Peroxy acetyl nitric anhydride (PAN) and peroxy acetic acid (PAA) over the Atlantic west of Africa during CAFE-Africa and the influence of biomass-burning†

Abstract

PAN (CH₃C(O)O₂NO₂) is often the most important chemical reservoir of reactive nitrogen compounds throughout the free- and upper troposphere and provides a means of transport of reactive nitrogen from source regions to more remote locations. Both PAN and PAA (peroxy acetic acid, CH₃C(O)OOH) are formed exclusively via reactions of the CH₃C(O)O₂ radical, with PAA favoured under low NO_X conditions. We present airborne measurements of PAN and PAA taken with a chemical-ionisation mass spectrometer on board the High Altitude-Long range (HALO) aircraft over the North and tropical Atlantic Ocean west of Africa in August–September 2018. Our observations showed that mixing ratios of PAN and PAA are enhanced in biomass-burning impacted air masses and we determined molar enhancement ratios for both trace gases relative to CO and CH₃CN. The PAA-to-PAN ratio was enhanced in biomass-burning impacted air masses compared to background air, which may reflect the continued photochemical formation of PAA in such plumes even after NO_X has been largely depleted. This was confirmed by the large ratio of PAN/(PAN + NO_X), which was on average ≈0.8 at 7–8 km altitude and approached unity in biomass burning impacted air masses. Although no measurements of total reactive nitrogen species (NO_y) or HNO₃ were available, a major fraction of NO_X was likely sequestered in the form of PAN in this region, especially in air masses that had been impacted by biomass burning.