Organic nitrates in upper tropospheric aerosol: Results from airborne measurements over the Amazon

We conducted aircraft-based aerosol composition measurements in the upper troposphere (UT) over the Amazonian rainforest using the HALO research aircraft during the CAFE-Brazil (Chemistry of the Atmosphere – Field Experiment in Brazil) mission. In December 2022 and January 2023, a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS; Schulz et al., 2018) was operated on 22 flights (including test and ferry flights) at altitudes up to 14 km. The measurements show that organic compounds dominate the aerosol composition in the tropical UT, with a significant contribution from organic nitrates. Organic nitrates can form during secondary organic aerosol (SOA) production via reactions of volatile organic carbon (VOC) precursors (e.g., isoprene) with OH and/or O₃ in the presence of NO_x. NO_x is observed to be abundant in the tropical UT (NO up to 300 pptv; Nussbaumer et al., 2024) with a major source of frequent lightning activity in convective thunderstorms, and the low temperatures aloft appear to favor organic nitrate formation (Curtius et al., 2024).

To distinguish inorganic from organic particulate nitrate, we use the NO₂⁺ (m/z 46) and NO⁺ (m/z 30) ion ratio in the C-ToF-AMS mass spectra, which has been shown to indicate the presence of organic nitrates (e.g., Day et al., 2022). Inorganic ammonium nitrate, used for calibration, exhibits a markedly higher NO₂⁺/NO⁺ ratio than organic nitrates. Our data show that in the UT, as sampled here at altitudes above 10 km, nitrate is predominately present as ammonium nitrate in the extratropics (> 23° N), whereas in the tropics (< 23° N), nitrate occurs mainly as organic nitrate. The ferry flights between Germany and Brazil clearly capture this transition when entering and leaving the tropical region.

Organic nitrates have also been identified as a key component in new particle formation from isoprene in the UT over the Amazon (Curtius et al., 2024; Shen et al., 2024; Russell et al., 2025). As the C-ToF-AMS detects particles larger than about 50 nm, our observations indicate that organic nitrates are essential not only for new particle formation but also for the subsequent particle growth in the tropical UT. They therefore represent a major source of cloud condensation nuclei for the middle and lower troposphere in tropical regions.

 

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