Overview of surface measurements and spatial characterization of submicrometer particulate matter during the DISCOVER-AQ 2013 campaign in Houston, TX.

The sources of submicrometer particulate matter (PM1) remain poorly characterized in the industrialized city of Houston, TX. A mobile sampling approach was used to characterize PM1 composition and concentration across Houston based on high-time-resolution measurements of nonrefractory PM1 and trace gases during the DISCOVER-AQ Texas 2013 campaign. Two pollution zones with marked differences in PM1 levels, character, and dynamics were established based on cluster analysis of organic aerosol mass loadings sampled at 16 sites. The highest PM1 mass concentrations (average 11.6 ± 5.7 µg/m3) were observed to the northwest of Houston (zone 1), dominated by secondary organic aerosol (SOA) mass likely driven by nighttime biogenic organonitrate formation. Zone 2, an industrial/urban area south/east of Houston, exhibited lower concentrations of PM1 (average 4.4 ± 3.3 µg/m3), significant organic aerosol (OA) aging, and evidence of primary sulfate emissions. Diurnal patterns and backward-trajectory analyses enable the classification of airmass clusters characterized by distinct PM sources: biogenic SOA, photochemical aged SOA, and primary sulfate emissions from the Houston Ship Channel. Principal component analysis (PCA) indicates that secondary biogenic organonitrates primarily related with monoterpenes are predominant in zone 1 (accounting for 34% of the variability in the data set). The relevance of photochemical processes and industrial and traffic emission sources in zone 2 also is highlighted by PCA, which identifies three factors related with these processes/sources (~50% of the aerosol/trace gas concentration variability). PCA reveals a relatively minor contribution of isoprene to SOA formation in zone 1 and the absence of isoprene-derived aerosol in zone 2. The relevance of industrial amine emissions and the likely contribution of chloride-displaced sea salt aerosol to the observed variability in pollution levels in zone 2 also are captured by PCA. IMPLICATIONS: This article describes an urban-scale mobile study to characterize spatial variations in submicrometer particulate matter (PM1) in greater Houston. The data set indicates substantial spatial variations in PM1 sources/chemistry and elucidates the importance of photochemistry and nighttime oxidant chemistry in producing secondary PM1. These results emphasize the potential benefits of effective control strategies throughout the region, not only to reduce primary emissions of PM1 from automobiles and industry but also to reduce the emissions of important secondary PM1 precursors, including sulfur oxides, nitrogen oxides, ammonia, and volatile organic compounds. Such efforts also could aid in efforts to reduce mixing ratios of ozone.

Adaptation of a proton transfer reaction mass spectrometer instrument to employ NO+ as reagent ion for the detection of 1,3-butadiene in the ambient atmosphere.

A proton transfer reaction mass spectrometer (PTR-MS) instrument was adapted to employ NO+ as a chemical reagent ion without any hardware changes by switching the reagent ion source gas from water vapor to dry air. Ionization of dry air within the hollow cathode ion source generates a very intense source of NO+ with only a minor impurity of NO2+. The intensities of the primary NO+ reagent ion and the unwanted impurity NO2+ are controllable and dependent on the operational conditions of the hollow cathode ion source. Ion source tuning parameters are described, which maintain an intense source of NO+ while keeping the impurity NO2+ signal to less than 2% of the total reagent ion intensity. This method is applied to the detection of 1,3-butadiene. NO+ reacts efficiently with 1,3-butadiene via a charge exchange reaction to produce only the molecular ion, which is detected at m/z 54. Detection sensitivities of the order of 45 pptv for a 1-s measurement of 1,3-butadiene are demonstrated. We present the first real-time on-line sub parts per billion measurement of 1,3-butadiene in the ambient atmosphere. The only likely interference is from 1,2-butadiene. Concurrent measurements of benzene are provided and suggest that the vehicular emissions are the predominant source of 1,3-butadiene in a suburban Boston area monitoring location.

Quantitatively resolving mixtures of isobaric compounds using chemical ionization mass spectrometry by modulating the reactant ion composition.

Acrolein (C(3)H(4)O) and 1-butene (C(4)H(8)) can both be individually detected by proton transfer chemical ionization mass spectrometry (CI-MS). However, because these compounds are isobaric, mixtures of these two compounds cannot be resolved since both compounds react with H(3)O(+) via a proton-transfer reaction to form a protonated molecule that is detected at a nominal mass-to-charge ratio of 57 (m/z 57). While both compounds react with H(3)O(+) only acrolein reacts to any significant extent with H(3)O(+)(H(2)O). Recognizing that the electrical potential applied to a drift tube reaction mass spectrometer provides a simple and effective means for varying the relative intensity of the H(3)O(+) and H(3)O(+)(H(2)O) reactant ions we have developed a method whereby we make use of this reactivity difference to resolve mixtures of these two compounds. We demonstrate a technique where the individual contributions of acrolein and 1-butene within a mixture can be quantitatively resolved by systematically changing the reagent ion from H(3)O(+) to H(3)O(+)(H(2)O) through control of the electric potential applied to the drift tube reaction region of a proton transfer reaction mass spectrometer.