Night-time reactions of biogenic volatile organic compounds (BVOCs) and nitrate radicals (NO3) can lead to the formation of NO3-initiated biogenic secondary organic aerosol (BSOANO3). Here, we study the impacts of light exposure on the chemical composition and volatility of BSOANO3 formed in the dark from three precursors (isoprene, α-pinene, and β-caryophyllene) in atmospheric simulation chamber experiments. Our study represents BSOANO3 formation conditions where reactions between peroxy radicals (RO2 + RO2) and between RO2 and NO3 are favoured. The emphasis here is on the identification of particle-phase organonitrates (ONs) formed in the dark and their changes during photolytic ageing on timescales of ∼ 1 h. The chemical composition of particle-phase compounds was measured with a chemical ionization mass spectrometer with a filter inlet for gases and aerosols (FIGAERO-CIMS) and an extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF). Volatility information on BSOANO3 was derived from FIGAERO-CIMS desorption profiles (thermograms) and a volatility tandem differential mobility analyser (VTDMA). During photolytic ageing, there was a relatively small change in mass due to evaporation (< 5 % for the isoprene and α-pinene BSOANO3, and 12 % for the β-caryophyllene BSOANO3), but we observed significant changes in the chemical composition of the BSOANO3. Overall, 48 %, 44 %, and 60 % of the respective total signal for the isoprene, α-pinene, and β-caryophyllene BSOANO3 was sensitive to photolytic ageing and exhibited decay. The photolabile compounds include both monomers and oligomers. Oligomers can decompose into their monomer units through photolysis of the bonds (e.g. likely O–O) between them. Fragmentation of both oligomers and monomers also happened at other positions, causing the formation of compounds with shorter carbon skeletons. The cleavage of the nitrate functional group from the carbon chain was likely not a main degradation pathway in our experiments. In addition, photolytic degradation of compounds changes their volatility and can lead to evaporation. We use different methods to assess bulk volatilities and discuss their changes during both dark ageing and photolysis in the context of the chemical changes that we observed. We also reveal large uncertainties in saturation vapour pressure estimated from parameterizations for the ON oligomers with multiple nitrate groups. Overall, our results suggest that photolysis causes photodegradation of a substantial fraction of BSOANO3, changes both the chemical composition and the bulk volatility of the particles, and might be a potentially important loss pathway of BSOANO3 during the night-to-day transition.

Highly oxygenated organic molecules (HOMs) are formed from the oxidation of biogenic and anthropogenic gases and affect Earth's climate and air quality by their key role in particle formation and growth. While the formation of these molecules in the gas phase has been extensively studied, the complexity of organic aerosol (OA) and lack of suitable measurement techniques have hindered the investigation of their fate post-condensation, although further reactions have been proposed. We report here novel real-time measurements of these species in the particle phase, achieved using our recently developed extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF). Our results reveal that condensed-phase reactions rapidly alter OA composition and the contribution of HOMs to the particle mass. In consequence, the atmospheric fate of HOMs cannot be described solely in terms of volatility, but particle-phase reactions must be considered to describe HOM effects on the overall particle life cycle and global carbon budget.

The formation of new atmospheric particles involves an initial step forming stable clusters less than a nanometre in size (<similar to 1 nm), followed by growth into quasi-stable aerosol particles a few nanometres (similar to 1-10 nm) and larger (>similar to 10 nm). Although at times, the same species can be responsible for both processes, it is thought that more generally each step comprises differing chemical contributors. Here, we present a novel analysis of measurements from a unique multi-station ground-based observing system which reveals new insights into continental-scale patterns associated with new particle formation. Statistical cluster analysis of this unique 2-year multi-station dataset comprising size distribution and chemical composition reveals that across Europe, there are different major seasonal trends depending on geographical location, concomitant with diversity in nucleating species while it seems that the growth phase is dominated by organic aerosol formation. The diversity and seasonality of these events requires an advanced observing system to elucidate the key processes and species driving particle formation, along with detecting continental scale changes in aerosol formation into the future.