Investigating Carbonyl Compounds above the Amazon Rainforest using PTR-ToF-MS with NO⁺ Chemical Ionization

Abstract. The photochemistry of carbonyl compounds significantly influences tropospheric chemical composition by altering the local oxidative capacity, free radical abundance in the upper troposphere, and formation of ozone, PAN, and secondary organic aerosol particles. Carbonyl compounds can be emitted directly from the biosphere into the atmosphere and are formed through photochemical degradation of various precursor compounds. Aldehydes have atmospheric lifetimes of hours to days, in contrast to ketones, which persist for up to several weeks. While standard operating conditions for proton transfer time‑of‑flight mass spectrometer (PTR-ToF-MS) using H₃O⁺ ions are unable to separate aldehydes and ketones, the use of NO⁺ reagent ions allows for the differential detection of isomeric carbonyl compounds with a high time resolution. Here we study the temporal (24 h) and vertical (80–325 m) variability of individual carbonyl compounds in the Amazon rainforest atmosphere with respect to their rainforest-specific sources and sinks. We found strong sources of ketones within or just above the rainforest canopy (acetone, MEK, and C₅-ketones). A common feature of the carbonyls was nocturnal deposition observed by loss rates, most likely since oxidized volatile organic compounds are rapidly metabolized and utilized by the biosphere. With NO⁺ chemical ionization, we show that the dominant carbonyl species include acetone and propanal, which are present at a ratio of 1:10 in the wet–to–dry transition and 1:20 in the dry season.