Suchergebnisse

Formaldehyde (CHO) is a tracer of the photochemical activity of the atmosphere. Linked to air quality, CHO is an ozone (O) precursor and serves as a proxy for natural and anthropogenic reactive organic emissions. As a product of the photooxidation of methane (CH) and other hydrocarbons (e.g., isoprene), CHO represents an important source of radicals in the remote free troposphere. This work aims at improving the characterization of this part of the troposphere where data are scarce. In particular, this study assesses the presence of CHO at two high-altitude remote sites: El Teide (TEI, 3570 m a.s.l., Tenerife, Canary Islands, Spain) and Pic du Midi (PDM, 2877 m a.s.l., French Pyrenees). Through ground-based remote sensing measurements performed during two field campaigns in July (TEI) and September (PDM) 2013, this study presents the vertical distribution of CHO at both locations. Results at PDM show that CHO mixing ratios follow a decreasing vertical profile with a mean maximum of 0.5 ± 0.2 nmol mol (i.e., ppbv) at the instruments' altitude. At TEI, observations indicate an uplifted layer of CHO with a mean maximum of 1.3 ± 0.3 nmol mol at 3.8 km a.s.l. (i.e., 300 m above the instrument's altitude). At both remote sites, the observed CHO levels are higher than expected for background methane oxidation (a threefold increase in the case of TEI). Air mass back trajectory analysis links CHO observations with abundant natural (e.g. forests) and/or anthropogenic isoprene emissions from the region nearby PDM, while the high CHO levels detected at TEI indicate in-plume formation of CHO resulting from its precursors emitted from west-African and Canadian fires. Finally, as a key trace gas for O and HO chemistries, we estimate the upper limit of bromine monoxide (BrO) in the free troposphere at TEI and PDM to be 0.8 and 1.5 pmol mol (i.e., pptv) respectively. ; The TEI campaign was funded through the AMISOC project (Atmospheric MInor Species relevant to the Ozone Chemistry, Spanish National RCD Funding Agency, CGL2011-24891). The PDM campaign was funded by the European Research Council Executive Agency under the European Union's Horizon 2020 Research and innovation program (ERC-2010-STG 258537) to JES. LGM would like to the MINECO support under grant VHODCA (CTM2017–83199P). The authors would like to thank the assistance of the personal working at the Izaña and at the Pic du Midi observatories. The authors acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and READY website (http://www.ready.noaa.gov) used in this publication. Also, we acknowledge the use of the aerosol product (AOD) from AERONET (http://aeronet.gsfc.nasa.gov/.) and the fire counts imagery from LANCE FIRMS operated by NASA's Earth Science Data and Information System (ESDIS) with funding provided by NASA Headquarters. The UV aerosol index visualization used in this work was produced with the Giovanni online data system, developed and maintained by the NASA GES DISC. We also acknowledge the OMI mission scientists and associated NASA personnel for the production of the UV aerosol index data used in this study (Bhartia P. K. 2012, OMTO3d-OMI/Aura TOMS-Like Ozone, Aerosol Index, Cloud Radiance Fraction L3 1 day 1° x 1° V3).