Atmospheric formaldehyde at El Teide and Pic du Midi remote high-altitude sites

Abstract

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.