Heterogeneous and multiphase reactions on aerosol particles

Project Overview: Aerosol particles play a critical role in Earth’s energy budget through the absorption and scattering of radiation, and/or through their ability to form clouds and alter cloud lifetime.  Heterogeneous and multi-phase reactions alter the climate-relevant properties of aerosol particles and catalyze reaction pathways that are energetically unfavorable in the gas phase (Ravishankara, 2005).  The chemical composition of aerosol particles dictates the kinetics of heterogeneous and multi-phase reactions.  At present, the vast majority of the molecular level information on these processes has been determined in laboratory investigations on model aerosol systems.  At present, we are involved in three projects on heterogenenous and multiphase chemistry that span from laboratory measurements, field investigations, and model parameterizations. This work focuses on the uptake kinetics and product yields for the reactions of reduced and oxidized nitrogen compounds (amines and ammonia) and various sulfur containing molecules found in the oxidation of dimethyl sulfide.

Group Members: Rachel Bergin, Chris Jernigan, Tom Derrah, and Steve Kregel

Representative Publications:

Ryder et al. Role of Organics in Regulating ClNO₂ Production at the Air-Sea Interface. JPC-A 2015 Link to the article.

Ryder et al. Role of Organic Coatings in Regulating N2O5 Reactive Uptake to Sea Spray Aerosol​. JPC-A 2015 Link to the article.

Staudt et al. Sulfate and Carboxylate Suppress the Formation of ClNO₂ at Atmospheric Interfaces. ACS Earth and Space Chemistry 2019 Link to the article.

Collaborators: Gil Nathanson (UW Madison), Tracey Holloway (UW Madison)

Funding Sources: NSF CCI CAICE (laboratory studies), NOAA (field studies), and EPA (model development)

Production, Emission, and Oxidation of Marine Trace Gases 

Our research interests in this area are threefold.  The first project focuses on the production of volatile organic compounds in the surface ocean, the second project focuses on chemical reactions occurring at the air-sea interface, and the third focuses on bacteria mediated production pathways for small molecules in the surface ocean. 

Project A Overview: Volatile organic compounds (VOC) play a controlling role in both regulating oxidant loadings and setting the production rate of secondary organic aerosol (SOA) in both terrestrial and marine environments.  To date, the vast majority of research has focused on terrestrial sources of VOC, with specific attention to the factors that control the emission rates of isoprene (C₅H₈), monoterpenes (C₁₀H₁₆), and sesquiterpenes (C₁₅H₂₄).  In comparison, considerably less is known about marine VOC emissions (Shaw et al., 2010).  Similar to terrestrial processes, the spatio-temporal distribution of marine VOC emissions is thought to be highly variable, depending on the number concentration and species of phytoplankton and/or bacteria.  It has been suggested that marine VOC emissions in highly productive regions of the oceans: 1) impact oxidant loadings in the marine boundary layer (MBL), 2) contribute to SOA production, and 3) alter particle size and microphysical properties, thus impacting cloud formation and persistence in the MBL.  At present, there exists an extreme paucity of experimental data of BVOC fluxes to constrain global models of ocean BVOC emissions and their subsequent impact on climate and atmospheric chemistry.  

