Articles | Volume 15, issue 18
Atmos. Chem. Phys., 15, 10567–10579, 2015
Atmos. Chem. Phys., 15, 10567–10579, 2015

Research article 25 Sep 2015

Research article | 25 Sep 2015

NO2 seasonal evolution in the north subtropical free troposphere

M. Gil-Ojeda1, M. Navarro-Comas1, L. Gómez-Martín1,2, J. A. Adame1, A. Saiz-Lopez3, C. A. Cuevas3, Y. González4, O. Puentedura1, E. Cuevas4, J.-F. Lamarque5, D. Kinninson5, and S. Tilmes5 M. Gil-Ojeda et al.
  • 1Instituto Nacional de Técnica Aeroespacial, Torrejón de Ardoz, Spain
  • 2Groupe de Spectrométrie Moléculaire et Atmosphérique, URM CNRS 7331, UFR Sciences Exactes et Naturelles, Moulin de la Housse, BP 1039, 51687 Reims CEDEX 2, France
  • 3Atmospheric Chemistry and Climate Group, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, Spain
  • 4Izaña Atmospheric Research Center, AEMET, Tenerife, Spain
  • 5Atmospheric Chemistry Division, NCAR, Boulder, CO, USA

Abstract. Three years of multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements (2011–2013) have been used for estimating the NO2 mixing ratio along a horizontal line of sight from the high mountain subtropical observatory of Izaña, at 2370 m a.s.l. (NDACC station, 28.3° N, 16.5° W). The method is based on horizontal path calculation from the O2–O2 collisional complex at the 477 nm absorption band which is measured simultaneously to the NO2 column density, and is applicable under low aerosol-loading conditions.

The MAXDOAS technique, applied in horizontal mode in the free troposphere, minimizes the impact of the NO2 contamination resulting from the arrival of marine boundary layer (MBL) air masses from thermally forced upwelling breeze during middle hours of the day. Comparisons with in situ observations show that during most of the measuring period, the MAXDOAS is insensitive or very slightly sensitive to the upwelling breeze. Exceptions are found for pollution events during southern wind conditions. On these occasions, evidence of fast, efficient and irreversible transport from the surface to the free troposphere is found.

Background NO2 volume mixing ratio (vmr), representative of the remote free troposphere, is in the range of 20–45 pptv. The observed seasonal evolution shows an annual wave where the peak is in phase with the solar radiation. Model simulations with the chemistry–climate CAM-Chem model are in good agreement with the NO2 measurements, and are used to further investigate the possible drivers of the NO2 seasonality observed at Izaña.

Short summary
The NO2 seasonal evolution in the free troposphere (FT) has been established for the first time, based on a remote sensing technique (MAXDOAS) and thus avoiding the problems of the local pollution of in situ instruments. A clear seasonality has been found, with background levels of 20-40pptv. Evidence has been found on fast, direct injection of surface air into the free troposphere. This result might have implications on the FT distribution of halogens and other species with marine sources.
Final-revised paper