Influence of humidity and iron(iii) on photodegradation of atmospheric secondary organic aerosol particles

The absorption of solar actinic radiation by atmospheric secondary organic aerosol (SOA) particles drives condensed-phase photochemical processes, which lead to particle mass loss by the production of CO, CO2, hydrocarbons, and various oxygenated volatile organic compounds (OVOCs). We examined the influence of relative humidity (RH) and Fe(III) content on the OVOC release and subsequent mass loss from secondary organic aerosol material (SOM) during UV irradiation. The samples were generated in a flow tube reactor from the oxidation of D-limonene by ozone. The SOM was collected with a Micro Orifice Uniform Deposit Impactor (MOUDI) on CaF2 windows. To selected samples, a variable amount of FeCl3 was added before irradiation. The resulting SOM samples, with or without added FeCl3, were irradiated with a 305 nm light-emitting diode and the release of several OVOCs, including acetic acid, acetone, formic acid and acetaldehyde, was measured with a Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS). The release of OVOCs from photodegradation of SOM at typical ambient mid-values of RH (30–70%) was 2–4 times higher than under dry conditions. The release of OVOCs was slightly enhanced in the presence of low concentrations of iron (0.04 Fe molar ratio) but it was suppressed at higher concentrations (0.50 Fe molar ratio) of iron indicating the existence of a complicated radical chemistry driving the photodegradation of SOM. Our findings suggest that the presence of iron in atmospheric aerosol particles will either increase or decrease release of OVOCs due to the photodegradation of SOM depending on whether the relative iron concentration is low or high, respectively. At atmospherically relevant RH conditions, the expected fractional mass loss induced by these photochemical processes from limonene SOA particles would be between 2 and 4% of particle mass per hour. Therefore, photodegradation is an important aging mechanism for this type of SOA.