Sulfur Dioxide Point Sources Remain Important for Daytime Plume-Scale New Particle Formation in the United States: Case Study for Gary, Indiana During the Summer 2023 AEROMMA Aircraft Campaign

Abstract

Plumes from sulfur dioxide (SO₂) point sources have been shown to be significant contributors to new particle formation (NPF), but SO₂ emissions in the United States (US) have decreased drastically by ∼10× since the early 2000s. We present observations of plume-scale NPF from SO₂ point sources over the industrial hub in Gary, Indiana and assess the importance of SO₂ plumes to NPF using the P6 subgrid NPF parameterization and model the subsequent aerosol growth using the Statistical Oxidation Model-TwO Moment Aerosol Sectional (SOM-TOMAS) kinetic gas/aerosol model. We identify 29 SO₂-rich NPF plumes downwind of SO₂ point sources in our observations. Across eight separate flight passes over Gary, the total plume-scale particle production rate via NPF is estimated to be in the range of 8.5 × 10¹³–3.5 × 10¹⁸ # s⁻¹ and is typically comparable to or greater than primary particle number emissions from traffic in nearby Chicago. On a single case day, subsequent growth of newly formed particles is mainly driven by regional condensation of organics and coagulation. These results demonstrate the continued importance of SO₂ point sources for plume-scale NPF and regional aerosol number concentrations, including those of climate-relevant sizes (<50–100 nm diameter), despite more than a factor-of-10 reduction in SO₂ emissions in recent decades. Further, organics are critical in sustaining the growth of these newly formed particles, and organics may be more important for this growth compared to past decades due to reduced sulfuric acid (H₂SO₄).