Daniel Knopf (center) with two members of his team, technician Tracey Evans (l)
and Ph.D. student Luping Su (r).
This January, atmospheric chemistry Professor Daniel A. Knopf began his fourth year at the School of Marine and Atmospheric Sciences. In his relatively short tenure at SoMAS, Knopf has embarked on a number of projects focusing on the role of aerosols in the atmosphere. An aerosol is a suspension of fine particles or tiny liquid droplets in a gas. Smog, smoke and air pollution are aerosols. One of Knopf’s projects involves ice nucleation; another deals with the heterogeneous kinetics of aerosols and a third and most recent (2010) treats of the measurement of volatile organic compounds in the Manitou Experimental Forest in Colorado. These projects, although developed separately, are thoroughly interconnected from Knopf’s perspective.
“You could see (these projects) as separate,” said Knopf. “But the overall idea or aim for me personally is to have characterized a suite of different aerosols and their potential to interact with ice clouds or to form ice clouds. Then, if we have a wider range of different particles, maybe there’s a good hope to have a good physical parameterization of these processes.”
Aerosol particles are important, Knopf asserts, because they interact with solar and terrestrial radiation. Aerosol particles both absorb and reflect parts of the electromagnetic spectrum, including visible light, infrared radiation and ultraviolet radiation, and they play a role in the radiative budget that exists between the Earth and the Sun. Clouds, which are important in maintaining the Earth’s radiative budget, are also a function of the presence of aerosols in the environment. A cloud requires aerosol particles to form, giving the particles seed-like stature in the workings of the atmosphere. As Knopf explains it, without aerosols, while Earth would have an atmosphere and the Greenhouse Effect would still be possible, clouds would not exist. Thus, there would be no rain. In addition to their effect on atmospheric temperature, aerosol particles such as pollen or industrial soot play a role in air quality, which can impact human health.
In the on-going discussion of climate change, the role of ice clouds remains uncertain.
“If you see the whole climate discussion, in the actual climate models, we have no ice clouds implemented yet,” said Knopf. “They are not able to predict them accurately enough to say, ‘Oh, there is this temperature and, with that relative humidity, an ice cloud should form that shields the Earth from the Sun a little bit or not or, what is its radiative input.’ So the aim in doing these different studies and then combining them is that, maybe, you can get a bigger framework for predicting the ice nucleation capability of these particles.”
Knopf’s ice nucleation project, funded by NOAA, the National Oceanic and Atmospheric Administration, examines the phase transition of aerosol particles from liquid to solid phases and the formation of ice in the atmosphere. The project requires an optical microscope with a cooling stage that can freeze pure water, which Knopf designed himself. Pure water does not freeze at 0o Celsius, but at -40o Celsius, explained Knopf. For Knopf’s work, a device which is capable of precisely freezing water down to -90o Celsius in atmospherically relevant conditions is necessary.
“There are some commercial cryo-cooling stages available but they do not fit all our experiments and so, many of them, we built by ourselves,” said Knopf. “We have experiments where we can control relative humidity and temperature that really, like in the atmosphere, can adjust the water’s partial pressure and relative humidity at 10 kilometers [altitude] and temperature and then look what the aerosol particle is doing. And that [type of instrument] you cannot buy. You have to completely design it by yourself.”
The study utilizes the Lawrence Berkeley National Laboratory’s light source and Pacific Northwest National Laboratory’s environmental scanning electron microscope to analyze the composition of these particles.
Knopf is also investigating the organic monolayer on the surface of water in its aqueous phase using a Langmuir-Blotcher trough, which allows for the examination and measurement of the monolayer by compressing it. When this organic matter enters the atmosphere, it has been found that this particle has a comparatively high freezing point, which could potentially allow ice clouds to form at lower altitudes or at higher temperatures.
“These are the things we want to know,” said Knopf. “We want to predict when and under which atmospheric conditions these clouds form.”
The heterogeneous kinetics project also requires a custom-made instrument. The chemical ionization mass spectrometer, which measures atmospheric radicals in situ, like OH, ozone and nitrate radicals (ex., NO3), was completely constructed at Stony Brook University, with Knopf designing the instrument and the Physics Department’s machine shop fashioning its components. Funded by a Faculty Early Career Development Award from the National Science Foundation, the project examines the chemical and physical changes that occur as gas molecules react with aerosol particles. Knopf received the grant in recognition of his efforts to meld the worlds of research and education.
“We measure the chemical kinetics between gas phase species and an aerosol surface,” explained Knopf. “You measure how quickly an ozone molecule would, for example, react with an organic particle. We can give that a number.”
This reaction plays a role in the loss of atmospheric ozone, manifested in the Antarctic ozone hole.
In another realm of his research, Knopf uses one of the few high resolution proton transfer reaction time-of-flight mass spectrometers in the United States to measure and identify biogenic volatile organic compounds (VOCs), or organically-generated environmental gases emitted by liquids and solids, in Colorado’s Manitou Experimental Forest.
“Now, coming back to the aerosols, from these VOCs if they are in the atmosphere, they can react with other gas phase species, like ozone, something which is reactive can transfer these VOCs into bigger molecules which condense and again form molecules; And so, at the moment, the big interest is how, for example, a forest, which emits VOCs (plants emit VOCs) can form new particles as a source of aerosol particles,” explained Knopf. “And with this instrument, we want to measure VOC fluxes and emission rates. It’s more a gas phase measurement but it can lead to insight on how aerosol particles form. We measure the precursors of some of the aerosol particles…We want to modify the instrument to enable us to use it for aerosol measurements but, as of now, it’s mostly for [examining the] gas phase.”
The instrument, which was procured via a Major Research Instrumentation grant from NSF, is also being used by SoMAS Professor John E. Mak.
To pursue these projects which rely so heavily on instruments, Knopf has several teams of students from various levels of study. He specifically makes a point of involving undergraduates in his lab work.
“You cannot compare [lab work] to philosophy, human science or theoretical work. So you are really bound to your instrument, you know what your instruments can perform, can they ask the question you pose, then you have to work around that but usually the experiments are all not done in 10 minutes,” said Knopf. “So, yea from that point, you need a couple of students if you want to address several of these questions, which makes it then also, it gives you a comprehensive picture in a way because if you only attack one of these questions, you may miss other aspects.”
