New Detector Will Provide Better Understanding of Atmospheric Pollutant
Berkeley Lab's Ron Cohen has been trying to better understand the chemical equations of smog during his seven years as an atmospheric chemist. With the help of a Laboratory Directed Research and Development grant, he has now developed a new technique of measuring the molecules that control the production of smog and greenhouse gases.
The atmosphere is made up of about eighty percent nitrogen and twenty percent oxygen, Cohen explains, but an atmospheric chemist worries about the last 0.05 percent of the air, which contains CO2, free radicals, and ozone. These are the molecules that are strongly affected by energy consumption and other human activity. For Cohen, a researcher in the Energy and Environment Technology Division (EETD), making sense of that .05 percent and averting further smog and greenhouse gas production is a matter of early detection. His detector measures the concentration of NO2 and NO, collectively known as NOx, a pollutant which catalyzes the processes that produce smog. Armed with the powerful new system, Cohen hopes to better understand the role that NOx (pronounced "knox") plays in the atmosphere.
NOx is an important cog in the complex set of chemical steps known as the "smog reactions" whose main products are ozone and NO2, both which contribute to global warming through the greenhouse effect. Though in higher altitudes ozone serves the crucial purpose of protecting the planet from harmful ultraviolet rays, in the lower atmosphere ozone serves as the main component of smog as well as a greenhouse gas.
While laboratory-based models that explain atmospheric chemistry abound, data taken from nature on the role of NOx is lacking. Cohen's new device, which will be adapted into a compact, sensitive and cheap detector, should provide a plentiful source of data with which to test predictions. The technique utilizes laser induced fluorescence (LIF) to measure the concentration of NO2. LIF works by first tuning a laser to the frequency at which NO2 absorbs light, then aiming the tuned laser at the air sample. This causes the NO2 molecules in the air mixture to fluoresce, emitting photons that are counted by a detector to determine the concentrationthe more photons, the more NO2 that is present in the sample. In order to measure the concentration of the precursors to NO2, the air is heated until the molecules fall apart, yielding NO2 that can be detected with LIF.
Though LIF is a long-standing method used to detect gases, the experiment marks the first time that the technique is being applied to measure NO2 in the atmosphere. Cutting edge lasers, super-sensitive photon detectors, and a special method of cooling the gas will allow Cohen's group to measure concentrations with unprecedented accuracy of five parts per trillion in ten seconds, improving on older methods by a factor of ten.
"High precision allows you to look at data in a more chemical way, to use data to critically evaluate our models," Cohen says.
While the equipment is still in the preliminary stages of testing, the project faced its first trial earlier this year. At a collaborative field test conducted fifty miles outside Sacramento, Cohen took measurements alongside fellow EETD chemist Allen Goldstein, whose group focuses on natural emissions of hydrocarbons from forests. With support from the California Air Resources Board, the team hopes to examine how the smog of Sacramento interacts with nearby woodlands.
After establishing the reliability of the detector, Cohen's team will work to make the detection systemwhich now takes up the space of two large suitcasesmuch smaller. A sensitive, compact NOx detector could pave the way for a global network of detectors, augmenting the data from existing satellites which provide NOx readings over vast stretches of the lower atmosphere.
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