- For Staff
Organic aerosols are the dominant constituents of submicron aerosols worldwide, and yet our understanding of their atmospheric lifecycle is limited. Current regional-scale and global models describe the emissions, photodissociation and chemistry of trace gases important to secondary organic aerosol (SOA) formation, as well as the removal by direct deposition and precipitation. However, these models largely ignore photodissociation in the condensed phase and the subsequent chemical reactions. Many SOA exhibit strong absorption in the ultraviolet part of the solar spectrum (λ < 400 nm), where photon energies are sufficient to break chemical bonds.
We have used the hyper-explicit chemical model GECKO-A (Generator of Explicit Chemistry and Kinetics for Organics in the Atmosphere) to quantify the sensitivity of SOA mass evolution to photolysis reactions, in both the gas-phase and condensed-phase under several assumptions. The results (Fig. 1) show that condensed-phase photolysis can reduce SOA concentrations by a factor of 4 or more. Incorporation of condensed-phase photolysis in a global model (Fig. 2) suggests that condensed-phase photolysis may be a major global sink for SOA, being comparable to losses by dry and wet deposition. The relative importance of photolytic losses increases with exposure time, leading to especially large effects in aged air far from sources or at high altitudes.
Figure 1: Secondary Organic Aerosol (SOA), for different photolysis assumptions:
Blue – no photolysis of organic vapors. Red – photolysis of organic vapors only.
Yellow – photolysis of gas and condensed organics, all with gas-phase J-values.
Black – photolysis of gas and condensed organics, the latter with J = 0.04% JNO2.
[from ref. 1]
Figure 2: Change (%) in SOA mass concentration in the PBL (0-1.5 km) due to
hypothesized photolytic loss, GEOS-Chem model 2005-2008 average. [from ref. 2]
More information about the model GECKO-A can be found online at https://www2.acom.ucar.edu/modeling/gecko.
 Hodzic, A., S. Madronich, P. S. Kasibhatla, G. Tyndall, B. Aumont, J. L. Jimenez, J. Lee-Taylor, and J. Orlando, Organic photolysis reactions in tropospheric aerosols: effect on secondary organic aerosol formation and lifetime, Atmos. Chem. Phys., 15, 9253-9269, 2015.
 Hodzic, A., P. S. Kasibhatla, D. S. Jo, C. Cappa, J. L. Jimenez, S. Madronich, and R. J. Park, Rethinking the global secondary organic aerosol (SOA) budget: stronger production, faster removal, shorter lifetime, Atmos. Chem. Phys., 16, 7917-7941, 2016.