New Particle Formation Events Observed during the COALA-2020 Campaign

Abstract. Aerosols play an important role in atmospheric processes influencing cloud formation, scattering and absorbing solar radiation, and as a part of the chemical reactions affecting the abundance of trace gases in the atmosphere. Ultimately aerosols affect the radiative balance of the earth modifying climate. A large fraction of aerosols is formed through chemical reactions following gas-to-particulate processes in the atmosphere: nucleation, condensation and growth. Biogenic Secondary Organic Aerosols (BSOA) are formed when plant produced volatile organic compounds (VOCs) react in the atmosphere through heterogeneous reactions. South-east Australia is one of the locations with the highest emissions of biogenic VOCs in the world, due to the high density of Eucalyptus species, which are high emitters of VOCs. The COALA-2020 (Characterizing Organics and Aerosol Loading over Australia) campaign worked towards a better understanding of biogenic VOCs in quasi-pristine conditions in the atmosphere and their role in particle formation.

The observations showed a highly reactive atmosphere with frequent new particle formation occurring (50 % days with data) often associated with pollution plumes. Analysis of NPF events indicated that SO₂ and NOx plumes triggered particle formation, while particle growth depended on available VOCs, OH concentration (influenced by relative humidity), and the presence of multiple SO₂ and NOx intrusions promoted growth of smaller clusters. Nighttime NPF events correlated with NOx but the limited night-time data hindered conclusive interpretations. These findings highlight the significant role of biogenic VOCs, especially isoprene, in driving NPF and SOA formation in South-east Australia, even after major wildfires. The COALA-2020 campaign provided valuable insights into local atmospheric chemistry and its potential impact on regional air quality and climate. However, longer-term observations are crucial to understand seasonal variations, trends and extreme events.