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Novel Approach for Evaluating Secondary Organic Aerosol from Aromatic Hydrocarbons: Unified Method for Predicting Aerosol Composition and Formation Lijie Li, Ping Tang, Shunsuke Nakao, Mary Kacarab, and David R. Cocker Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05778 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016
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Environmental Science & Technology
CnH2n hv, NOx
Aromatic Hydrocarbons
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C6+nH6+2nO4
Aerosol
Environmental Science & Technology
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Novel Approach for Evaluating Secondary Organic Aerosol from Aromatic Hydrocarbons: Unified Method for Predicting Aerosol Composition and Formation
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Lijie Li†, ‡, Ping Tang†,‡, Shunsuke Nakao†,
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III* †,
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† University of California, Riverside, Department of Chemical and Environmental
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Engineering, UC Riverside, CA 92521, USA
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‡College of Engineering – Center for Environmental Research and Technology
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‡, §
, Mary Kacarab†, ‡, David R. Cocker
‡
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(CE-CERT), UC Riverside, CA 92507, USA
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§ Currently at Clarkson University, Department of Chemical and Biomolecular
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Engineering, Potsdam, NY 13699, USA
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KEYWORDS
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SOA yield; Elemental ratio; Aromatic hydrocarbon; Isomer
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ABSTRACT
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Innovative SOA composition analysis methods normalizing aerosol yield and
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chemical composition on an aromatic ring basis are developed and utilized to explore
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aerosol formation from oxidation of aromatic hydrocarbons. SOA yield and chemical
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composition are revisited using fifteen years of UC Riverside/CE-CERT
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environmental chamber data on 17 aromatic hydrocarbons with HC:NO ranging from
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11.1 to 171 ppbC:ppb. SOA yield is redefined in this work by normalizing the
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molecular weight of all aromatic precursors to the molecular weight of the aromatic
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ring ( = ×
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normalization process demonstrates that the amount of aromatic rings present is a
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more significant driver of aerosol formation than the vapor pressure of the precursor
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aromatic. Yield normalization also provided a basis to evaluate isomer impacts on
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SOA formation. Further, SOA elemental composition is explored relative to the
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aromatic ring rather than on a classical mole basis. Generally, four oxygens per
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aromatic ring are observed in SOA, regardless of the alkyl substitutes attached to the
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ring. Besides the observed SOA oxygen to ring ratio (O/R ~4), a hydrogen to ring
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ratio (H/R) of 6+2n is observed, where n is the number of non-aromatic carbons.
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Normalization of yield and composition to the aromatic ring clearly demonstrates the
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greater significance of aromatic ring carbons compared with alkyl carbon substituents
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in determining SOA formation and composition.
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1 INTRODUCTION
; i- aromatic hydrocarbon precursor). The yield
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Challenges remain in predicting anthropogenic secondary organic aerosol (SOA)
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with anthropogenic SOA sources underestimated in current models by a factor of upto
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10.1-9 It is imperative to develop SOA predictive frameworks in order to help models
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predict SOA formation from a wide variety of aerosol precursors given limited
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experimental resources. Aromatic hydrocarbons are an example of a major class of
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anthropogenic SOA precursors6,10-12 that can benefit from such a predictive
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framework as their SOA formation is explored for increasingly relevant atmospheric
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conditions.