Project B Overview: The production rate of tropospheric ozone, depends critically on the concentrations of nitrogen oxides (NO_x ≡ NO + NO₂), volatile organic compounds (VOCs), trace oxidants (e.g., OH, NO₃, and Cl) and the wavelength dependent actinic flux.  Accurate model representation of O₃ mixing ratios and the sensitivityof O₃ to changes in NO_x and VOC emissions rely heavily on a complete description of the factors that control NO_x lifetimes and in turn the concentrations of atmospheric oxidants.  Modeling studies, constrained by laboratory and field observations, suggest that nocturnal processes involving the nitrate radical (NO₃) and N₂O₅, both products of NO_x oxidation, can account for as much as 50% of the NO_x removal rate (Alexander et al., 2009).  Incorporation of the heterogeneous reaction of N₂O₅ on chloride containing aerosol particles (Finlayson-Pitts et al., 1989; Behnke et al., 1997) serves as both an efficient NO_x recycling and halogen activation mechanism via the production of photo-labile nitryl chloride (ClNO₂) in both coastal (Osthoff et al., 2008) and continental airmasses (Thornton et al., 2010).  To date, study of the impact of nocturnal processes on the lifetime of NO_x and the production of reactive halogen species in the marine boundary layer has concentrated on gas-phase reactions and heterogeneous and multiphase processes occurring on/within aerosol particles, with little attention paid to reactions occurring at the air-sea interface.  This project focuses on direct measurements of the vertical flux of N₂O₅ and ClNO₂ obtained via eddy covariance at a polluted coastal site to provide observation-based constraints on the role of the air-sea interface in setting the lifetime of reactive nitrogen and the production rate of reactive halogens in the marine boundary layer.

Project C Overview:  Volatile organic compounds (VOC) play a controlling role in both regulating oxidant loadings and setting the production rate of secondary organic aerosol (SOA).  To date, the vast majority of research has focused on terrestrial sources of VOC, while studies of marine VOC emissions have concentrated on a select few molecules (e.g., DMS, organic halides), and nearly exclusively on the role of phytoplankton in regulating their production rates.  Here, we focus on the production of VOC and reduced nitrogen compounds (e.g., NH₃) that are mediated by bacteria and or oxidation/photochemistry of DOM in the SML with the objective of determining production rates related to bacteria cell count and product yields for small organic molecules (e.g., aldehydes and ketones) formed following deposition of O₃ to the SML.     

Representative Publications:

Kim et al. A controlling role for the air-sea interface in the chemical processing of reactive nitrogen in the coastal marine boundary layer in PNAS 2014. Link to the article.

Kim et al. Air-Sea exchange of biogenic volatile organic compounds and the impact on aerosol particle size distributions in GRL 2017. Link to the article.

Novak and Bertram, Reactive VOC Production from Photochemical and Heterogeneous Reactions Occuring at the Air-Ocean Interface Accounts of Chemical Research 2020. Link to the article.

Group Members: Delaney Kilgour and Chris Jernigan

Funding Sources: NSF

Instrument development for the next generation of atmospheric chemistry measurements

Development and characterization of chemical ionization – time-of-flight mass spectrometry for atmospheric measurements

Direct, simultaneous measurements of a wide array of trace gases has been made possible by the recent development of a new chemical ionization time-of-flight mass spectrometer (T.H. Bertram et al., 2011).  Over the past few years, in a collaborative project with the University of Washington (Thornton), University of Colorado (Jimenez) Aerodyne Inc. (Billerica, MA), and Tofwerk (Thun, Switzerland), we have developed and characterized a compact, field-deployable CI-TOFMS.  As applied to trace acids, the sensitivity (> 300 counts pptv^-1) is an order of magnitude better than that reported for similar quadrupole based instruments (Veres et al., 2008), which we attribute to large, novel advances in the high pressure interface described below.  In the laboratory we have used a host of reagent ions (e.g., C₆H₆⁺, NO⁺, H₃O⁺, I^-, and CH₃C(O)O^-) to demonstrate the versatility of the system toward detection of a wide array of target molecules.  Details of the instrument can be found in Bertram et al., 2011.  This instrument provides the basis for most of our measurements (e.g., research foci 1 and 2).  We have recently received funding from NASA to fly the instrument as part of a series of summer flights over the next three years.

Representative Publications:

Bertram et al. A field-deployable, chemical ionization time-of-flight mass spectrometer in Atmos. Meas. Tech. Link to the article.

Kim et al., Revisiting benzene cluster cations for the chemical ionization of dimethyl sulfide and select volatile organic compounds in Atmos. Meas. Tech. Link to the article.