Investigating vertical gradients of trace gases and aerosol at the Amazon Tall Tower Observatory (ATTO) using MAX-DOAS measurements

Multi-AXis (MAX)-Differential Optical Absorption Spectroscopy (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). A major advantage of this kind of measurements is the possibility to observe multiple trace gases e.g., formaldehyde (HCHO), glyoxal (CHOCHO) and nitrogen dioxide (NO₂), for the same air mass simultaneously with one instrument. A first MAX-DOAS instrument was installed at the Amazon Tall Tower Observatory (ATTO) at an altitude of 80 m above ground in October 2017. Since March 2019, a second instrument is operational at an altitude of 298 m. Besides the individual profile retrievals for both instruments, this measurement strategy allows the identification of (small scale) vertical gradients of trace gas and aerosol abundances by directly comparing the VCDs and concentrations (at instrument altitude) measured by both instruments. Such (small scale) vertical gradients provide important insights into the chemical processing of the different species. Located in a pristine rain forest region in the central Amazon Basin about 150 km north-east of Manaus, the ATTO site offers a rare possibility to study the chemical processing of tropospheric trace gases far from major anthropogenic emission sources. 

In the presented study, a general overview of the trace gas and aerosol results covering several years is provided. Thereby, a specific focus is put on the HCHO and glyoxal results including the annual variations of their abundances in the course of the characteristic alternation between wet and dry seasons. Based on the ratio of their abundances, our measurements indicate that different precursor compositions of both species prevail in the different seasons, whereby in the wet season the relative amount of precursors favouring the formation of glyoxal, e.g. monoterpenes, appears to be larger than in the dry season. In addition, (small scale) vertical gradients in the altitude range between both instruments are presented. Our results suggest that HCHO is mostly formed in the lowest 200 m above the canopy, while glyoxal is already degraded in this altitude range. Together with their characteristic profile shapes, these findings indicate different chemical processing (production and degradation) of HCHO and glyoxal, despite similar sources of their precursors.