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Importing reasonable SOA formation parameters into models is essential for
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accurate estimates of global and regional SOA budgets. SOA yield is a function of the
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extent of gas-to-particle conversion, which depends on the vapor pressure of the
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absorbing species.13 Recent studies also demonstrate the importance of aqueous
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chemistry14 and heterogonous reaction.15 However, collecting reliable SOA
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parameters for every aerosol precursor requires significant effort and time. Therefore,
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a general rule to predict SOA yield and chemical composition by precursor category
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is valuable for SOA prediction. Traditionally, two-product models simulate SOA
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yield by categorizing all oxidation products into two lumped semi-volatile products
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according to their gas-particle phase partitioning coefficient.13, 16 The volatility basis
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set (VBS) stems from the two-product model by assigning VOC oxidation products to
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volatility “bins”, which spans across multiple ambient organic effective saturation
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mass concentrations (C∗).17 Several models have also been established based on the
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two product and/or VBS model by adding tunable parameters that estimate chemical
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polarity, polymerization, fragmentation, functionalization and elemental ratio.18-21
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More complicated models are also developed considering explicit gas and particle
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phase reaction mechanisms22-24 and phase state impact.5,
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models are calibrated and tuned with alkanes21, 24 and biogenic SOA precursors.19, 20
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Ensuring the accuracy of SOA prediction form aromatic hydrocarbon photooxidation
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under ambient conditions improves the SOA prediction in current models.27 Due to
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the limited understanding of aromatic oxidation chemistry, the applicability of these
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models to a variety of aromatic hydrocarbons requires further work.
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Newly developed
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Earlier two-product model fittings to aromatic hydrocarbon oxidation are not
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applicable to ambient atmospheric conditions28, 29 as they were conducted at NOx
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concentrations that are not atmospherically relevant (>100ppb~1000ppb). Recent
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studies on the photooxidation of aromatic hydrocarbons at lower NOx levels only
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focus on a few selected aromatic precursors.30-32 Therefore, a new systematic study on
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SOA formation as a function of aromatic molecular structure is required at more
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relevant atmospheric conditions to elucidate how these compounds are truly behaving
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in the atmosphere. Odum’s (1997) work summarizes the SOA yields formed from the
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oxidation of aromatics into two groups (low yield and high yield group). This work
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improves upon this earlier work by providing SOA yields at more realistic conditions
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(e.g., NOx concentration simulating urban regions) with tighter controls on the
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environmental conditions (e.g., temperature and light intensity). Our earlier work33
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found traditional mass based SOA yields for aromatic hydrocarbons are associated
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with the number of alkyl substitutes in aromatic precursor. Further studies are still
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needed to explore the underlying mechanisms leading to the relationship in between
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SOA formation and aromatic hydrocarbon molecular structure.
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Elemental analysis34,
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can help elucidate SOA chemical composition and
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formation mechanisms.36, 37 Previous SOA product studies observed a decrease in O/C
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and an increase in H/C with the addition of alkyl substitutes attached to the precursor
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aromatic ring,37-39 resulting in a lower measured oxidation state of carbon (OSc)40
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with increasing number of alkyl substitutes. However, the low H/C ratio or high
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degree of unsaturation (Double Bond Equivalent=4) of the aromatic ring makes the
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H/C of aromatic hydrocarbons more dependent on the number of substituted alkyl
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carbons than more saturated precursors. This dependence of H/C on alkyl carbon
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substitutes is likely retained in aromatic SOA products. Further, the carbon on the
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aromatic ring may behave differently than the alkyl substitute carbon with respect to
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overall O/C. Therefore, evaluating the extent of aromatic hydrocarbon oxidation
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solely relying on average SOA H/C and O/C without distinguishing between alkyl
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substitute and aromatic ring carbons may conceal the compositional similarity among
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SOA from different aromatics. Refinement of the SOA elemental ratio interpretation
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is required to clarify the contribution of different functional groups to the SOA
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formation from aromatic hydrocarbons.
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The motivation of this study is to improve the understanding of aromatic
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hydrocarbon SOA formation using innovative methods developed within this study.
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SOA yield and chemical composition are reanalyzed on the basis of the aromatic ring
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using fifteen years of UC Riverside/CE-CERT chamber data on 17 aromatic
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hydrocarbons with HC:NO ratio ranging from 11.1 ppbC:ppb to 171 ppbC:ppb.
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Specific and general alkyl substitute or molecular structure impacts to SOA formation
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from single ring aromatic hydrocarbons are explored.
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2 METHODS
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All experiments (Table S1) were conducted in the UC Riverside/CE-CERT indoor
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dual 90 m3 environmental chambers.41 Experiments were conducted at dry conditions
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(RH
