ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-11179-2012 Formaldehyde and nitrogen dioxide over the remote western Pacific Ocean: SCIAMACHY and GOME-2 validation using ship-based MAX-DOAS observations Peters E. 1 Wittrock F. 1 Großmann K. 2 Frieß U. 2 Richter A. 1 Burrows J. P. 1 Institute of Environmental Physics (IUP), University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany Institute of Environmental Physics (IUP), University of Heidelberg, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany 27 11 2012 12 22 11179 11197 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/11179/2012/acp-12-11179-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/11179/2012/acp-12-11179-2012.pdf

In October 2009, shipborne Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were performed during the TransBrom campaign over the western Pacific Ocean (≈ 40&deg; N to 20&deg; S). Vertical tropospheric trace gas columns and profiles of nitrogen dioxide (NO<sub>2</sub>) and formaldehyde (HCHO) as well as stratospheric NO<sub>2</sub> columns were retrieved in order to validate corresponding measurements from the GOME-2 and SCIAMACHY satellite instruments and to estimate tropospheric background concentrations of these trace gases. <br><br> All instruments reproduced the same characteristic, latitude-dependent shape of stratospheric NO<sub>2</sub>. SCIAMACHY and GOME-2 data differ by about 1% from each other while yielding lower vertical columns than MAX-DOAS morning values as a consequence of measurement time and stratospheric NO<sub>2</sub> diurnal cycle. Due to this diurnal cycle, an increase of 8.7 ± 0.5 × 10<sup>13</sup> molec cm<sup>−2</sup> h<sup>−1</sup> of stratospheric NO<sub>2</sub> was estimated from MAX-DOAS data at low latitudes during the day. <br><br> Tropospheric NO<sub>2</sub> was above the detection limit only in regions of higher anthropogenic impact (ship traffic, transport of pollution from land). A background column of 1.3 × 10<sup>14</sup> molec cm<sup>−2</sup> (or roughly 50 ppt boundary layer concentration) can be estimated as upper limit for the remote ocean, which is in agreement with GOME-2 monthly mean values. In the marine boundary layer close to the islands of Hokkaido and Honshu, up to 0.8 ppbv were retrieved close to the surface. <br><br> Background HCHO concentrations over the remote ocean exhibit a diurnal cycle with maximum values (depending strongly on weather conditions) of 4 × 10<sup>15</sup> molec cm<sup>−2</sup> for the vertical column at noontime. Corresponding peak concentrations of up to 1.1 ppbv were retrieved in elevated altitudes (≈ 400 m) around noon while maximum concentrations in the evening are close to the ground. An agreement between MAX-DOAS and GOME-2 data was found for typical vertical columns of 3 × 10<sup>15</sup> molec cm<sup>−2</sup> over the remote ocean at the time of overpass.

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