Searching for Hydroxyl Radicals at the Air-Ice Interface
The photolysis of compounds such as NO2-, NO3-, and H2O2 in ice samples is thought to produce hydroxyl radicals at rates similar to those in liquid water. The subsequent reactions of OH are expected to have significant impacts on the fate of pollutants in ice, as well as on the composition of the boundary layer above polar snowpacks. We have developed a method of monitoring the formation of OH in situ at the air-ice interface using spectroscopic detection of a molecular OH-trap. We have validated this technique for use at the air-ice interface, and quantified OH production rates in aqueous solutions using this method, but we do not observe OH formation via photolysis of these precursor compounds on ice surfaces under any experimental conditions. We conclude that either hydroxyl radicals are less efficiently formed in this manner at the air-ice interface than in aqueous solution, or that hydroxyl radicals formed at the air-ice interface are not available for reaction with organic compounds present at the air-ice interface.
Advances in Laser-Induced Emission Based Instrumentation for Fast, Direct, High- Sensitivity In-Situ Measurement of Formaldehyde and Alpha-Dicarbonyls
We present advances in instrumentation for fast, direct, high-sensitivity in situ measurement of formaldehyde and alpha-dicarbonyls. Glyoxal, the smallest dicarbonyl, is a molecule of emerging importance. It is a tracer of the oxidative chemistry of volatile organic compounds (VOCs) and has been implicated in secondary organic aerosol (SOA) formation. We have developed a laser-induced phosphorescence (LIP) instrument for measurement of ambient glyoxal, taking advantage of the unusually high intersystem crossing yield of alpha-dicarbonyls. This is the first instrument for atmospheric field measurements based on LIP. Measurement of the slow (30 microsecond) phosphorescence offers the advantage of temporal separation from laser scatter and fluorescence, obviating the need for filters to eliminate particulate matter. The instrument has been characterized in two field campaigns and the precision during the PROPHET 2008 field campaign was 2 pptv/min. We present an extension of instrumental capabilities to measuring glyoxal fluxes via eddy-correlation, and of the LIP method to measuring methylglyoxal. The Madison LIP instrument can thus provide alpha-dicarbonyl datasets essential to insight into the oxidative chemistry of VOCs and SOA formation. We have also developed a laser-induced fluorescence (LIF) based instrument for fast, high-sensitivity measurements of formaldehyde with a precision as high as 20 pptv/min. We present characterization of the instrument during the PROPHET 2008 field campaign. Since then, we have replaced the Ti:Sapphire laser with a novel pulsed fiber laser, which has considerable advantages in many important aspects, such as size and power requirements, and it offers an opportunity to significantly advance laser based instrumentation in the UV- Vis range. Fiber-coupling also eliminates problems with optics contamination. Finally, the laser can be tuned rapidly, as required for formaldehyde flux measurements via eddy correlation. The entire instrument (excluding the pump) requires roughly 3'x3'x2' and is ideally suited for ground-based and aircraft measurements.
Online formaldehyde measurement using proton transfer reaction mass spectrometry. Results of intercomparison with the Hantzsch monitor and correction for humidity effects
Formaldehyde is an important atmospheric species that is both emitted directly or produced in-situ via oxidation of hydrocarbons. HCHO measurements can provide useful information about photochemical activity in ambient air, given that it is formed via numerous oxidation processes. Proton transfer reaction mass spectrometry (PTR-MS) is an online technique that allows measurement of VOC at the sub-ppbv level with good time resolution. PTR-MS detection of formaldehyde is hampered by the humidity dependence of the instrument sensitivity, with higher humidity leading to loss of PTR-MS signal. In this study we develop an analytical, first principles approach to correct the PTR-MS signal according to the concentration of water vapor in sampled air. The results of the correction are validated by inter-comparison of the PTR-MS results with those from a Hantzsch fluorescence monitor which does not have the same humidity dependence. In particular, results are presented for an intercomparison made during two field campaigns, one in rural Ontario at Environment Canada's Centre for Atmospheric Research Experiments and one in a forested environment at Whistler, BC.
