Like many Coloradoans, the team members that make up Access Sensor Technologies’ engineering department love to go on hut trips. A “hut” is a cabin in the Colorado backcountry where the main attractions are skiing and snowboarding. There’s not much that beats panoramic views of snow-capped 14,000-foot mountains, fresh champagne powder, and cooked-from-scratch dinners with friends, but…the old-fashioned, off-the-grid nature of a hut—coupled with winter weather in the Colorado high country—presents some indoor air quality challenges! The data shown below illustrate how Access Sensor Technologies’ UPAS v2.1 PLUS can detect indoor air pollution from a variety of sources.
Huts typically have solar-powered electric lighting (no indoor air quality concerns there 😀), wood-burning heating stoves and ovens (which, despite being vented outdoors, still emit some pollution into the indoor living area 🙁), and gas cooking burners (which emit CO2 and oxides of nitrogen [NOx] directly into the indoor living area ☹️).
We used Access Sensor Technologies’ UPAS v2.1 PLUS to measure air pollution inside a hut for 41 hours during a weekend trip. The UPAS was placed in the same room as the wood-burning stove and gas cooking burners, on a shelf adjacent to the dining table and cushioned benches where most hut trippers sat when they weren’t skiing, cooking, or sleeping. The UPAS was approximately 0.75 m above the floor, 2 m from the wood-burning stove, and 0.5 m from a wall. The UPAS sampled air through our 2 LPM PM2.5 inlet and a PTFE filter. Data from the sensors included in the UPAS v2.1 PLUS were written to a microSD card at 30-s intervals.
This is what the sample filter from the UPAS looked like (shown next to a clean filter for comparison). Did we mention that, even when wood-burning stoves are vented to the outside, some particulate matter leaks into the living area?
The 41-hour average fine particulate matter (PM2.5) concentration in the main living area of the hut was 70 micrograms per cubic meter of air. This filter-derived concentration was 1.5× the 41-hour average PM2.5 concentration reported by the Sensirion SPS30 sensor inside the UPAS v2.1 PLUS (46 μg m-3). Approximately 7.5 μg m-3 (11%) of the PM2.5 sampled onto the filter was estimated to be black carbon. The 41-hour average PM2.5 concentration measured in the hut was more than 2× the mean 24-hour average PM2.5 concentration of 30.7 μg m-3 that was measured in wood-heated homes in Montana (McNamara et al., 2011). The 41-hour average PM2.5 concentration measured in the hut did fall within the range of six-day average PM2.5 concentrations that Walker et al. (2021) measured in U.S. homes that were heated with wood-burning stoves (1.7 to 200 μg m-3, n = 92 homes).
Data from the Sensirion SPS30 sensor included in the UPAS v2.1 PLUS indicated that indoor PM2.5 pollution reached the highest concentration when the wood stove was started upon arrival. The guest book indicated that the hut had been unoccupied for several days, so both the stove and the building were cold. Once the stove was warmed up, transient peaks in the indoor PM2.5 concentration got progressively smaller. A second large peak was observed around 19:00 on the first evening, when the new occupants cooked dinner and continued their efforts to warm up the chilly hut. Another peak was measured around 7:00 the next morning, when occupants woke up, rekindled the fire in the still-warm stove, and began cooking breakfast. A fourth peak was measured around 13:00, when hut trippers returned for lunch after a morning of skiing. A fifth peak occurred around 18:00, when hut trippers were cooking dinner and eating at the table by the stove. The last peak was measured at 7:00 the following morning. Each of these peaks was most likely associated with occupants opening the door to the wood stove to add fuel to fire so that the temperature in the main living area remained comfortable while occupants were cooking and eating. It’s likely that the cooking food emitted additional particulate matter. Indoor PM2.5 pollution was lowest overnight when occupants were sleeping and neither tending the fire nor cooking.
In addition to the Sensirion SPS30 PM sensor, the UPAS v2.1 PLUS includes a Sensirion SCD41 photoacoustic CO2 sensor. We also brought an Aranet4 CO2 monitor, which includes a non-dispersive infrared (NDIR) sensor, to the hut. These two monitors were collocated for most of the weekend. The UPAS v2.1 PLUS and the Aranet4 captured the same trends in indoor air quality and both monitors indicated that CO2 concentrations in the hut were VERY HIGH! When collocated, the UPAS v2.1 PLUS and Aranet4 reported similar values when CO2 concentrations were relatively low; however, the UPAS v2.1 PLUS reported higher values than the Aranet when CO2 concentrations were very high. The high CO2 concentrations measured inside the hut were attributed to regular use of four gas-fueled cooking burners in a relatively small, well-sealed (i.e., poorly ventilated) living area.
The UPAS v2.1 PLUS also includes a Sensirion SGP41 metal oxide NOx and TVOC sensor. This sensor can provide semi-quantitative information on indoor NOx pollution. As shown below, the raw NOx signal from this sensor, which is supposed to increase linearly with the logarithm of the oxidizing gas concentration, increased sharply during cooking activities for which the gas burners were used.
Figure 3: The raw NOx signal from the Sensirion SGP41 metal oxide TVOC and NOx sensor inside the UPAS v2.1 PLUS. This raw signal increases linearly with the logarithm of the NOx concentration.
Overall, these results illustrate how UPAS filter samples can provide information on time-averaged particulate matter and black carbon concentrations while time-resolved data from the particulate matter sensor in the UPAS v2.1 PLUS can indicate the timing and relative severity of individual pollution events. Additionally, the photoacoustic CO2 sensor in the UPAS v2.1 PLUS can capture quantitative variations in CO2 concentrations over time and the metal oxide TVOC/NOx sensor in the UPAS v2.1 PLUS can capture qualitative variations in NOx concentrations, thus indicating when severe indoor CO2 and NOx pollution is present.
But, really, the moral of this story is that the best place to be on a hut trip is outside skiing!
You can try using the UPAS v2.1 PLUS for FREE before you buy! If you’d like to borrow a demo unit, please email us at contact@accsensors.com or fill out this form. Want to read about some prior studies in which the UPAS was used for stationary indoor air monitoring? Check out our public library. This post was made using R Markdown and UPAS data were read into R using our astr package, which you can install from GitHub!