Highlights of OH, H₂SO₄, and methane sulfonic acid measurements made aboard the NASA P-3B during Transport and Chemical Evolution over the Pacific

Measurements of hydroxyl radical (OH), sulfuric acid (H₂SO₄), and methane sulfonic acid (MSA) were performed aboard the NASA P-3B using the selected ion chemical ionization mass spectrometry technique during the Transport and Chemical Evolution over the Pacific (TRACE-P) study. Photochemical box model calculations of OH concentrations yielded generally good agreement with an overall tendency to overestimate the measured OH by ∼20%. Further analysis reveals that this overestimation is present only at altitudes greater than ∼1.5 km, with the model underestimating OH measurements at lower altitudes. Boundary layer H₂SO₄ measurements, performed in a volcanic plume off the southern coast of Japan, revealed some of the largest marine boundary layer H₂SO₄ concentrations ever observed and were accompanied by new particle formation. Nighttime measurements of OH, H₂SO₄, and MSA in the remote pacific off Midway Island revealed significant boundary layer concentrations of H₂SO₄ and MSA, indicating evidence of nighttime boundary layer oxidation processes but in the absence of OH. A cursory exploration of the sources of production of the H₂SO₄ and MSA observed at night is presented.