PM_2.5 chemistry, organosulfates, and secondary organic aerosol during the 2017 Lake Michigan Ozone Study

The Lake Michigan Ozone Study from 21 May to 23 June 2017 (LMOS 2017) aimed to better understand the anthropogenic and biogenic sources that contribute to ozone and fine particles (PM_2.5) along the coast of Lake Michigan. Here, we focus on the chemical composition of daytime and nighttime PM_2.5—especially organic carbon, inorganic ions and organosulfates—at a ground-based supersite in Zion, Illinois. PM_2.5 mass concentrations ranged from 1.5 to 12.9 μg m⁻³ with an average (±standard error) of 5.2 ± 0.4 μg m⁻³. The most significant contributor to PM_2.5 mass was organic matter (OM; calculated as 1.7 × organic carbon [OC]; contributing an average of 59 ± 2%), followed by sulfate (17 ± 1%), ammonium (6.3 ± 0.3%), nitrate (3.5 ± 0.4%), and elemental carbon (EC; 3.4 ± 0.2%). During each of the three periods of high ozone, PM_2.5 had different regional characteristics. Period A (2–3 June) was impacted by lake breeze and south-easterly air masses that travelled over major urban areas. Period A had the highest daily PM_2.5 mass concentrations (11.4 ± 1.5 μg m⁻³) and EC with a relatively low OC:EC ratio of 7.0, indicating the influence of sources with low OC:EC ratios, which includes the anthropogenic combustion of fossil fuels and biomass. Period B (10–13 June) was impacted by air masses traveling from the southern US. It had a relatively high OC:EC ratio of 18, the highest PM_2.5 sulfate concentrations and aerosol acidity, and elevated mixing ratios of isoprene along with its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR). Peak concentrations of organosulfates, including methyltetrol sulfate (m/z 215; C₅H₁₁SO₇⁻), were also observed throughout period B. Period C (13–17 June) followed a change to northerly winds. PM_2.5 concentrations decreased along with decreases in sulfate, acidity, and most organosulfates. Throughout the study, organosulfates accounted for an average of 4% of OM and up to 15% of OM in Period B. Organosulfates were largely isoprene-derived, with lessor contributions from monoterpenes (0.3%) and anthropogenic sources (0.5%). Through these measurements of organosulfates in the Great Lakes region, we demonstrate the importance of anthropogenic sulfate emissions and aerosol acidity on SOA formation, and establish that isoprene-derived organosulfates, in particular, contribute significantly to PM_2.5. With other LMOS observations, the chemical signatures of PM_2.5, and back trajectories show that ozone episodes cooccur with localized lake-breeze meteorology within air masses that vary from episode to episode in chemical history and source region.