Spatial and temporal variability of ground and satellite column measurements of NO₂ and O₃ over the Atlantic Ocean during the Deposition of Atmospheric Nitrogen to Coastal Ecosystems Experiment

Abstract

In situ measurements of O₃ and nitrogen oxides (NO + NO₂ ≡ NOx) and remote sensing measurements of total column NO₂ and O₃ were collected on a ship in the North Atlantic Ocean as part of the Deposition of Atmospheric Nitrogen to Coastal Ecosystems (DANCE) campaign in July–August 2014, 100 km east of the mid-Atlantic United States. Relatively clean conditions for both surface in situ mixing ratio and total column O₃ and NO₂ measurements were observed throughout the campaign. Increased surface and column NO₂ and O₃ amounts were observed when a terrestrial air mass was advected over the study region. Relative to ship-based total column measurements using a Pandora over the entire study, satellite measurements overestimated total column NO₂ under these relatively clean atmospheric conditions over offshore waters by an average of 16%. Differences are most likely due to proximity, or lack thereof, to surface emissions; spatial averaging due to the field of view of the satellite instrument; and the lack of sensitivity of satellite measurements to the surface concentrations of pollutants. Total column O₃ measurements from the shipboard Pandora showed good correlation with the satellite measurements (r = 0.96), but satellite measurements were 3% systematically higher than the ship measurements, in agreement with previous studies. Derived values of boundary layer height using the surface in situ and total column measurements of NO₂ are much lower than modeled and satellite-retrieved boundary layer heights, which highlight the differences in the vertical distribution between terrestrial and marine environments.