Linking MAX-DOAS measurements of formaldehyde and glyoxal to precursor substances at the canopy-atmosphere-interface at ATTO

Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) uses trace gas absorptions in spectra of scattered sun light recorded at different elevation angles to retrieve vertical profiles of trace gas concentrations and aerosol extinctions in the lower troposphere as well as their total tropospheric vertical column densities. A major advantage of MAX-DOAS is the possibility to observe multiple trace gases, such as formaldehyde and glyoxal, simultaneously for the same air mass. We operate two MAX-DOAS instruments at the Amazon Tall Tower Observatory (ATTO) at altitudes of 80 m (since 2017) and 298 m (since 2019) above ground. Besides the full profile retrievals for both instruments, this setup allows the determination of (small-scale) vertical gradients of trace gas abundances in the altitude range between both instruments (ca. 200 m) by directly comparing their measurements.

Located in a pristine rain forest region in the central Amazon Basin about 150 km north-east of Manaus, the ATTO site offers a unique opportunity to study the chemical processing of tropospheric trace gases far from major anthropogenic emission sources. Further, the site hosts long-term and campaign-based measurements of a large variety of different atmospheric constituents and parameters. Combining these measurements allows investigating chemical processes at the canopy-atmosphere-interface and directly above it. Comparisons with model data yield further insights, e.g. the identification of processes that are not (fully) represented by the simulations or the confirmation of surprising observational results. 

In this study, the MAX-DOAS results of formaldehyde and glyoxal are compared to measurements of their major precursor substances, i.e. isoprene and monoterpenes. This includes assessments of their respective seasonal and diel variations as well as their (small-scale) vertical gradients. For selected time periods, the results of these atmospheric measurements are also compared to model simulations performed with WRF-Chem, using the MOZART-4 chemical mechanism, in order to investigate whether the characteristic variations and vertical gradients found for the measurement data are also reflected in model simulations.