James Smith (Group Leader)
The UA group conducts measurements to understand how particles form and grow in the atmosphere to be important participants in atmospheric chemistry and climate. Ultrafine aerosols are particles with diameters smaller than 100 nm. They can be formed in the atmosphere over spatial scales of hundreds of kilometers by a process known as nucleation. Measurements by the UA group and its collaborators, performed in atmospheric environments as diverse as Mexico City and the boreal forests in central Finland, confirm that new particle formation from nucleation occurs frequently and often is the fundamental process that controls particle number. Understanding and modeling the nucleation process and the mechanisms by which newly formed particles grow to become cloud condensation nuclei (CCN) are essential for reducing uncertainties in the roles that aerosols play in climate.
The research activities of the UA group during the past year have addressed three fundamental questions:
- Can we develop new measurements for characterizing ultrafine aerosol physico-chemical properties and their impacts on climate?
- How do newly formed aerosols grow to become important participants in chemistry/climate?
- What are the impacts of biogenic emissions on secondary organic aerosol formation and growth?
As Table 1 shows, these activities address the ESSL strategic scientific priorities of investigating the interaction of the atmosphere with the broader Earth system and improving prediction of weather, climate, and other atmospheric phenomena.
Table 1. Summary of UA group research activities as they relate to ESSL 2008 strategic priorities.
|UA Group Activity||Interaction of atmosphere with broader Earth system||Improving prediction of weather, climate, and other phenomena|
|Develop new measurements for characterizing ultrafine aerosol physico-chemical properties and their impacts on climate.||Better measurements of the physico-chemical properties of aerosols are needed to understand and model the impacts of aerosols on air quality and climate.||Better measurements of the physico-chemical properties of aerosols are needed to understand how aerosols form, grow, and impact climate through direct and indirect radiative effects as well as by the modification of clouds and precipitation.|
|Investigate how newly formed particles grow to become important participants in chemistry and climate.||New particle formation is the main global source of particle number, and subsequent condensational growth makes these particles important to the earth system by allowing them to become cloud condensation nuclei (CCN).||Understanding and modeling particle formation and growth will allow better predictions of the impact of this process on direct radiative scatter, cloud formation, and precipitation.|
|Investigate secondary organic aerosol (SOA) formation from biogenic emissions.||The interactions between anthropogenic oxidants and emissions and biogenic trace gases are poorly understood, and may be important in gas-phase chemistry and particle formation and growth.||SOA formation from biogenic emissionss surpasses that from anthropogenic precursors and can modify cloud properties. Cloud formation/precipitation can cause feedbacks on emissions and particle formation and growth, due to their influence on biogenic processes.|
1.Developing new measurements for characterizing ultrafine aerosol physico-chemical properties and their impacts on climate
a.The chemical composition of atmospheric nanoparticles
The ability to characterize the molecular constituents in atmospheric nanoparticles, which is the subset of ultrafine particles whose diameters are smaller than 50 nm, is one of the greatest challenges in aerosol science. This is because these measurements require the characterization of highly non-volatile compounds, which are generally not amenable to study. In addition, rapid timescales for the growth of nanoparticles require measurements to be made at temporal resolutions as small as 10 minutes. The size of these nanoparticles conspires against such efforts, since many existing instruments require micrograms of aerosol and thus require collection times of at least 3 hours. Because of this there is still much that is not known about the species contribute to the formation and growth of particles in the atmosphere. Without this knowledge, we cannot adequately predict the impacts of aerosols in chemistry and climate. Meeting these challenges is of fundamental importance not only to the atmospheric sciences, but also to various engineering applications relating to the development of new materials and the control of particulate contaminates in microelectronics and nano-machine technologies.
To address the need for measuring the chemical composition of atmospheric nanoparticles, the UA group has teamed with Peter McMurry at the University of Minnesota to develop the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS). The TDCIMS is an instrument that is capable of measuring the molecular composition of particles as small as 4 nm (typically 8 nm in ambient air). It accomplishes this with a sensitivity that makes it possible to measure the molecular composition of nanoparticles at ambient concentrations in the atmosphere. Figure 1 shows a flow chart that describes TDCIMS operation. Aerosols are first charged, then size resolved and collected by electrostatic deposition onto a platinum (Pt) filament. The collection time varies with particle size and concentration, but usually ranges from 5 – 15 min. Then the filament is slid into the ionization region of an Atmospheric Pressure Chemical Ionization Mass Spectrometer (APCIMS), where it is resistively heated to evaporate the particles. The desorbed molecules are ionized by proton and electron transfer with protonated water clusters or oxygen anion/water clusters. Ions are then transferred to a triple quadrupole mass spectrometer for mass analysis.
