The Worst Air Quality in Recent Chicago History: A Look at the 27-29 June 2023 Air Quality Event

PM_2.5, tiny particles or droplets in the air that measure less than 2.5 micrometers in diameter, can have serious health effects, not just limited to the respiratory system. Additionally, PM_2.5 affects Hispanics and Blacks disproportionally [King K. Population Environment 37, 1-21 (2014)] and they often have improper monitoring in their neighborhoods [Illgner T. & Lad N. Front Public Health 10, (2022)]. As such, the Community Research on Climate and Urban Science (CROCUS) project investigates this relationship and establishes sensors in these poorer neighborhoods. While on a deployment at Northeastern Illinois University (NEIU), CROCUS sensors recorded record amounts of PM_2.5, with amounts as high as 250 µg/m³ recorded. Along with PM_2.5, observations of other precursor gases, O₃ and NO₂, were collected. PM_2.5 observations are also analyzed for a range of environmental factors, including humidity, temperature, and wind speed and direction. Data collected from the CROCUS project at NEIU and Environmental Protection Agency (EPA) sensors scattered throughout the city are used, and an analysis on which sensors is better reveals that the CROCUS sensors are much more sensitive to changes in PM_2.5 concentrations. Numerous processes are observed to have occurred, including deliquescence, advection both of temperature and PM_2.5 and depression of temperature. While this paper attempts to investigate the physical processes behind what happened on the 27-29 June, CROCUS attempts to combine socioeconomic factors as well. As such, further work is needed to investigate these factors. Additionally, further analysis is required on other mesoscale phenomena, like effects on the Lake Breeze and Boundary Layer, Mixing, etc.