In situ measurements of 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.  This project is a comprehensive investigation into the reactivity of complex, ambient and chamber generated aerosol particles with the intent of determining: 1) how representative laboratory investigations of heterogeneous and multi-phase processes conducted on model, simple systems are of the real atmosphere, 2) the impact of heterogeneous and multi-phase processes on ambient particle optical properties and their ability to nucleate clouds, and 3) the impact of multi-phase reactions on the oxidative capacity of the atmosphere. This work focuses on the uptake kinetics and product yields for the reactions of reduced nitrogen compounds (amines and ammonia), dinitrogen pentoxide (N2O5), and hypochlorous acid (HOCl).  These investigations focus on the controlling role of particle chemical composition, particle mixing state, morphology, and physical phase state.  

Representative Publication:

Ryder et al. On the role of particle inorganic mixing state in the reactive uptake of N2O5 to ambient aerosol particles Environ. Sci. & Tech. 2014 Link to the article.

Group Members: Olivia Ryder and Nicole Campbell

Funding Sources: NSF CCI CAICE (laboratory studies) DOE Early Career Award (ambient measurements)

Air-sea exchange of trace gases and reactions at the air-sea interface

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 (NOx ≡ 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 sensitivity of O3 to changes in NOx and VOC emissions rely heavily on a complete description of the factors that control NOx 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 NOx oxidation, can account for as much as 50% of the NOx 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 NOx 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 NOx 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 Publication:

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. Link to the article.

Group Members: Michelle Kim and Nicole Campbell

Funding Sources: NSF CAREER Award, Dreyfus Postdoctoral Fellowship

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 Publication:

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

Group Members: Matt Zoerb

Funding Sources: DOE SBIR (Aerodyne) and NASA NIP

A compact, low-cost, network accessible, optical particle counter for the real time measurement of submicron aerosol particle size distributions

Atmospheric aerosol particles play a critical role in Earth’s radiation budget, act to limit visibility through the scattering and absorption of radiation, and represent a significant respiratory health hazard in urban environments.  However, the existing network of aerosol particle measurements is significantly sparse, and unable to capture the strong heterogeneity in particles that exists in urban locations.  In addition, current 24-hour air quality standards of particulate matter are based solely on the total mass of particles with diameters less than 2.5 mm, and do not account for variations in particle size or total number.  As a result, air quality assessments and local and regional modeling efforts are: 1) limited by a paucity of data, and 2) unconstrained by routine observations of particle number and size, which are both critical metrics for assessing the impact of aerosol particles on visibility and human health.  The objective of this project is the development of a miniature, wireless optical particle counter (OPC) capable of measuring and transmitting submicron aerosol particle number and size distributions to a remote server in real-time.  The proposal aims to provide the framework for significant improvements in the spatial and temporal resolution of continuous aerosol particle measurements on the city scale, while dramatically improving the availability of these data in real time.

Group Members: James Brady

Funding Sources: EPA STAR