Investigating small-scale vertical concentration gradients of formaldehyde and glyoxal above the canopy at the Amazon Tall Tower Observatory (ATTO) using two MAX-DOAS instruments

Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements use trace gas absorptions in spectra of scattered sun light recorded under different elevation angles to retrieve vertical profiles of trace gas concentrations and aerosol extinctions in the lower troposphere as well as the corresponding total tropospheric vertical column densities (VCDs). These measurements allow observation of multiple trace gases e.g., formaldehyde (HCHO) and glyoxal (CHOCHO), for the same air mass simultaneously with one instrument. We operate two MAX-DOAS instruments at the Amazon Tall Tower Observatory (ATTO) at altitudes of 80 and 298 m above ground. Besides the full profile retrievals for both instruments, this measurement setup allows the determination of vertical gradients of trace gas and aerosol abundances in the altitude range between both instruments by directly comparing the VCDs and concentrations at instrument altitude. Such small-scale vertical gradients provide important insights into the chemical processing of the different species. Located in a pristine rainforest region in the central Amazon Basin about 150 km north-east of Manaus, the ATTO site offers a unique possibility to study the chemical processing of tropospheric trace gases far away from major anthropogenic emission sources.

Here, we present an overview of these small-scale vertical gradients of formaldehyde and glyoxal abundances at ATTO. We investigate their seasonal variations, the effects of meteorological parameters and compare them to the vertical concentration gradients of isoprene and monoterpenes both being precursor substances of formaldehyde and glyoxal. Also, a comparison to model simulations might yield interesting insights. The main result of our work is that formaldehyde is net formed in the altitude range (around 200 m) between both instruments, while glyoxal is already net degraded in this altitude range. Together with their characteristic profile shapes, these findings indicate different chemical processing (production and degradation) of formaldehyde and glyoxal although both compounds can be produced from isoprene. In particular, glyoxal is likely formed and then photolyzed very rapidly in that height range.