Isoprene nitrates drive new particle formation in Amazon’s upper troposphere

For several decades intense new particle formation (NPF) events have been observed by aircraft measurements in the upper tropical troposphere (UTT) (Brock et al. 1995, Weigel et al. 2011, Williamson et al. 2019). These events typically occur above 8 km altitude in the outflow of mesoscale convective systems. The resulting particles can grow further and be transported downwards where they enhance cloud condensation nuclei (CCN) levels over large geographic areas in the tropics. However, the chemical mechanism driving these events remained unclear, as no direct measurements of the involved low-volatility gaseous precursors were possible.

Here, we present in-situ aircraft observations taken on board the High Altitude LOng Range (HALO) aircraft (operated by the German Aerospace Center, DLR) over the Amazon rainforest with the goal of deciphering the chemical mechanism behind NPF in the UTT. The measurements were taken during the CAFE Brazil campaign in December 2022/January 2023, where HALO was stationed in Manaus, Brazil. The aircraft was equipped with a comprehensive suite of instruments measuring both gas- and particle-phase properties. To detect low volatility organic compounds, we operated a purpose-built nitrate Chemical Ionization Mass spectrometer (CIMS).

We show that isoprene nitrates drive new particle formation after sunrise in the upper tropospheric outflow of mesoscale convective systems (Curtius et al. 2024). Isoprene (C₅H₈) is carried from the boundary layer to high altitudes within deep convective cells, while NO_x is produced in these cells via lightning. After sunrise, oxidation of isoprene by OH is initiated, as well as photolytic conversion of NO₂ to NO, which leads to the formation of second generation isoprene nitrates. At the cold temperatures in the upper tropical troposphere (around -60 °C) these molecules have sufficiently low saturation vapour pressure to drive strong new particle formation events, leading to several tens of thousands of particles per cubic centimetre. We find that this process happens frequently over the Amazon at high altitude (>8 km) and might have far reaching consequences for tropical aerosol and CCN production.

We also compare our results with recent findings from the CLOUD experiment (Shen et al. 2024), which studied the capability of isoprene to nucleate at low temperature conditions and find good agreement between field and laboratory measurements.

 

 

 

References:

Brock C. A., et al. (1995), Science, 270, 1650-1653

Weigel, R, et al. (2011), Atmos. Chem. Phys., 11, 9983-10010

Williamson, C. J., et al. (2019). Nature, 574(7778), 399-403.

Curtius, J, et al. (2024). Nature, 636(8041), 124-130.

Shen, J, et al. (2024). Nature, 636(8041), 115-123.