Photodegradation of Secondary Organic Aerosols by Long-Term Exposure to Solar Actinic Radiation

Abstract

Sunlight-driven chemical transformations of secondary organic aerosol (SOA) are important for understanding the climate- and health-relevant properties of atmospheric particulate matter, but these photochemical processes are not well understood. We measured the photodegradation rates of SOA by observing condensed-phase photochemical processes over many days of UV exposure. The experiments relied on a quartz crystal microbalance to quantify the mass loss rate from SOA materials prepared by ozonolysis of d-limonene and α-pinene and photo-oxidation of toluene under either high or low NO x conditions. We observed that 254 nm irradiation degraded SOA almost entirely after 24 h. The mass loss rates were higher for toluene-derived SOA, which absorbs strongly at 254 nm. Irradiation at 305 nm, which is more relevant for the troposphere, resulted in larger mass loss rates from SOA generated from α-pinene and d-limonene, even though toluene-derived SOA had a higher absorption coefficient. In all 305 nm irradiation experiments, the initial mass loss rate was high (corresponding to 1–5% fractional mass loss per hour), but it slowed down after 24 h of irradiation, with a photorecalcitrant fraction of SOA degrading much slower (<1% fractional mass loss per hour). The mass loss rates were observed to increase at a higher relative humidity because volatile photoproducts could diffuse out of SOA faster. Long-term changes in the chemical composition of limonene ozonolysis SOA were examined using high-resolution electrospray ionization mass spectrometry and revealed a more complex mixture of species after photodegradation compared to the initial SOA. The compounds in the photodegraded sample had on average lower molecular weights, lower H/C ratios, and higher O/C ratios compared to the compounds in the un-photolyzed sample. These experiments confirm that condensed-phase photochemistry is an important aging mechanism for SOA during long-range transport.

Fates of Organic Hydroperoxides in Atmospheric Condensed Phases

“…This result is consistent with the finding that peroxide groups are not the main photolabile groups in SOA generated via α-pinene ozonolysis . The implication is that the peroxide moiety does not contribute to solar photolytic aging of SOA. , …”

“…5 The implication is that the peroxide moiety does not contribute to solar photolytic aging of SOA. 78,79 An experimental investigation has revealed that ROOH (R = CH 3 or C 2 H 5 ) + Fe 2+ produces Fe 3+ + RO • + OH − in water at 279 K and at pH 2 with k = 16 ± 5 and 24 ± 9 M −1 s −1 , respectively. 80 Given that [Fe 2+ ] ≈ 10 −6 M, that [ROOH] ≈ 10 −6 M in atmospheric aqueous aerosol, and that k ROOH+Fe2+ ≈ 20 M −1 s −1 , 80 the lifetime of ROOH decomposed by Fe 2+ is estimated to be 14 h, which is much longer than the lifetime associated with the H + -catalyzed decomposition (i.e., a few seconds).…”

Aging of Atmospheric Brown Carbon Aerosol

“…247 Based on absorption cross sections from the UV into the visible range for SOA derived from a wide range of precursors, photolysis is likely a broadly relevant process in the aging of particulate BrC in the atmosphere. 115 Photoreactions may also result in changes to mass absorption coefficients, in addition to mass, 248 for particulate BrC. For 2,4-dinitrophenol, a representative BrC aerosol constituent, mixed into deposits of α-pinene and limonene SOA generated under a range of conditions including exposure to ammonia to produce strongly absorbing limonene SOA, irradiation from 290 to 400 nm resulted in excitation to a triplet state followed first by abstraction of hydrogen from another molecule and then an array of products.…”

Environmental effects of stratospheric ozone depletion, UV radiation, and interactions with climate change: UNEP Environmental Effects Assessment Panel, Update 2020

“…New research indicates that UV radiation also plays a role in the destruction of some of these particles. Measurements in the laboratory show that many SOA are photodegraded by UV irradiation [ 366 ], making UV radiation an essential factor in both their production and removal from the atmosphere. According to model calculations [ 367 ], these complex UV-driven processes could represent the largest global sink for SOA, exceeding removal by rain-out (Fig.…”

“…12 ). However, a fraction of SOA appears not to be degraded by UV radiation [ 366 , 368 , 369 ], with the overall effect of the UV radiation depending on the chemical composition of the many different aerosols present in the atmosphere. No attempt has yet been made to quantify the sensitivity of the lifetimes of different SOA to changes in UV radiation as would occur with changes in stratospheric ozone.…”