A recent study by Yale researchers has found that a type of air pollution is much more complicated than previous research had indicated.

Organic aerosol — which includes clouds and fog — can alter the climate by scattering light and changing Earth’s reflectivity, according to NASA. Such aerosol can also have adverse effects on human health. As a result, understanding the composition of aerosol, also known as particulate matter, is important to our understanding of how it might affect our bodies, according to Jenna Ditto GRD ’21, the lead author of the study.

“Organic aerosol is everywhere, indoors and outdoors, so understanding what this aerosol is made of chemically and how that composition changes over time is important for understanding how that might impact human health,” Ditto said.

Prior research has suggested that samples of air taken at a specific temperature or humidity are representative of the overall air composition in those weather conditions. But in their new study, Yale researchers demonstrated that there is high variability in aerosol compositions even in days that appear to have similar baseline conditions.

The team looked at particulate matter composition at three different field sites and in a laboratory setting, finding that the composition varied with time much more than previously published data suggested. Although the researchers expected differences in air composition throughout the day due to the chemical effects of sunlight, Ditto said, they also found that there were still large changes in chemical composition from day to day.

There are a variety of potential explanations for the variability, according to Ditto. If the sources of the compounds are different — from cars or from trees, for example — they will form different final products.

Another possible reason is that composition depends on the origin of the air flow. The researchers were able to determine where a specific air flow originates through backward trajectory modeling, a method used to figure out where air has come from based on factors such as wind patterns — the opposite of a forecast, Ditto said.

Julia Lee-Taylor, research associate at the University of Colorado, Boulder and co-author of the study, added that the study helped identify a wide variety of compounds.

Ditto said she believes the research will help inform approaches going forward. Many other researchers tend to collect data from a few representative days and characterize trends they see, but such days are not truly representative of the true extent of air composition variability, she said.

“We want to be a little conscious when selecting a particular set of conditions and saying they’re representative of something,” added Emily Barnes ’17, another researcher on the paper. “Even in days that seem to be very similar in air quality conditions, they’re actually much more distinct than we think.”

Barnes added that the study can also contribute to improving modeling capabilities, explaining that modelers need simple ways of characterizing air. Future directions will include determining how exactly the specific compounds in aerosol differ and affect the environment, as well as developing more advanced models for measuring aerosol composition.

Important next steps for the research include developing more comprehensive explanations for the variability in air composition, as well as investigating if the differences in aerosol composition have an impact on human health and the environment.

Kelly Wei | kelly.wei@yale.edu