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In October of 2021, people in the city of Carson, California noticed a bad smell in the air. It gave some of them headaches and seemed to come from the Dominguez Channel, which begins in Carson and eventually empties into the Los Angeles harbor.
Did drought and decaying plants in the water cause the smell of rot? Or were there other causes? As thousands of people complained to the South Coast Air Quality Management District, Atmospheric Chemist Wing-Sy DeRieux and her colleagues got to work.
Hydrogen sulfide (H2S), part of what makes the gas people and other animals pass smell when we fart, was responsible for the stench. But where did it come from? People working at the Air Quality Management District (AQMD) used different equipment to test the air around the Dominguez Channel. They also brought samples back to their labs. Both types of tests helped them figure out where the H2S was most concentrated. Read to the end to learn what they discovered.
Perhaps because Wing-Sy DeRieux’s family moved a lot when she was a child, she focused her energy on school, a reliable presence in her life. “We would change houses often but I could always go to school.” Wing Sy (pronounced “wing see”) excelled at math and was very curious about the world around her, so “science was a natural fit.”
For an elementary school assignment, Wing-Sy read through cards listing different careers. The card for “physicist” appealed to her: she liked the idea of studying the “basic building blocks of matter and how they interact.” And so when she headed to Harvard College years later, she majored in physics.
During college, DeRieux (pronounced “deroo”) spent a summer in Arizona, at an observatory. The people running the program took everyone on a number of hikes in the Southwest, whose beauty “really made an impression on me.”
With her mind focused on the natural environment upon her return to school, DeRieux took a course on tropical rainforest ecology. That class “made me very interested in the environment, conservation, and sustainability.” She decided to seek work in those areas.
After teaching science in elementary schools and working for an environmental study abroad program, DeRieux spent some time working at a medical device company. “They needed someone to perform chemical analysis. I had never done it but was the closest they had to someone who could,” so she learned how to do chromatography, separating the different parts of a mixture. And she continued taking classes, now in chemistry. As she became aware that chemical research could impact the environment and energy production, DeRieux became inspired to go back to school for a PhD in chemistry.
For her dissertation at the University of California, Irvine, DeRieux studied aerosols. “Think of an aerosol as a mixture of particles and the gas that’s around them.” The gas and the particles interact and are “constantly exchanging chemicals.” DeRieux’s research group did kinetic modeling. People collected samples from different places and created their own aerosols in laboratories, “to see how they are formed and how they behave.”
The stratosphere, the layer of Earth’s atmosphere that is from about 15 to about 50 kilometers up,1 contains the ozone layer. That ozone (O3) blocks some of the Sun’s ultraviolet light. This is useful, because ultraviolet light which reaches the troposphere (the layer of the atmosphere closest to the ground — the layer we live in, just below the stratosphere) can cause sunburn, skin cancer, and other problems. Aerosol products people used in the past contained chemicals which damaged the ozone layer, creating holes in it which let through more ultraviolet light.2
Aerosols in the troposphere also affect our health. If we breathe in many of these particles, they can cause asthma and other health problems.
DeRieux and her colleagues found that aerosols behave differently over Southern California than they do over a forest or over the ocean. Their behavior changes with changes in temperature, location, elevation, season, and weather. The scientists created models “to try to simplify the system, allowing you to make predictions” about how aerosols might behave in different situations.
Using computerized models and also doing hands-on laboratory work both are important, DeRieux explains. “Models are only as good as the data that we put into them. You need the other approaches [gathering samples in the field and doing experiments in the lab]. Without them, modeling won’t give you answers. You need to go back to the real world. But [computer models] provide something of real value, especially about something as complicated as the atmosphere.”
These days, DeRieux and her colleagues at California’s South Coast Air Quality Management District work on reducing VOCs (volatile organic compounds — chemicals which can increase ground-level ozone). While ozone in the stratosphere helps us by blocking u-v rays from the Sun, ozone on the ground can cause health problems. The AQMD is “constantly monitoring to make sure that there are not sources that emit more VOCs than allowed out there.” They make sure that oil refineries and other industrial operations as well as smaller companies, including auto shops, do not release significant amounts of damaging chemicals into the air.
Inspectors from the AQMD collect samples from companies, businesses, and stores and bring them back to the lab. That’s where DeRieux helps “analyze them to determine their VOC content.” She does this in different ways. One of the first steps: determining how much water is in a sample. This “helps you figure out which portion is not water…. You’re looking at concentrations.” DeRieux uses an electrode to measure the amount of water in a known mass of sample. The relationship between the reading of the electrode and the amount of water is linear.”It needs to be linear to be valid.”
Spectrometers help DeRieux and her colleagues count the different organic carbons in a sample. “After all of that analysis is done, it will give us a measure of how much of the sample was made up of volatile organic compounds.” There are levels of VOCs that are allowed. If samples indicate that a company is releasing more than that, the AQMD contacts them and works with them to meet standards for cleaner air. “Does something need to be changed in the manufacturing process?”
“We will also analyze air samples and aerosols in the region if there’s an odor complaint,” DeRieux told Math4Science, which brings us back to the situation in Carson involving the stinky hydrogen sulfide.
It took a few months of research to figure out the chain of events that led to the bad smell. A late September fire had broken out in a warehouse where wellness and beauty products were stored. Chemicals flowed into the channel during efforts to put out the fire. “It had been a chemical spill,” explains DeRieux. These chemicals “were interacting with organic material in the channel.”
Thanks to a drought in the area, liquids were not moving through the channel as quickly as they would when storm water flowed through it. The situation was “not something that would have improved on its own.” The companies whose chemicals had been stored in the warehouse had to work with the AQMD to clean up the chemicals which had leaked into the channel. Thanks in part to Atmospheric Chemist DeRieux and her colleagues, the air in Carson cleared up.
1 How many miles is this? If one km is about .62 of a mile, what math would tell you where to find the stratosphere, in miles?
2 Learn more about the ozone layer and chemicals which damage it here.
