Oxidation of isoprene, the biogenic volatile organic compound with the highest emissions globally, is a large source of secondary organic aerosol (SOA) in the atmosphere. Organosulfates, particularly methyltetrol sulfates, formed from acid-driven reactions of isoprene epoxydiols (IEPOX), a key oxidation product, are important contributors to SOA mass. To date, most studies have focused on organosulfate formation on ammonium sulfate particles at low pH. However, recent work has shown sea spray aerosol (SSA) in the accumulation mode (~100 nm) is often quite acidic (pH ~ 2) and IEPOX-derived organosulfates have been identified in marine environments. Herein, we demonstrate that substantial SOA, including organosulfates, are formed on acidic sodium sulfate particles (pH = 1.3), representative of marine aerosol heterogeneously reacting with H2SO4 to form Na2SO4. For acidic sodium and ammonium sulfate particles, 31 and 28% (±1%), respectively, of inorganic sulfate is incorporated into organosulfate species, even though acidic particles with sodium versus ammonium as the primary cation formed 5% (±0.2) less SOA volume and 45% (±6%) less methyltetrol sulfates, suggesting other organosulfates may form. Even though both exhibited core-shell morphology after IEPOX uptake, physicochemical differences were observed via Raman microspectroscopy, with organosulfates identified in both the core and shell of acidic ammonium sulfate SOA particles, but only in the core for acidic sodium sulfate SOA via Raman microspectroscopy. Our results suggest that isoprene-derived SOA formed on aged SSA is potentially an important, but underappreciated, source of SOA and organosulfates in marine and coastal regions and could modify SOA budgets in these environments.
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Organosulfate Formation in Proxies for Aged Sea Spray Aerosol: Reactive Uptake of Isoprene Epoxydiols to Acidic Sodium Sulfate
Oxidation of isoprene, the biogenic volatile organic compound with the highest emissions globally, is a large source of secondary organic aerosol (SOA) in the atmosphere. Organosulfates, particularly methyltetrol sulfates formed from acid-driven reactions of the oxidation product isoprene epoxydiol (IEPOX) onto particulate sulfate, are important contributors to SOA mass. To date, most studies have focused on organosulfate formation on ammonium sulfate particles at low pH. However, recent work has shown that sea spray aerosol (SSA) in the accumulation mode (~100 nm) is often quite acidic (pH ~ 2). Marine biota are well-established sources of isoprene, with annual global oceanic fluxes of isoprene estimated to range from 1-12 Tg, and IEPOX-derived organosulfates have been identified in marine environments. Herein, we demonstrate that substantial SOA, including organosulfates, are formed on acidic sodium sulfate particles, representative of marine aerosol heterogeneously reacting with H2SO4 to form Na2SO4. We compare SOA formed from the reactive uptake of IEPOX onto particulate sulfate and find that the cation (sodium vs. ammonium) impacts the physical properties and chemical composition of the SOA formed. Additionally, we investigate the formation of SOA derived from sodium sulfate based on key properties including particle acidity and the extent of exposure to oxidation via OH radicals. Our results suggest that isoprene-derived SOA formed on aged SSA is potentially an important, but underappreciated, source of SOA and organosulfates in marine and coastal regions and could modify SOA budgets and composition in these environments.
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- Award ID(s):
- 2001027
- NSF-PAR ID:
- 10392030
- Date Published:
- Journal Name:
- 2022 Fall AGU Meeting
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Isoprene has a strong effect on the oxidative capacity of the troposphere due to its abundance. Under low-NOx conditions, isoprene oxidizes to form isoprene-derived epoxydiols (IEPOX), contributing significantly to secondary organic aerosol (SOA) through heterogeneous reactions. In particular, organosulfates (OSs) can form from acid-driven reactive uptake of IEPOX onto preexisting particles followed by nucleophilic addition of inorganic sulfate, and they are an important component of SOA mass, primarily in submicron particles with long atmospheric lifetimes. Fundamental understanding of SOA and OS evolution in particles, including the formation of new compounds by oxidation as well as corresponding viscosity changes, is limited, particularly across relative humidity (RH) conditions above and below the deliquescence of typical sulfate aerosol particles. In a 2-m3 indoor chamber held at various RH values (30 – 80%), SOA was generated from reactive uptake of gas-phase IEPOX onto acidic ammonium sulfate aerosols (pH = 0.5 – 2.5) and then aged in an oxidation flow reactor (OFR) for 0 – 24 days of equivalent atmospheric ·OH exposure. We investigated the extent of inorganic sulfate conversion to organosulfate, formation of oligomers, single-particle physicochemical properties, such as viscosity and phase state, and oxidation kinetics. Chemical composition of particle-phase species, as well as aerosol morphological changes, are analyzed as a function of RH, oxidant exposure times, and particle acidity to better understand SOA and OS formation and destruction mechanisms in the ambient atmosphere.more » « less
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