Airborne measurements of peroxyacetyl nitric anhydride (PAN) and peroxyacetic acid (PAA) over the Amazon rainforest: the role of isoprene and the fate of the peroxyacetyl radical

Abstract

PAN (CH₃C(O)O₂NO₂) and PAA (CH₃C(O)OOH) are formed in the atmosphere uniquely and competitively from the same precursor radical (CH₃C(O)O₂), whereby the relative rates of formation of PAN and PAA are determined by the abundances of HO₂ and NO₂. Here, we present airborne measurements of PAN and PAA in an altitude range from 0.3–15 km over the Amazon rainforest during the CAFE Brazil campaign, which took place from December 2022 to January 2023. Median PAA mixing ratios (ca. 340 pptv) across all analysed flights exceed those of PAN (ca. 65 pptv) by more than a factor of 5 in the mid troposphere around 6 km altitude, underlining the dominance of HO₂ over NO₂ chemistry under low NO_X conditions of the Amazon rainforest. Within experimental uncertainty, the median vertical profiles of PAA and PAN throughout the entire measurement campaign are reproduced by the EMAC global chemical transport model. The EMAC analysis reveals that the oxidation of isoprene (emitted from the rainforest) is the primary source of the CH₃C(O)O₂ radical. With an average contribution of ∼31%, the main direct precursor of CH₃C(O)O₂ is methylglyoxal (CH₃C(O)CHO), followed by acetaldehyde (17%) and methyl vinyl ketone (10%). At altitudes between 12 and 14 km, only a few percent (<5%) of CH₃C(O)O₂ radicals formed result in the production of PAN and PAA. The dominant sink of CH₃C(O)O₂ in the upper troposphere is NO, while at lower altitudes, its reaction with other organic peroxy radicals produces acetic acid and CH₃O₂. The CH₃O₂ production resulting (via reactions of CH₃C(O)O₂) from isoprene oxidation over the Amazon rainforest contributes between 9 and 44% to total CH₃O₂ production and surpasses the contribution from methane oxidation at altitudes below 4 km. Through the formation of CH₃C(O)O₂, isoprene oxidation in this region thus significantly influences not only PAN and PAA formation but also the yields of acetic acid as well as methylperoxynitrate, methylhydroperoxide and formaldehyde, which are products of CH₃O₂ reactions with HO₂ and NO, and photochemical precursors to HO_x.