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 (H 2 SO 4 ), 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 H 2 SO 4 measurements, performed in a volcanic plume off the southern coast of Japan, revealed some of the largest marine boundary layer H 2 SO 4 concentrations ever observed and were accompanied by new particle formation. Nighttime measurements of OH, H 2 SO 4 , and MSA in the remote pacific off Midway Island revealed significant boundary layer concentrations of H 2 SO 4 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 H 2 SO 4 and MSA observed at night is presented.