During 2008, development efforts with the TDCIMS have focused on quantifying its sensitivity to various organic acids and amines. A manuscript summarizing the response to organic acids was published in the past year . Our studies show that the instrument sensitivity is relatively constant for most compounds that are ionized by our chemical ionization reagents (that is, for those species for which the proton or electron transfer process is exothermic).
During 2008, the UA group has developed other new instruments to study the formation of atmospheric aerosols and their impacts on climate. An ion trap mass spectrometer was built by visiting German Research Foundation postdoc Andreas Held and interfaced with a redesigned electrostatic nanoparticle sampler for measuring nanoparticle chemical composition using TDCIMS. Figure 2 shows results using this new instrument from a recent study of the formation and deposition of ultrafine aerosols at NCAR’s Marshall Field Site. A manuscript describing this new instrument is in review . Dr. Held also completed the design of a conditional sampling inlet which works with the TDCIMS to obtain size-resolved nanoparticle chemical flux. This instrument was deployed at Marshall Field Site and at the BEACHON Southern Rocky Mountain experimental site in 2008. The latter measurements were performed by Washington State University graduate student Rasa Grivicke, who will also analyze the data.
b.Other instrument development activities
Work continued on developing a scanning mobility particle sizer for the GV aircraft. Design of the instrument is nearing completion with the goal of flight testing the system in 2009. Figure 3 shows an assembly drawing of this instrument, which features two differential mobility analyzers that allow the measurement of size distributions from 3 to 500 nm at atmospheric pressure, or 7 to 300 nm at the maximum flight altitude of 51000 feet.
A hygroscopicity and volatility tandem differential mobility analyzer was designed and assembled in 2008 that can measure size-resolved aerosol hygroscopic growth factors at 90% RH at user selectable residence times of 1, 2, 5 and 28 s, as well as size-resolved aerosol volatility at user selectable temperatures from room temperature to 300 °C and at user selectable residence times of 0.8 and 10.5 s. This instrument operates autonomously and will be used in the 2009 OASIS field study in Barrow, AK. Those measurements will be conducted in collaboration with Dr. Jon Abbatt’s research group from the University of Toronto. A schematic of this instrument is shown in Figure 4.
2.Studying the growth of particles formed by nucleation
a.TDCIMS measurements from the MILAGRO field study in Tecamac, Mexico
There are presently huge uncertainties in predictions of the role of aerosols in climate, especially as related to cloud formation and precipitation . The use of global models to assess these impacts is at its infancy, yet one such study suggests that new particle formation can contribute up to 40% of the cloud condensation nuclei (CCN) at the boundary layer, and 90% in the remote troposphere . Field observations of new particle formation and, most importantly, the subsequent growth are needed to support such model developments.
During 2008 the UA group has continued work on the analysis of TDCIMS data acquired during the March 2006 MILAGRO campaign, in which TDCIMS measurements and particle size distributions were acquired at the ground-based “T1” site in Tecamac, Mexico . The particle formation events observed in Tecamac were unlike any they had seen. New particle formation was so vigorous that sub-10 nm particles were constantly being formed even as the particles were growing rapidly by condensation of low-volatility vapors. This phenomenon, looks like a tomato when plotted as a time series of particle size distribution, so we refer to these as “tomato events.” Figure 5 shows one such tomato event that occurred on 16 March 2006. The shape of this size distribution made it particularly challenging to calculate the growth rates, since the peak of the distribution of the growing nanoparticles was obscured by the new sub-10 nm particles that were simultaneously being formed. A new technique, developed by UMN graduate student Kenjiro Iida , allowed the accurate determination of the growth rate during these tomato events that employed an additional measurement, the fraction of ambient nanoparticles that were charged. The result was a growth rate for the 16 March event of 22 nm hr-1, among the highest growth rates reported anywhere.
