Photolytically Induced Changes in Composition and Volatility of Biogenic Secondary Organic Aerosol from Nitrate Radical Oxidation during Night-to-day Transition
- 1Department of Environmental Science, Stockholm University, Sweden
- 2Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland
- 3Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, 00014, Finland
- anow at: Department of Chemistry and Applied Biosciences, ETH Zurich, Switzerland
Abstract. Night-time reactions of biogenic volatile organic compounds (BVOCs) and nitrate radicals (NO3) can lead to the formation of 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 favored. The emphasis here is on the identification of particle-phase organonitrates (ONs) formed in the dark and their changes during photolytic aging on timescales of ~1 h. Chemical composition of particle-phase compounds was measured with a chemical ionization mass spectrometer with filter inlet for gases and aerosols (FIGAERO-CIMS) and an extractive electrospray ionisation time-of-flight mass spectrometer (EESI-TOF). Volatility information of BSOANO3 was derived from FIGAERO-CIMS desorption profiles (thermograms) and a volatility tandem differential mobility analyser (VTDMA). During photolytic aging, there was a relatively small change in mass due to evaporation (< 5 % for the isoprene and α-pinene BSOANO3, 12 % for the β-caryophyllene BSOANO3), but we observed significant changes in the chemical composition of the BSOANO3. Overall, 53 %, 45 %, and 62 % of the total mass for the isoprene, α-pinene, and β-caryophyllene BSOANO3 was sensitive to photolytic aging 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 used different methods to assess bulk volatilities and discuss their changes during both dark aging and photolysis in context of the chemical changes we observed. We also reveal large uncertainties in saturation vapor pressure estimated from parameterization 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.
Cheng Wu et al.
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Cheng Wu et al.
Cheng Wu et al.
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