Observations of Nocturnal Sulfuric Acid Formation in Pittsburgh, PA

Measurements of sulfuric acid (H₂SO₄) and sulfur trioxide (SO₃) were conducted in Pittsburgh, Pennsylvania, during field campaigns in Fall 2023 and Fall 2024. These measurements identified nocturnal concentrations of H₂SO₄ comparable to those of daytime values. Nocturnal H₂SO₄ concentrations were observed to increase by 5 × 10⁵ to 5 × 10⁷ molecules cm^-3 above background on 16 of the 31 measurement nights. The median peak concentration during events was 6.5 × 10⁶ molecules cm^-3, with a maximum of 1.0 × 10⁸ molecules cm^-3, exceeding previously reported nighttime concentrations. Increases in H₂SO₄ concentrations were positively correlated with the anomalously high SO₃ concentrations and condensation sink rates, indicating that the formation of H₂SO₄ increased to overcome the loss rates to particles. Increases in particulate mass and the mass fraction of metals commonly emitted from coal combustion and steel production were also observed. The air masses were traced back to the southeast of Pittsburgh, a region home to a steel mill, coke plant, and a steel processing plant. The observations indicate a previously unrecognized nighttime formation pathway for H₂SO₄, potentially from heterogeneous catalysis with metal or black carbon, originating from steel and coke plant emissions. Further measurements are needed to identify key compounds and chemical processes driving these increases in nocturnal H₂SO₄ concentrations.