Soot particles are generally considered to be poor ice nucleating particles. Involvement of soot in clouds and their release back into the atmosphere can form residual particles with altered cloud forming potential. The impact and extent of such different cloud processing scenarios on ice nucleation is, however, not well understood. In this work, we present the impact of cloud processing of soot aerosols on subsequent ice nucleation cycles at T≤233 K. Coupling of two continuous flow diffusion chambers allows the simulation of different cloud processing scenarios and investigation of subsequent ice nucleation activity of the processed particles. The processing scenarios presented here encompass contrail, cirrus, and mixed‐phase cloud processing, mimicking typical pathways that soot particles can be exposed to in the atmosphere. For all scenarios tested, the processed particles showed an enhanced ice active fraction for T<233 K. The relative humidity with respect to water for the ice nucleation onset was observed to be on average approximately 10% (relative humidity with respect to ice, RHi≈16%) lower for the cloud‐processed particles compared to the unprocessed soot for which ice nucleation was observed close to or at homogeneous freezing conditions of solution droplets. We attribute the enhanced ice nucleation abilities of the cloud‐processed soot to a pore condensation and freezing mechanism and have identified key parameters governing these changes. Enhanced ice nucleation abilities of soot in cirrus clouds can have significant impacts, given the importance of the atmospheric ice phase for precipitation formation and global climate.

Ice nucleation one of the most uncertain microphysical processes, as it occurs in various ways and on many different types of particles that can host a mix of various types...

• Cloud processing can increase organic particle viscosity enhancing their ice nucleation activity 10 • Coal fly ash particles show a decreased ice nucleation ability after cloud process-11 ing 12 • Aerosol generation methods impact the observed ice nucleation abilities of coal 13 fly ash particles 14

&lt;p&gt;Heavy aerosol loading threatens human health across the globe and is typically related to photochemical processing associated with emission of organic, inorganic and trace metal compounds. Aerosol particles dominated by organic solutes may attain a high or ultra-high viscosity (&gt; 10&lt;sup&gt;12&lt;/sup&gt; Pa s) becoming solid-like in cold and dry air, limiting diffusion of organic and reactive molecules through the particle volume, thus slowing chemistry. In contrast, illumination and thus photochemistry to produce radicals may occur through the bulk of light absorbing particles irrespective of diffusion limitations, but its efficiency is not well constrained. We investigated iron oxidation state changes in particles containing various concentrations of citric acid, iron(III)-citrate, copper(II)-citrate and copper(II) salts using environmental X-ray spectromicroscopy with control of relative humidity, &lt;em&gt;RH&lt;/em&gt;, and temperature, &lt;em&gt;T&lt;/em&gt;. Chemical images of single aerosol particles with resolution currently as low as 35 &amp;#215; 35 nm&lt;sup&gt;2&lt;/sup&gt; were acquired in a humidified microreactor revealing spatial gradients in the concentration of iron(II), iron(III), copper(I) and copper(II) compounds. We have also quantified the CO&lt;sub&gt;2&lt;/sub&gt; formation from coated wall flowtube experiments due to decarboxylation subsequent to ligand to metal charge transfer and the condensed and gas phase products using proton-transfer reaction mass spectrometry to characterize the complex chemical reaction scheme. We observed that iron was largely reduced in particles despite being in an oxygen atmosphere immediately after exposure to atmospherically relevant UV light exposure i.e. using 375 nm LED and a measured intensity of 3 W m&lt;sup&gt;&amp;#8209;2&lt;/sup&gt; for 15 min. This implies that oxygen uptake, diffusion, reactive oxygen species generation and metal reoxidation reactions were slow compared to photochemical reduction. When relative humidity, &lt;em&gt;RH &lt;/em&gt;&lt; 50%, there was significant oxidation only near the surface of particles extending over scales of tens of nanometers. At higher &lt;em&gt;RH&lt;/em&gt;, particles became more homogeneously oxidized. We concluded that O&lt;sub&gt;2&lt;/sub&gt; reaction and diffusion is limited and results in organic radical persistence in particles. In the presence of copper, iron was immediately oxidized after UV exposure, which is in sharp contrast to particles without copper. If oxygen is limited, and therefore cannot quickly reoxidize iron, then copper oxidation reactions or cross iron-copper redox reactions must generate more radicals than expected. We aim to improve the kinetic treatment of radical production from copper and iron, which can affect redox cycling in organic aerosol. Such information is necessary for the accurate prediction of aerosol phase radical generation, chemical loss of oxygenated organic aerosol dominated by carboxyl functionalities and identifying diffusion limitations leading to the preservation of reactive oxygen species and free radicals.&lt;/p&gt;