[research interests]



Atmospheric chemistry plays a critical role in governing both the abundance and distribution of greenhouse gases, air pollutants and aerosol particles. The skill with which climate and chemistry models represent the current state of the atmosphere and respond accurately to chemical and physical perturbation is limited by our understanding of the underlying chemical processes. Direct ambient observations, supported by detailed laboratory investigations, provide unparalleled constraints for, and confidence in climate and chemistry simulations and ultimately the policy recommendations derived from them.

Research in the Bertram group is designed to provide direct experimental and observation based constraints for a host of chemical processes. Projects include:

I. In situ measurements of heterogeneous reactions on ambient aerosol particles

Project Summary: The objective of this project is to identify the mechanistic drivers that control the variability in heterogeneous aerosol processes through direct in situ measurement of reaction kinetics on ambient aerosol particles. Heterogeneous reactions, occurring between gases and aerosol particles, alter the climate-relevant properties of aerosols and catalyze reaction processes that are energetically unfavorable in the gas phase. In this project, the heterogeneous reactivity of complex, ambient aerosol particles will be investigated to determine: 1) how representative laboratory investigations of heterogeneous processes conducted on model, simple systems are of the real atmosphere, 2) the impact of heterogeneous processes on ambient particle hygroscopicity and optical properties, and 3) the uptake kinetics for a host of atmospheric trace gases as a function of particle composition and phase. The results of these investigations will be used to directly improve the representation of heterogeneous processes in global climate models.

Funding: DOE Early Career Award


Figure: Observations of the N2O5 reactive uptake coefficient (red) as measured on ambient particles from the SIO pier. Simultaneous measurements of aerosol surface area are shown in black. Changes in the g(N2O5) strongly correlate with air mass type, where the uptake coefficient is elevated in clean marine air chracterized by fresh sea-spray.

II. Air-sea exchange of volatile organic compounds: Impacts on climate and atmospheric chemistry

Project Summary: Biogenic volatile organic compounds (BVOC), emitted from photosynthetic organisms, 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. It has been suggested that marine BVOC 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. This project represents a multi-scale investigation designed to link detailed laboratory investigations of isoprene and total monoterpene production from isolated monocultures of an array of relevant marine organisms with direct measurements of isoprene and total monoterpene emissions made via eddy correlation during coastal algae blooms from the SIO pier and from the NSF sponsored research cruise (HiWinGS) in regions of high biological activity and high wind speeds.

Funding: NSF CAREER Award


Figure: Phaeodactylum tricornutum (diatom) growing happily in the laboratory.

III. Chemical processing of sea-spray aerosol under controlled laboratory conditions

Project Summary: As part of the Phase I CCI program CAICE, we are investigating the production and chemical evolution of nascent sea-spray aerosols in a controlled laboratory setting. In collaboration with physical and biological oceanographers at SIO, we have created a coupled ocean-atmosphere system in which aerosols ejected from breaking waves can be studied in situ in a clean environment. In this study, we are investigating the following research hypotheses: i) Organic species produced by marine microorganisms in the ocean impact the formation pathways for sea spray aerosols, ultimately dictating the partitioning of inorganic and organic species between individual particles, resulting in particle chemistries that are unique from one another as well as from bulk seawater, ii) The surface tension, solubility, and heterogeneous reactivity of the primary sea spray particle types are different from each other and from that of bulk sea water, and iii) The oxidation of ocean-derived organics within the ejected submicron sea spray particles, and surfactant coatings at the surface of the particles, will lead to an evolving distribution of particles that have different surface tension, solubility, and reactivity than particles emitted directly by bubble bursting.

More Info: CAICE Blog

Funding: NSF CCI Program (PI: Prather)


Figure: Professor Mario J. Molina and Assistant Prodessor Timothy H. Bertram watching an individual wave break in ocean-atmosphere wave flume (SIO Hydraulics Lab). As part of the NSF CCI center we are measuring gas-aerosol interaction on nascent sea-spray particles generated in the wave flume.

IV. Development of field-deployable chemical ionization mass spectrometric techniques

Project Summary: Most recently, in collaboration with Tofwerk AG, Aerodyne, and UW, we have constructed a new chemical ionization time-of-flight mass spectrometer (CI-TOFMS) that measures atmospheric trace gases in real time with high sensitivity. In our initial demonstration, we applied the technique to the measurement of formic acid via negative-ion proton transfer, using acetate as the reagent ion. A novel high pressure interface, incorporating two RF-only quadrupoles is used to efficiently focus ions through four stages of differential pumping before analysis with a compact TOFMS. The high ion-duty cycle (>20%) of the TOFMS combined with the efficient production and transmission of ions in the high pressure interface results in a highly sensitive (>300 ions s-1 pptv-1 formic acid) instrument capable of measuring and saving complete mass spectra at rates faster than 10 Hz. We demonstrate the efficient transfer and detection of both bare ions and ion-molecule clusters, and characterize the instrument during field measurements aboard the R/V Atlantis as part of the CalNex campaign during the spring of 2010. The in-field short-term precision is better than 5% at 1 pptv (pL/L), for 1-second averages. The detection limit (3 , 1-second averages) of the current version of the CI-TOFMS, as applied to the in situ detection of formic acid, is limited by the magnitude and variability in the background determination and was determined to be 4 pptv. Ongoing work includes the application of new ion-molecule reactions using CI-TOFMS to the detection of other inorganic and organic acids in addition to a suite of VOC and atmospheric radicals.

Funding: DOE SBIR (Joint with Aerodyne, UW, and CU)


Figure: Front end of the field deployable chemical ionization time-of-flight mass spectrometer.

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

Project Summary: 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 m, 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 proposal 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.

Funding: EPA

VI. Understanding Primary Organic Aerosol Volatility at Atmospherically Realistic Concentrations

Project Summary: The objective of this research is to identify the dominant partitioning mechanism for primary organic aerosol emitted from a diesel-powered and from gasoline-powered vehicles at atmospherically realistic concentrations in the range from < 5 - 30 ug m-3. Recent emissions tests have determined that primary organic aerosol (POA) generated from combustion sources behaves like a series of semi-volatile compounds when the particulate phase concentrations range between 100 - 10,000 ug m-3. The data available for atmospherically relevant concentrations below 30 ug m-3 are sparse. The simple absorption theory that appears to explain the behavior of gas-particle distribution of condensable organics at high concentrations may not be accurate at atmospherically relevant concentrations. It is likely that other processes such as chemical and physical adsorption onto elemental carbon, partitioning into the aqueous phase, and /or the formation of chemical bonds with POA functional groups play significant roles at lower concentrations. The results of experiments conducted at atmospherically relevant concentrations will determine if the simple absorption theory can be extrapolated to the real atmosphere. These findings will have broad application within regional air quality models and global climate models used to predict the efficiency of emissions control programs on ambient organic aerosol concentrations and/or to predict the response of organic aerosol concentrations to climate change.

Funding: California Air Resources Board (CARB, PI Kleeman, UC Davis)

home | [research]| group members | news and links | publications | contact