Abstract
Nighttime oxidation of biogenic volatile organic compounds (BVOCs) by nitrate radicals (NO3·) represents one of the most important interactions between anthropogenic and natural emissions, leading to substantial secondary organic aerosol (SOA) formation. The direct climatic effect of such SOA cannot be quantified because its optical properties and atmospheric fate are poorly understood. In this study, we generated SOA from the NO3· oxidation of a series BVOCs including isoprene, monoterpenes, and sesquiterpenes. The SOA were subjected to comprehensive online and offline chemical composition analysis using high-resolution mass spectrometry and optical properties measurements using a novel broadband (315–650 nm) cavity-enhanced spectrometer, which covers the wavelength range needed to understand the potential contribution of the SOA to direct radiative forcing. The SOA contained a significant fraction of oxygenated organic nitrates (ONs), consisting of monomers and oligomers that are responsible for the detected light absorption in the 315–400 nm range. The SOA created from β-pinene and α-humulene was further photochemically aged in an oxidation flow reactor. The SOA has an atmospheric photochemical bleaching lifetime of >6.2 h, indicating that some of the ONs in the SOA may serve as atmosphere-stable nitrogen oxide sinks or reservoirs and will absorb and scatter incoming solar radiation during the daytime.
Highlights
Washenfelder et al.[38] measured aerosol optical properties at a forest site in rural Alabama during the 2013 Southern Oxidant and Aerosol Study (SOAS) campaign. They reported that ∼7% of brown carbon (BrC) absorption could be attributed to the less oxidized oxygenated organic aerosol (LO-OOA) that reached a diel maximum at night and was correlated with particle-phase organic nitrates (ONs), formed by nighttime reactions between monoterpenes and NO3·
They reported that ∼7% of BrC absorption could be attributed to the less oxidized oxygenated organic aerosol (LO-OOA) that reached a diel maximum at night and was correlated with particle-phase ONs, formed by nighttime reactions between monoterpenes and NO3·.23. These findings suggest that the secondary organic aerosol (SOA) produced from reactions of NO3· with biogenic volatile organic compounds (BVOCs) may be a nighttime source of BrC that may affect the direct radiative effect of the SOA through the scattering and absorption of solar radiation
We investigated the relationship between the SOA formation mechanism, the SOA’s chemical composition, and the measured optical properties
Summary
Atmospheric secondary organic aerosols (SOAs) affect radiative forcing by aerosol−radiation interactions and through aerosol−cloud interactions.[1,2] SOAs contain lightabsorbing compounds, called brown carbon (BrC), and play a significant role in the direct climate forcing on regional and local scales.[3,4] Owing to the high emission rates and high reactivities with primary atmospheric oxidants, such as ozone, the hydroxyl radical (OH·), and the nitrate radical (NO3·), vegetation-emitted biogenic volatile organic compounds (BVOCs), such as isoprene (C5H8), monoterpenes (C10H16), ganlodbasel sSqOuiAterbpuerndeesn(.5C−195H24), are the major contributors to the Whereas OH· and ozone (O3) play a key role during daytime atmospheric oxidation, NO3· is a dominant oxidant at night, especially in environments affected by anthropogenic emissions.[10]. The few studies that examined the BSOANO3 reached partially contradictory conclusions.[34−37] For instance, the real part of the refractive index (RI) for the SOA from the NO3· oxidation of β-pinene and limonene was higher than those observed following OH- and ozone-initiated terpene oxidation.[35,37] the real part of the RI for the SOA from the NO3· oxidation of isoprene seems to be similar to that of OH· and ozone-initiated oxidation.[34] absorption was not detected for the SOA from the NO3· oxidation of isoprene, βpinene, and limonene, but significant light absorption at 355 and 405 nm was detected for the SOA formed by NO3·+ αpinene.[36] Washenfelder et al.[38] measured aerosol optical properties at a forest site in rural Alabama during the 2013 Southern Oxidant and Aerosol Study (SOAS) campaign They reported that ∼7% of BrC absorption could be attributed to the less oxidized oxygenated organic aerosol (LO-OOA) that reached a diel maximum at night and was correlated with particle-phase ONs, formed by nighttime reactions between monoterpenes and NO3·.23. This study emphasizes the role of this important chemistry in the climate, air quality, and atmospheric nitrogen cycle
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