Deliquescence behavior of photo-irradiated single NaNO 3 droplets

Abstract Nitrate-containing particles are ubiquitous in the troposphere because of their secondary production due to anthropogenic emissions of NO x from the combustion of fossil fuels. Nitrate ions are recognized as photoactive species that may contribute to the formation of oxidants in the atmosphere through heterogeneous photochemical reactions. The chemical transformation of aerosol particles in the atmosphere often leads to modification of the particles’ hygroscopic properties. Although the photo-transformation of nitrate ions into nitrite within aerosol particles has been investigated, the influence of the photoproducts formation on the hygroscopic behavior of particles has not been reported. In this study, we examined the hygroscopic properties of single, ultraviolet-irradiated NaNO 3 droplets using Raman microspectrometry. We are the first demonstrated that irradiating NaNO 3 particles affects their hygroscopic behavior. For short-term exposures, regarding hygroscopic behavior, the irradiated particles exhibited two-stage transitions that were clearly reproduced in the experimental NaNO 3 -NaNO 2 phase diagram. The production of NO 2 − decreased the deliquescence relative humidity values. For long irradiation times (>5 h), these values are even more affected by the additional production of peroxynitrite and carbonate ions in individual droplets. The NaNO 3 -NaNO 2 deliquescence phase diagram cannot explain the hygroscopic behavior of long-term irradiated particles. Finally, we demonstrated the influence that CO 2 has on the photo-transformation process in NaNO 3 droplets.