Measurements of OH, H₂SO₄, and MSA during tropospheric Ozone production about the Spring Equinox (TOPSE)

Data from OH, H₂SO₄, and MSA measurements performed during Tropospheric Ozone Production About the Spring Equinox (TOPSE) are presented. Model simulations of OH results made at solar zenith angles of 60° or less showed a tendency of the model to overestimate OH concentrations. This overestimation was a factor of 1.7 at the lower latitudes (<57°N) of the study. At higher latitudes (>57°N) the model tended toward agreement and ultimately an underestimation of OH concentrations by a factor of 0.7. Comparisons of measurements and model showed the model underestimates OH concentrations at solar zenith angles greater than 70°. The results of the present study are discussed in the context of previous studies. Possible model discrepancies are discussed. Measurements of H₂SO₄ showed the highest concentrations at the lowest altitudes (<2000 m) and the lowest latitudes of the study. Larger H₂SO₄ concentrations observed at higher latitudes were accompanied at times by particle nucleation as indicated by the presence of UCN (ultra-fine condensation nuclei) with diameters of 3-4 nm. Concentrations of MSA were generally low with typical values of <2 × 10⁵ molecule cm^-3. High concentrations (>10⁷ molecule CM^-3) were found in layers and were accompanied by other compounds such as NO_x and NO_y indicating MSA may also have an industrial source.