Photolysis Controls Atmospheric Budgets of Biogenic Secondary Organic Aerosol

Abstract

Secondary organic aerosol (SOA) accounts for a large fraction of the tropospheric particulate matter. Although SOA production rates and mechanisms have been extensively investigated, loss pathways remain uncertain. Most large-scale chemistry and transport models account for mechanical deposition of SOA but not chemical losses such as photolysis. There is also a paucity of laboratory measurements of SOA photolysis, which limits how well photolytic losses can be modeled. Here, we show, through a combined experimental and modeling approach, that photolytic loss of SOA mass significantly alters SOA budget predictions. Using environmental chamber experiments at variable relative humidity between 0 and 60%, we find that SOA produced from several biogenic volatile organic compounds undergoes photolysis-induced mass loss at rates between 0 and 2.2 ± 0.4% of nitrogen dioxide (NO 2 ) photolysis, equivalent to average atmospheric lifetimes as short as 10 h. We incorporate our photolysis rates into a regional chemical transport model to test the sensitivity of predicted SOA mass concentrations to photolytic losses. The addition of photolysis causes a ∼50% reduction in biogenic SOA loadings over the Amazon, indicating that photolysis exerts a substantial control over the atmospheric SOA lifetime, with a likely dependence upon the SOA molecular composition and thus production mechanisms.

New SOA Treatments Within the Energy Exascale Earth System Model (E3SM): Strong Production and Sinks Govern Atmospheric SOA Distributions and Radiative Forcing

“…It is likely that there could be compensating errors in SOA‐related to stronger photolysis and weaker wet deposition in E3SM. Laboratory measurements by Zawadowicz et al (2020) suggest that isoprene and monoterpene SOA decays at the rate of 1.5% and 0.8% of JNO 2 , respectively, which are higher than the 0.04% of JNO 2 values used in this study and previous studies (Hodzic et al, 2016; Malecha et al, 2018). However, Zawadowicz et al (2020) also reported that 10% and 30% of isoprene and monoterpene SOA, respectively, did not undergo photolytic loss, which should be taken into account in future studies.…”

“…Although the model suggests almost complete photolytic removal of SOA at high altitudes, recent studies showed that a significant fraction of SOA does not photolyze, and this nonphotolabile SOA depends on SOA composition (O'Brien & Kroll, 2019; Zawadowicz et al, 2020). Although not considered in this study, heterogeneous chemistry with OH radicals becomes slower at higher altitudes as OH concentrations (molecule cm −3 ) decrease with altitudes (Figure S4b in the supporting information).…”

“…Zawadowicz et al (2020) measured a higher photolysis rate constant for SOA generated from some precursors (up to 2% of the NO 2 photolysis) as compared to the Hodzic et al (2016) rate constant of 0.04%. However, previous studies also showed that a fraction of SOA does not photolyze (Braman et al, 2020; O'Brien & Kroll, 2019; Zawadowicz et al, 2020); for example, 20% of α‐pinene and β‐caryophyllene SOA, as well as 10% of isoprene SOA, were measured to be nonphotolabile in their environmental chamber measurements. In addition to wet removal and photolytic loss, recent studies suggest that the heterogenous reactions of gas‐phase oxidants (i.e., ozone and hydroxyl radical) with SOA could be additional sinks of SOA (Hu et al, 2016; Yu et al, 2019).…”

“…Laboratory studies also suggest that photolysis of SOA particles can remove tropospheric aerosols on time scales comparable to those of wet scavenging, and the photolysis rate depends on SOA composition, for example, differences in photolysis rates of isoprene versus terpene precursor‐derived SOA and O/C ratios (Epstein et al, 2014; Henry & Donahue, 2012; Malecha et al, 2018; O'Brien & Kroll, 2019; Wong et al, 2014; Zawadowicz et al, 2020). Hodzic et al (2016) investigated the role of photolysis on simulated SOA loadings and lifetimes in the atmosphere using a global model (GEOS‐Chem).…”

“…Recently, using WRF‐Chem simulations, Zawadowicz et al (2020) showed that photolysis decreases simulated SOA concentrations by 50–66% within the boundary layer in the Amazon. Zawadowicz et al (2020) measured a higher photolysis rate constant for SOA generated from some precursors (up to 2% of the NO 2 photolysis) as compared to the Hodzic et al (2016) rate constant of 0.04%. However, previous studies also showed that a fraction of SOA does not photolyze (Braman et al, 2020; O'Brien & Kroll, 2019; Zawadowicz et al, 2020); for example, 20% of α‐pinene and β‐caryophyllene SOA, as well as 10% of isoprene SOA, were measured to be nonphotolabile in their environmental chamber measurements.…”