Chemical Composition of Secondary Organic Aerosols Generated from the Dark Ozonolysis of Isoprene - A High Resolution Mass Spectrometric Analysis
Isoprene (C5H8) is the single largest contributor to secondary organic aerosol (SOA) formation in the atmosphere due to oxidation by the hydroxyl radical, ozone, and to a lesser extent, the nitrate radical. Obtaining detailed information about the constituents in the aerosol phase is the first step in constraining the chemical pathways of condensed and heterogeneous-phase chemistry in aerosols, a still largely unexplored frontier. The polydispersed and heterogeneous nature of organic aerosols necessitate exact mass resolution and parts-per-billion sensitivity. In this presentation, we re-examine the SOA generated from the ozonolysis of isoprene by ultrahigh resolution mass spectrometry. An Orbitrap instrument (105 m/Δm) coupled to an electrospray ionization (ESI) source was used to probe the richness in the composition of the solvent- extractable aerosol mixture. The isolated ozonolysis pathway produced several hundred assignable peaks, the majority of which are oligomeric compounds containing several isoprene units. High-resolution mass analysis techniques like van Krevelen diagrams, Kendrick mass plots, double bond equivalency ratio and aromaticity index, when applied to the analysis of SOA, offer a novel and useful approach to identifying many monomeric compounds in the aerosol phase. A high degree of oxidation in the SOA components and significant contribution from particular subsets of organic compounds are also discussed.
Investigating the role of VOCs in secondary organic aerosol production during the PROPHET 2008 field intensive by proton transfer reaction linear ion trap (PTR-LIT) mass spectrometry
A major aim of the PROPHET 2008 field intensive conducted at the University of Michigan Biological Station was to more completely understand the local formation of secondary organic aerosol from oxidation of biogenic volatile organic compounds (BVOCs). This oxidation was monitored at every step including gas phase reactant VOCs, oxidants, reaction products, and finally aerosol number and size distribution. Several low molecular weight VOCs occur in the atmosphere in the low ppb to ppt range, including methyl vinyl ketone (MVK) and (MACR) from isoprene oxidation, and aerosol precursor monoterpenes and sesquiterpenes. A proton transfer reaction - linear ion trap (PTR-LIT) mass spectrometer was developed and utilized to quantify and distinguish isomeric VOCs as well as test for interferents by allowing for MSn experiments while retaining a LOD in the 100 ppt range for most compounds. The PTR-LIT was deployed from July to mid-August 2008 to sample above canopy air from a mixed deciduous forest. The extended capabilities of the PTR-LIT were used to address local ozone formation from the oxidation of isoprene by monitoring speciated MVK and MACR. Possible interferences at common masses were directly addressed by comparing the MS2 spectra of atmospheric masses to those of standards. Aerosol formation and size distribution data were compared to VOC oxidation rates to determine the correlation between VOC oxidation and SOA nucleation events. In particular isoprene oxidation was monitored using MVK, MACR, and glyoxal, a fifth generation oxidation product, and SOA precursor. Furthermore, total monoterpene concentration was monitored and oxidation rates were calculated. Finally, total sesquiterpenes were not observed above the limit of detection making them an unlikely source either aerosol growth or the missing OH reactivity at UMBS.
Analysis and Interpretation of Single Particle Mass Spectra by the Aerosol Time-of-Flight Mass Spectrometer
In recent years there have been many advances in instrumentation technologies for the analysis of atmospheric aerosols. The Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) offers several advantages over many of the other aerosol analysis instruments. It has the capabilities to qualitatively asses the composition of individual atmospheric particles at high time resolution which allows for increased sensitivity for source apportionment analysis and is essential for understanding the mixing state of ambient aerosol. The ATOFMS has been monitoring ambient aerosol particles in downtown Toronto since 2006. In this study, the Adaptive Resonance Theory (ART-2a) clustering algorithm was used to classify 700,000 particles measured from February 2007 until February 2008. Results from this analysis demonstrate the presence of a variety of particle types and how each type varies in number throughout the year. The ATOFMS does not, however, come without its limitations. Ambient aerosol data obtained from the ATOFMS can be quite complex and difficult to interpret. There is also significant variability in the mass spectra obtained from identical particles due to variations in laser intensity. Ion signal intensities may also vary depending on what species are present in the particle. Laboratory experiments were conducted to elucidate these limitations, and to better interpret the ambient aerosol particle data. A number of known particle types of various sizes and compositions were analyzed by the ATOFMS. Spectral variation obtained for specific known particle types, due to fluctuations in laser intensity, was studied. Also investigated was the variation in spectra due to the presence of other species within the particle. Lastly, the sensitivity of the ATOFMS to detect certain particles depending on their composition was investigated.