Photochemical degradation of iron(III)-citrate/citric acid aerosol quantified with the combination of three complementary experimental techniques and a kinetic process model

Abstract

Abstract. Iron(III) carboxylate photochemistry plays an important role in aerosol aging, especially in the lower troposphere. These complexes can absorb light over a broad wavelength range, inducing the reduction of iron(III) and the oxidation of carboxylate ligands. In the presence of O2, ensuing radical chemistry leads to further decarboxylation, and the production of ·OH, HO2·, peroxides, and oxygenated volatile organic compounds, contributing to particle mass loss. The ·OH, HO2·, and peroxides in turn re-oxidize iron(II) back to iron(III), closing a photocatalytic cycle. This cycle is repeated resulting in continual mass loss due to the release of CO2 and other volatile compounds. In a cold and/or dry atmosphere, organic aerosol particles tend to attain highly viscous states. While the impact of reduced mobility of aerosol constituents on dark chemical reactions has received substantial attention, studies on the effect of high viscosity on photochemical processes are scarce. Here, we choose iron(III)-citrate (FeIII(Cit)) as a model light absorbing iron carboxylate complex that induces citric acid (CA) degradation to investigate how transport limitations influence photochemical processes. Three complementary experimental approaches were used to investigate kinetic transport limitations. The mass loss of single, levitated particles was measured with an electrodynamic balance, the oxidation state of deposited particles was measured with X-ray spectromicroscopy, and HO2· radical production and release into the gas phase was observed in coated wall flow tube experiments. To quantitatively compare these experiments and determine important physical and chemical parameters, a numerical multi-layered photochemical reaction and diffusion (PRAD) model that treats chemical reactions and transport of various species was developed. We observed significant photochemical degradation, with up to 80 % mass loss within 24 hours of light exposure. Interestingly, we also observed that mass loss always accelerated during irradiation, resulting in an increase of the mass loss rate by about a factor of 10. When we increased relative humidity, the observed particle mass loss rate also increased. This is consistent with strong kinetic transport limitations for highly viscous particles. The PRAD model was tuned to reproduce all experimental results and captured the essential chemistry and transport during irradiation. In particular, the photolysis rate of FeIII, the re-oxidation rate of FeII, HO2· production, and the diffusivity of O2 in aqueous FeIII(Cit)/CA system as function of relative humidity and FeIII(Cit) / CA molar ratio could be constrained. Photochemical degradation under atmospheric conditions predicted by the PRAD model shows that release of CO2 and re-partitioning of organic compounds to the gas phase may be very significant to accurately predict organic aerosol aging processes.

Photolytic radical persistence due to anoxia in viscous aerosol particles

“…We used a triad of photochemical experiments to observe photochemical reaction cycles on the nanoscale and microscale in single particles and films including scanning transmission X-ray microscopy coupled with near-edge X-ray absorption fine structure (STXM/NEXAFS) spectroscopy, a single particle electrodynamic balance (EDB), and a coated-wall flow tube (CWFT). Our multifarious observations were supported by a recent photochemical reaction and diffusion (PRAD) model 28 , 29 . We observed that photochemically produced CCRs induce anoxia in the interior of viscous particles, effectively preserving CCRs under dry conditions.…”

“…These low β values are indicative of LMCT, i.e., charge transfer from the citrate ligand to Fe 3+ , forming Fe 2+ and a primary radical product, which immediately decays by decarboxylation (i.e., the removal of a carboxyl group via loss of CO 2 ). Then, an organic, carbon-centered radical, the primary CCR, is left behind 28 . Following the reaction steps in Fig.…”

“…1 . We used the PRAD model 28 to simulate radical persistence due to photochemical reduction and dark reoxidation experiments performed with STXM/NEXAFS (solid lines in Fig. 1c–e ).…”

“…This small mass loss was due to the fact that ROS and CCR, though continuously produced, did not produce and release significant amount of CO 2 or volatile compounds to the gas phase. In general, EDB observations are master constraints for modeling radical chemistry occurring in the condensed phase of citric acid initiated by photolysis of iron carboxylate complexes 28 . All modeled radii decrease (solid lines) for all RH shown in Fig.…”

“…Error bars indicate the standard deviation of multiple measurements or the propagated random error from solution preparation, whichever is larger. Dotted and dashed lines were determined from the PRAD model with k SR = 8.3 × 10 5 M −1 s −1 in dilute aqueous solution, or using the parameterization respectively 28 . Here, M is molarity in units of mole L −1 .…”