCl from equilibrium partitioning out of the particle. The highest concentrations of HCl (>5,000 pptv) were measured within freshly emitted coal‐fired power plant plumes (Lee et al, ).…”

“…The background noise for chlorine‐containing compounds are typically lower than those composed only of carbon, oxygen, and hydrogen atoms since chlorine‐containing compounds have large negative mass defects. Therefore, the DLs of chlorine‐containing compounds are mainly a function of their sensitivity to iodide‐adduct ionization and noise detector (Lee et al, ). The estimated uncertainty for the reported data is ±30% for HCl, ClNO 2 , Cl 2 , HOCl, N 2 O 5 , and HNO 3 , for which a direct calibration was performed, and the uncertainty represents a conservative estimate of the variability in measured sensitivity.…”

Comparison of Airborne Reactive Nitrogen Measurements During WINTER

“…A high‐resolution time‐of‐flight chemical ionization mass spectrometer with iodide‐adduct ionization (HRToF‐CIMS, referred to as I ‐ CIMS here) measured N 2 O 5 , ClNO 2 , and HNO 3 (Lee et al, ; Lee, Lopez‐Hilfiker, et al, ). Intake is at 22 slpm through a 1.6 cm i.d., 40‐cm‐long polytetrafluoroethylene Teflon inlet after which compounds are ionized with iodide and then detected according to their mass to charge ratios.…”

“…The output was sampled off‐line by the independently calibrated CRDS and TD‐LIF instruments and found to agree to within 1.3%. For further details see Lee et al (). During WINTER, the 1‐s 1‐σ limits of detection for N 2 O 5 , ClNO 2 , and HNO 3 were 0.4, 0.6, and 7.0 pptv, respectively, with a calibration uncertainty of 30% for each (Lee et al, ).…”