# 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 (NOinline-formula3) can lead to the formation of NOinline-formula3-initiated biogenic secondary organic aerosol (BSOAinline-formula $M4inlinescrollmathml{}_{chem{\mathrm{normal NO}}_{normal 3}}$ 18pt10ptsvg-formulamathimgcf0020b7b4e8a6179d194146c9353e8f acp-21-14907-2021-ie00001.svg18pt10ptacp-21-14907-2021-ie00001.png ). Here, we study the impacts of light exposure on the chemical composition and volatility of BSOAinline-formula $M5inlinescrollmathml{}_{chem{\mathrm{normal NO}}_{normal 3}}$ 18pt10ptsvg-formulamathimg8fb603d12fdbacfaf100344bb539bdac acp-21-14907-2021-ie00002.svg18pt10ptacp-21-14907-2021-ie00002.png formed in the dark from three precursors (isoprene, inline-formulaα-pinene, and inline-formulaβ-caryophyllene) in atmospheric simulation chamber experiments. Our study represents BSOAinline-formula $M8inlinescrollmathml{}_{chem{\mathrm{normal NO}}_{normal 3}}$ 18pt10ptsvg-formulamathimg8df4dd929e497c6c5ac66a4a11c98c6a acp-21-14907-2021-ie00003.svg18pt10ptacp-21-14907-2021-ie00003.png formation conditions where reactions between peroxy radicals (ROinline-formula2inline-formula+ ROinline-formula2) and between ROinline-formula2 and NOinline-formula3 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 inline-formula∼ 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 BSOAinline-formula $M15inlinescrollmathml{}_{chem{\mathrm{normal NO}}_{normal 3}}$ 18pt10ptsvg-formulamathimg9336103c4723420e0a3c3ed1af7ad700 acp-21-14907-2021-ie00004.svg18pt10ptacp-21-14907-2021-ie00004.png 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 (inline-formula< 5 % for the isoprene and inline-formulaα-pinene BSOAinline-formula $M18inlinescrollmathml{}_{chem{\mathrm{normal NO}}_{normal 3}}$ 18pt10ptsvg-formulamathimgfca924adb7d7b3a8d7a20e2c7b026f15 acp-21-14907-2021-ie00005.svg18pt10ptacp-21-14907-2021-ie00005.png , and 12 % for the inline-formulaβ-caryophyllene BSOAinline-formula $M20inlinescrollmathml{}_{chem{\mathrm{normal NO}}_{normal 3}}$ 18pt10ptsvg-formulamathimg7136f5d502a1ef684b6c76ccb240a2a3 acp-21-14907-2021-ie00006.svg18pt10ptacp-21-14907-2021-ie00006.png ), but we observed significant changes in the chemical composition of the BSOAinline-formula $M21inlinescrollmathml{}_{chem{\mathrm{normal NO}}_{normal 3}}$ 18pt10ptsvg-formulamathimg43849c4644252f418b3af1dc5c32280d acp-21-14907-2021-ie00007.svg18pt10ptacp-21-14907-2021-ie00007.png . Overall, 48 %, 44 %, and 60 % of the respective total signal for the isoprene, inline-formulaα-pinene, and inline-formulaβ-caryophyllene BSOAinline-formula $M24inlinescrollmathml{}_{chem{\mathrm{normal NO}}_{normal 3}}$ 18pt10ptsvg-formulamathimg480710eebb7b69032525ed6eec73c891 acp-21-14907-2021-ie00008.svg18pt10ptacp-21-14907-2021-ie00008.png 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 BSOAinline-formula $M25inlinescrollmathml{}_{chem{\mathrm{normal NO}}_{normal 3}}$ 18pt10ptsvg-formulamathimgfc3dba69adb2c7557485ad899802bf03 acp-21-14907-2021-ie00009.svg18pt10ptacp-21-14907-2021-ie00009.png , changes both the chemical composition and the bulk volatility of the particles, and might be a potentially important loss pathway of BSOAinline-formula $M26inlinescrollmathml{}_{chem{\mathrm{normal NO}}_{normal 3}}$ 18pt10ptsvg-formulamathimge1600841231d57f34d39f5f6c5cf6aff acp-21-14907-2021-ie00010.svg18pt10ptacp-21-14907-2021-ie00010.png during the night-to-day transition.

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Wu, Cheng / Bell, David M. / Graham, Emelie L. / et al: Photolytically induced changes in composition and volatility of biogenic secondary organic aerosol from nitrate radical oxidation during night-to-day transition. 2021. Copernicus Publications.

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