Published in the journal Environmental Science & Technology Letters, the study analyzed measurements of ultrafine particles, referred to as particle number concentration (PNC), that were collected before and during the first year of COVID at a rooftop site near Boston’s Logan International Airport. The findings revealed that during the state-of-emergency period from April-June 2020, average PNC was 48 percent lower than pre-pandemic levels, corresponding with flight activity that was 74 percent lower, highway traffic volume that was 51 percent lower, and local traffic volume that was 39 percent lower than pre-pandemic levels.