role in increasing gas phase atmospheric iodine concentrations during daylight hours. In the coastal mid-latitude regions during daytime at low tide, a large source of iodine species is believed to stem from photooxidation of freshly exposed oceanic biota. I 2 undergoes photolysis on the order of 5-10 s as it absorbs sunlight wavelengths of 400-700 nm, whereas CH 2 I 2 likely only undergoes photolysis on the order of 12 min to 13 hrs as it primarily absorbs in the 300-350 nm range. I atoms can react very efficiently with tropospheric O 3 to form IO, 21, with reported polar springtime tropospheric ozone depletion events of 15-30 ppb, which was predominantly attributed to iodine. IO can further undergo photolysis or react with other gas phase species to form larger iodine oxides. These somewhat larger iodine oxides encounter a similarly competitive fate, where photolysis leads to recycling of atmospheric iodine, but gas phase reactions can result in larger iodine oxide molecule formation which can potentially grow into iodine oxide particles (IOPs). Several studies have examined photodissociation of iodine oxide molecules including IO, OIO, and I 2 O 2-3 , along with kinetics/rates of these photolysis reactions. Larger iodine oxides (I 2-5 O 2-12 ) have also been studied, probing photolysis reaction rates and their absorption cross sections, and photolysis reactions for iodate have been proposed. Some of these iodine oxide molecules, such as I 2 O 3 , have small enough absorption cross sections in the actinic region that they likely live long enough to play a role in NPF and clustering. IOP formation appears to be driven by temperature and humidity: under cooler, drier conditions I 2 O 3 and I 2 O 4 are found to polymerize into larger and more abundant IOPs. A further study concluded I 2 O 4 is the most likely of the smaller iodine oxide molecules to initiate NPF, with some likely contribution from I 2 O 5 . Yet another study identified the presence of I 2 O 5 in IOP nanoparticles concluding that IOPs formed in coastal marine boundary layers are likely made up of I 2 O 5 . Anionic species are expected to be important in iodine-driven NPF. Chamber experiments found that ion-induced nucleation driven by IO - 3 was efficient, and suggested that in some conditions it may be the dominant mechanism. Observed clusters generally followed the pattern (I 2 O 5 ) 0-7 (HIO 3 ) 0-1 (IO - 1-3 ). Although neutrals are more abundant, anionic clusters have been found to be more stable against dissociation by ion mobility-mass spectrometry (IM-MS) during field observations, indicating these clusters could grow more quickly in the atmosphere. They are also directly observed in ambient sampling and chemical ionization mass spectrometry experiments, though the degree to which nitrate chemical ionization faithfully preserves the composition of new particles has been questioned. Furthermore, mixed IOP containing iodic acid and any less acidic species are likely to deprotonate iodic acid, and clusters including sulfuric acid and ammonia have been observed. Thus, these anionic clusters can also be viewed as likely subunits of larger pure or mixed IOPs. Despite the findings suggesting full conversion of I 2 O 5 to HIO 3 even at relative humidities below 65%, another study determined clustering of iodine oxides was still the primary IOP formation pathway even at high relative humidities. Here we use mass-resolved gas phase ultraviolet (UV) spectroscopy, and infrared (IR) 3 ) 2 compared to the standard solar spectrum AM1.5G are depicted. The yellow area highlighted between the two sets of spectra displays the region of overlap of atmospherically relevant wavelengths with the UV cluster spectra. Absorption shifts into the atmospherically-relevant window as cluster size increases. m/z, with and without 300 nm laser light. Figure shows a plot of difference mass spectra for absorption at 300 nm, calculated by subtracting the steady state mass spectrum with the laser on from that when the laser is blocked. The positive intensities, therefore, identify cluster compositions with increased population upon 300 nm irradiation and the negative intensities establish clusters that are depleted. Artifacts resulting from changes in peak shape are noted by a red *. All clusters of the composition (I 2 O 5 ) n>1 (IO While quantification of the absolute photolysis rates is difficult in the current experimental setup, we present conservative lower limits to these rates in Table only to estimate the degree to which photochemistry of this type is likely to be relevant to NPF. More careful Figure : The difference mass spectra computed by subtracting a mass spectrum with no laser light shining on the clusters from a mass spectrum where the clusters were irradiated by 300 nm light. Positive intensities, therefore, represent clusters or fragments whose intensity increased upon exposure to 300 nm light and negative intensities signify clusters that fragmented when irradiated with 300 nm light. Peaks labeled by a red * identify artifacts that resulted from the subtraction process due to small changes in peak shape and are not representative of an actual loss or gain in cluster/fragment intensity. kinetic measurements would be required to establish quantitative rates that could be incorporated into models. The calculation of these rates is described in the Supplementary Information, but briefly, we integrated the product of the experimental fragment yields (divided by photon flux) and the AM1.5G solar spectrum. Fragment yields are scaled to the O-loss fragment yield for (I 2 O 5 ) 2 (IO - 3 ), which was recorded with high signal to noise. We expect these rates to be lower bounds to the atmospheric rates because the AM1.5G reference spectrum is intended to represent a "typical" day in a terrestrial mid-latitude location at sea level. On a high-UV, low-ozone day in tropical coastal regions at altitude, the observed rates for clusters such as these are likely to be significantly higher, possibly by an order of magnitude. More sophisticated models to predict solar irradiance are available, but