Formation and aging of biogenic secondary organic aerosol in aqueous aerosol particles containing reactive nitrogen

Biogenic secondary organic aerosols (BSOA) are formed from oxidation of biogenic volatile organic compounds (BVOC). They are the main contributor to the global SOA fluxes with the values still remaining highly uncertain. The formation mechanism of BSOA has been studied extensively, however few studies up to date have been conducted at elevated relative humidity (85-95%) and in the presence of different aqueous inorganic seeds. These conditions are more realistic e.g., under night-time conditions or in coastal or tropical regions. As two typical and abundant BVOC, we focus on the SOA formation from isoprene and α-pinene. Experiments are conducted inside the AIDA aerosol and cloud simulation chamber under various humidities and the presence of different seed aerosols at atmospherically relevant conditions. During the campaign, NaCl, NH₄NO₃ or (NH₄)₂SO₄ seed aerosol particles are introduced into the chamber, followed by the oxidation of isoprene or α-pinene. Formation and aging of oxidation products are measured in the gas and condensed phase in the dark and with simulated solar radiation.

Our results show that the SOA mass production is enhanced under higher relative humidities. High-resolution aerosol mass spectrometry data show a higher oxidation state of SOA formed under higher humidities, suggesting further oxidation of SOA products within the condensed phase. Further molecular analysis on the particle phase oxidation products with FIGAERO-CIMS suggests that the increased oxidation state can mainly be explained by the production of HOMs and low-molecular weight dicarboxylic acids during the aging process, especially under illumination. Our observations indicate the critical influence of relative humidity and pre-existing seed aerosol composition on different secondary organic aerosol formation mechanisms, particularly through potential aqueous-phase reaction pathways.