Aerosols are an important part of the atmosphere, they are defined as liquid or solid particles suspended in air, ranging from one nanometer to tens of micrometers in diameter. Aerosols affect the climate directly, via aerosol radiation interactions, and indirectly, via aerosol-cloud interactions. While pollution in cities does not have the largest impact on global climate, it does affect local climate and weather. Aerosols can also be deadly; in 2019 lower respiratory infections were reported as the third leading cause of death globally, which are largely caused by aerosols. Since around 55% of the world’s population live in cities, it is important to understand the key drivers of urban aerosol formation and growth. Ammonium nitrate is an important component of aerosols, but not much is known about its contribution to aerosol formation and early growth. In this thesis, we aim to understand how nitric acid (HNO3) and ammonia (NH3) can impact aerosol formation in urban environments. Previous understanding of urban air conditions led to a puzzle of competing growth rates and loss rates, where it appeared that measured growth rates in cities were not high enough to explain the persistence of particle number concentrations in the face of high loss rates from coagulation with pre-existing large particles. Results from the CLOUD chamber at CERN presented in this thesis show a newly discovered mechanism of rapid growth by formation of ammonium nitrate onto pre-existing particles. We find that in situations of excess NH3 and HNO3, with respect to ammonium nitrate saturation ratios, particles can grow orders of magnitude faster than previously measured in ambient environments. Since this mechanism is consistent with the nano-Köhler theory, there is an activation diameter above which ammonium nitrate can form on the particles, and particles as small as a few nanometers can be affected. Furthermore, this mechanism was found to have a strong temperature dependence where at lower temperatures the same gas phase concentrations result in higher growth rates. At temperatures as low as −25°C, ammonia and nitric acid were found to be able to nucleate even in the absence of sulfuric acid or other known nucleating species. In order to determine whether these rapid growth rates are in fact high enough to overcome high coagulation loss rates, further experiments were undertaken at the CLOUD chamber at CERN at 5°C in the presence of a high condensation sink, analogous to haze. Experimental results showed that in experiments with higher NH3 and HNO3 concentrations, particle number concentrations were sustained with a steady formation of 2.5 nm particles. Newly formed particles are found to be effectively lost to the condensation sink, thus confirming that loss rates have not been over-estimated, and high growth rates are more likely to be the explanation for particle survival in haze conditions. Alongside experimental results, a kinetic model was developed which is capable of quantitatively reproducing growth from ammonium nitrate formation. We used this model to predict particle survival over a wide range of NH3 and HNO3 concentrations and condensation sinks. Results showed that survival of newly formed particles was drastically increased in the presence of supersaturated conditions of NH3 and HNO3.