During the 16 March event, the TDCIMS was tuned to measure the composition of particles in the size range from 10 – 35 nm, depicted by the black line plotted over the size distribution in Figure 5. The result, shown in Figure 6, is that about 84% of the detected ions are organic, comprised of organic acids, hydrocarbon-like species, and nitrogen-containing organic compounds. Particulate sulfate which arises from the condensation of sulfuric acid vapor constituted only 10% of the detected ions, and nitrate comprised 6%. Measurements of gaseous sulfuric acid, performed by Greg Huey’s research group at the Georgia Institute of Technology, showed that sulfuric acid levels were insufficient to account for the observed growth rate of 22 nm hr-1. The calculated growth rates based on these measurements of sulfuric acid, in which we assume that every sulfuric acid molecule that collides with a particle sticks to the particle and becomes particulate sulfate, amounted to 10% of the observed growth rate. This result that sulfuric acid contributes to 10% of particulate mass, based only on measurements of particle growth rate and sulfuric acid vapor concentration, is the exact same conclusion that one might draw from TDCIMS measurements if the detected ions correspond to the actual species present in particles. This latter issue, the extent to which detected ions correspond to the species in the particles, is the primary focus of current laboratory development with the TDCIMS.
The consequence of this research is that current models that are able to predict the growth of particles on that day would account for the 10% contribution of sulfuric acid to growth rates but entirely miss the 90% contribution of the organics. The result is that these models underestimate the impact of new particle formation on cloud processes. In addition, the prominence of the organic compounds in these particles highlights the need to understand the processes by which organic compounds transform into compounds with extremely low volatilities.
A manuscript on the TDCIMS measurements at the Tecamac site during MILAGRO is also being prepared on the observations of particulate amines. This is an important result as it provides additional support to the notion that amines could be stabilizing ambient nanoparticles through acid-base heterogeneous chemistry. Amines were also observed in Finnish boreal forest aerosols during the EUCAARI-07 campaign.
b.Thermodynamic modeling of organic salt formation as a mechanism for nanoparticle growth.
Based on our observations from Mexico City and Finland, we postulate that amines could be an important atmospheric base in ambient aerosol. Thermodynamic modeling of the organic acid/amine system is being performed to study the potential importance of this heterogeneous chemistry. The current modeling work focuses on two aspects of organic salt formation by organic acids and amines: 1) the relative importance of amines as a base in organic salt formation, as compared to the more commonly considered base, ammonia; and 2) the volatility of organic salts formed from organic acids and amines, particularly when curvature corrections are considered. Preliminary results suggest that amines are an important base, and may contribute significantly to ambient aerosol when their concentrations are within an order of magnitude (or more) of ammonia concentrations. A manuscript describing these studies is currently in preparation.
3.The impacts of biogenic emissions on aerosol formation and growth
During 2008 the UA group continued its close collaboration with the BAI group in order to assess the impact of biogenic emissions on aerosol formation and growth. Globally, secondary organic aerosol (SOA) formed from biogenic emissions surpasses those from anthropogenic precursors . These organic particles have important impacts on climate through their direct interactions with radiation, as well as their ability to modulate cloud condensation nuclei numbers and thus cloud properties and precipitation. These processes exert a substantial influence back upon the Earth system through links to the terrestrial carbon and water cycles (e.g., precipitation regulates plant growth and thus emissions of organic compounds).
To this end, the UA group has performed measurements on aerosol formation from the ozonolysis of Ponderosa pine emissions in their 10 m3 bioaerosol chamber. This work was conducted in the summer 2008 by ASP postdoc Kelley Barsanti along with visiting professor Dr. Rob Griffin and graduate student Meredith Cleveland from the University of New Hampshire. The experiments successfully demonstrated new particle formation from these reactions, which occurred at ambient concentrations. Data analysis is currently underway, and more experiments are planned for the Fall.
Another activity of the UA group in 2008 was to help establish the Manitou Experimental Forest Observatory (MEF) near Woodland Park, CO (Figure 7). The site was developed in close collaboration with several NCAR and university scientists associated with the TIIMES BEACHON project. The site will be used for long-term and intensive measurement campaigns to understand and quantify the role of biogenic aerosol in the complex interactions of the biological, physical and chemical components of the Earth system that regulate and link the terrestrial biogeochemical and water cycles in the southern Rocky Mountains.
To inaugurate the site, during 21 July - 19 September 2008 the UA group coordinated and participated in the TIIMES BEACHON Southern Rocky Mountains 2008 Study. The research objectives for the study included understanding and quantifying the formation of new particles in this setting, and determining the mechanism by which these particles grow to become important participants in atmospheric chemistry and climate. During this study we performed several measurements on new particle formation and nanoparticle composition, including the first measurements of 20 nm nanoparticle chemical flux using conditional sampling with the TDCIMS (see Sec. 1a).