Vertically resolved measurements of nighttime radical reservoirs in Los Angeles and their contribution to the urban radical budget.

Photolabile nighttime radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO(2)), contribute to the oxidizing potential of the atmosphere, particularly in early morning. We present the first vertically resolved measurements of ClNO(2), together with vertically resolved measurements of HONO. These measurements were acquired during the California Nexus (CalNex) campaign in the Los Angeles basin in spring 2010. Average profiles of ClNO(2) exhibited no significant dependence on height within the boundary layer and residual layer, although individual vertical profiles did show variability. By contrast, nitrous acid was strongly enhanced near the ground surface with much smaller concentrations aloft. These observations are consistent with a ClNO(2) source from aerosol uptake of N(2)O(5) throughout the boundary layer and a HONO source from dry deposition of NO(2) to the ground surface and subsequent chemical conversion. At ground level, daytime radical formation calculated from nighttime-accumulated HONO and ClNO(2) was approximately equal. Incorporating the different vertical distributions by integrating through the boundary and residual layers demonstrated that nighttime-accumulated ClNO(2) produced nine times as many radicals as nighttime-accumulated HONO. A comprehensive radical budget at ground level demonstrated that nighttime radical reservoirs accounted for 8% of total radicals formed and that they were the dominant radical source between sunrise and 09:00 Pacific daylight time (PDT). These data show that vertical gradients of radical precursors should be taken into account in radical budgets, particularly with respect to HONO.

Observation of ClNO2 in a mid-continental urban environment.

In the troposphere, nitryl chloride (ClNO2), produced from uptake of dinitrogen pentoxide (N2O5) on chloride containing aerosol, can be an important nocturnal reservoir of NO(x) (= NO + NO2) and a source of atomic Cl, particularly in polluted coastal environments. Here, we present measurements of ClNO2 mixing ratios by chemical ionization mass spectrometry (CIMS) in Calgary, Alberta, Canada over a 3-day period. The observed ClNO2 mixing ratios exhibited a strong diurnal profile, with nocturnal maxima in the range of 80 to 250 parts-per-trillion by volume (pptv) and minima below the detection limit of 5 pptv in the early afternoon. At night, ClNO2 constituted up to 2% of odd nitrogen, or NO(y). The peak mixing ratios of ClNO2 observed were considerably below the modeled integrated heterogeneous losses of N2O5, indicating that ClNO2 production was a minor pathway compared to heterogeneous hydrolysis of N2O5. Back trajectory analysis using HYSPLIT showed that the study region was not influenced by fresh injections of marine aerosol, implying that aerosol chloride was derived from anthropogenic sources. Molecular chlorine (Cl2), derived from local anthropogenic sources, was observed at mixing ratios of up to 65 pptv and possibly contributed to formation of aerosol chloride and hence the formation of ClNO2.

Quantification of nitryl chloride at part per trillion mixing ratios by thermal dissociation cavity ring-down spectroscopy.

Nitryl chloride (ClNO(2)) is an important nocturnal nitrogen oxide reservoir species in the troposphere. Here, we report a novel method, thermal dissociation cavity ring-down spectroscopy (TD-CRDS), to quantify ClNO(2) mixing ratios with tens of parts-per-trillion by volume (pptv) sensitivity. The mixing ratios of ClNO(2) are determined by blue diode laser CRDS of NO(2), produced from quantitative thermal dissociation of ClNO(2) in an inlet heated to 450 °C, relative to NO(2) observed in an unheated reference channel. ClNO(2) was generated by passing Cl(2) gas over a slurry containing a 1:10 mixture of NaNO(2) and NaCl. The TD-CRDS response was evaluated using parallel measurements of ClNO(2) by chemical ionization mass spectrometry (CIMS) using I(-) as the reagent ion and NO(y) (= NO + NO(2) + HNO(3) + ΣRO(2)NO(2) + ΣRONO(2) + HONO + 2N(2)O(5) + ClNO(2) + ...) chemiluminescence (CL). The linear dynamic range extends from the detection limit of 20 pptv (1 σ, 1 min) to 30 parts-per-billion by volume (ppbv), the highest mixing ratio tested. The ClNO(2) TD profile overlaps with those of alkyl nitrates, which has implications for nocturnal measurements of total alkyl nitrate (ΣAN = ΣRONO(2)) abundances by thermal dissociation (with detection as NO(2)) in ambient air.

Quantitative determination of biogenic volatile organic compounds in the atmosphere using proton-transfer reaction linear ion trap mass spectrometry.

Although oxidation of biogenic volatile organic compounds (BVOCs) plays an important role in tropospheric ozone and secondary organic aerosol production, significant uncertainties remain in our understanding of the impacts of BVOCs on ozone, aerosols, and climate. To quantify BVOCs, the proton-transfer reaction linear ion trap (PTR-LIT) mass spectrometer was previously developed. The PTR-LIT represents an improvement over more traditional techniques (including the proton-transfer reaction mass spectrometer), providing the capability to directly quantify and differentiate isomeric compounds by MS/MS analysis, with better time resolution and minimal sample handling, compared to gas chromatography techniques. Herein, we present results from the first field deployment of the PTR-LIT. During the Program for Research on Oxidants: Photochemistry, Emissions and Transport (PROPHET) summer 2008 study in northern Michigan, the PTR-LIT successfully quantified isoprene, total monoterpenes, and isomeric isoprene oxidation products methyl vinyl ketone and methacrolein at sub-parts per billion (nmol/mol) levels in a complex forest atmosphere. The utility of the fast time response of the PTR-LIT was shown by the measurement of rapid changes in isoprene, methyl vinyl ketone, and methacrolein, concurrent with changing ozone mole fractions. Overall, the PTR-LIT was shown to be a viable field instrument with the necessary sensitivity, selectivity, and time response to provide detailed measurements of BVOC mole fractions in complex atmospheric samples, at trace levels.

Development of a proton-transfer reaction-linear ion trap mass spectrometer for quantitative determination of volatile organic compounds.

Currently, proton-transfer reaction mass spectrometry (PTR-MS) allows for quantitative determination of volatile organic compounds in real time at concentrations in the low ppt range, but cannot differentiate isomers or isobaric molecules, using the conventional quadrupole mass filter. Here we pursue the application of linear quadrupole ion trap (LIT) mass spectrometry in combination with proton-transfer reaction chemical ionization to provide the advantages of specificity from MS/MS. A commercial PTR-MS platform composed of a quadrupole mass filter with the addition of end cap electrodes enabled the mass filter to operate as a linear ion trap. The rf drive electronics were adapted to enable the application of dipolar excitation to opposing rods, for collision-induced dissociation (CID) of trapped ions. This adaptation enabled ion isolation, ion activation, and mass analysis. The utility of the PTR-LIT was demonstrated by distinguishing between the isomeric isoprene oxidation pair, methyl vinyl ketone (MVK) and methacrolein (MACR). The CID voltage was adjusted to maximize the m/ z 41 to 43 fragment ratio of MACR while still maintaining adequate sensitivity. Linear calibration curves for MVK and MACR fragments at m/ z 41 and 43 were obtained with limits of detection of approximately 100 ppt, which should enable ambient measurements. Finally, the PTR-LIT method was compared to an established GC/MS method by quantifying MVK and MACR production during a smog chamber isoprene-NO x irradiation experiment.