Photolytically Induced Changes In Composition And Volatility Of Biogenic Secondary Organic Aerosol From Nitrate Radical Oxidation During Night-To-Day Transition

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 (BSOANO(3)). Here, we study the impacts of light exposure on the chemical composition and volatility of BSOANO(3) formed in the dark from three precursors (isoprene, alpha-pinene, and beta-caryophyllene) in atmospheric simulation chamber experiments. Our study represents BSOANO(3) 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 similar to 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 BSOANO(3) 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 alpha-pinene BSOANO3, and 12 % for the beta-caryophyllene BSOANO(3)), but we observed significant changes in the chemical composition of the BSOANO(3). Overall, 48 %, 44 %, and 60 % of the respective total signal for the isoprene, alpha-pinene, and beta-caryophyllene BSOANO(3) 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 BSOANO(3), changes both the chemical composition and the bulk volatility of the particles, and might be a potentially important loss pathway of BSOANO(3) during the night-to-day transition.