It’s my fourth year as a TA for our ‘Experimental Atmospheric Chemistry’ undergraduate and graduate course at MIT, and today we have loaded up the department’s van with nitric oxide (NO) and ozone (O3) monitors, a uv radiometer, and three particulate monitors (PM 10, 2.5, and 1.0 um). As part of the ‘Pollution Exposure’ unit, we will synchronize the monitors and drive around Boston noting changes in pollutant levels and keeping notes to identify possible pollutant sources. The field trip is a good time, and this year our class has grown to ten students, which is the biggest class we’ve had since I helped develop the course in 2007 with my advisor Professor Ron Prinn and group alumnus Arnico Panday, now at University of Virginia.
We explore tunnels (Boston’s Big Dig provides miles of them), construction sites, urban sites with high traffic congestion, and cleaner beach sites. The students note changes in particulate levels at different sites, which often have distinct particulate size distributions as you would expect from a variety of types of aerosol sources. We follow cars, trucks, and buses of all shapes and sizes. Diesel buses and accelerating vehicles have much higher particulate emissions than clean natural gas buses and stationary vehicles; we might already expect this, but students are able to witness it first-hand and real-time.
Tunnels provide a unique photochemical ‘experiment’. Outside air, under uv light from the sun, has certain levels of pollutants that are created and destroyed by ‘photochemical’ reactions. When this air is swept into a one-way tunnel by the traffic and moved slowly through the tunnel, the tunnel blocks the sun and air is no longer being acted on by uv light, so the photochemical reactions cease. Students can then watch what happens if certain reactions that don’t need uv light proceed (such as NO+O3-> NO2 + O2, which will decrease concentrations of O3) and certain reactions that need uv light are halted (such as NO2 + uv -> NO + O leading to O + O2 + M -> O3 + M, which would have regenerated concentrations of O3). In the tunnels, ozone concentrations decrease because O3 reacts with NO, and because there is no uv light, ozone cannot be regenerated; the students clearly see ozone concentrations fall to nearly zero by the end of long tunnels, such as the Ted Williams Tunnnel in Boston.
The study of atmospheric chemistry is often the study of invisible reactions producing invisible products in the atmosphere, so driving around with instruments and observing these phenomenon real-time have been invaluable teaching tools for students (and myself). Over the semester, the course includes the following sections and field exercises; 1) CO2 and climate, in which students deploy a CO2 monitor to Harvard Forest to understand the carbon cycle, 2) Pollution exposure, in which students monitor their own daily particulate exposure and also observe pollution around Boston as described here, 3) Photochemical cycles, in which a wide range of instruments are deployed to MIT’s Green Building roof, which is the tallest building in Cambridge, and the concentrations of chemicals linked by photochemical reactions are studied in detail, and 4) Isotopes and the carbon cycle, in which students learn the value of the added information provided by measuring the isotopic composition of atmospheric molecules, not just concentrations, and measure the isotopic composition of some atmospheric trace gases. Isotope expert, professor Shuhei Ono, has joined the course and spearheads this fourth topic on isotopes. I have enjoyed helping develop and teach this course, and along with the students I learn something new every year!