Abstract
Abstract. 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.
Highlights
Secondary organic aerosol (SOA), formed via the oxidation of volatile organic compounds (VOCs) emitted from human activities and vegetation, has important impacts on climate (Shrivastava et al, 2017) and human health (Daellenbach et al, 2020)
The variation in SOA yields can be caused by different chemical regimes, resulting, for example, in different branching ratios for the peroxy radicals (RO2) + hydroperoxyl radical (HO2), RO2 + NO3, and RO2 + RO2 pathways after the initial RO2 formation via NO3 + VOCs
As discussed in Bell et al (2021), even in experiments where the N2O5 : VOC ratio is high (∼ 3) and the MCM model predicts the RO2 + NO3 pathway to be dominant, the initial composition of BSOANO3 formed here in the dark is still dominated by RO2 + RO2 reactions, resulting in substantial fractions of oligomers and indicating that the rate of the RO2 + RO2 pathway is likely underestimated by the MCM model
Summary
Secondary organic aerosol (SOA), formed via the oxidation of volatile organic compounds (VOCs) emitted from human activities (anthropogenic) and vegetation (biogenic), has important impacts on climate (Shrivastava et al, 2017) and human health (Daellenbach et al, 2020). Reactions influencing particulate lifetime of ONs include oxidation, hydrolysis (Pye et al, 2015; Takeuchi and Ng, 2019), and photolysis (Müller et al, 2014; Suarez-Bertoa et al, 2012). They change both chemical and physical properties of SOA and ONs, and have different impacts on the atmospheric NOx budget
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