Nitric acid (HNO₃) is critically involved in aerosol formation, acid deposition, and atmospheric nitrogen cycling. However, the mechanisms underlying its daytime formation, particularly the influences of gaseous and interfacial water molecules, and its effect on methanesulfonic acid-methylamine (MSA-MA)-driven nucleation remains poorly understood. Here, we employed quantum chemical calculations, atmospheric cluster dynamics code (ACDC) simulations, and Born-Oppenheimer molecular dynamics (BOMD) to investigate how water catalyzes HNO₃ formation in both gaseous and interfacial environments, as well as the contribution of HNO₃ to MSA-MA-driven new particle formation (NPF). Results show that water-catalyzed HNO₃ formation in the gas phase proceeds through a barrierless pathway and becomes significantly faster than the uncatalyzed NO₂ + OH reaction under typical atmospheric water vapor concentrations. At the air-water interface, NO₂ and OH not only show pronounced surface affinity, but also can lead to the rapid formation of HNO₃ within picoseconds. Against the backdrop of declining global SO₂ emissions, this work further reveals that HNO₃ substantially promotes NPF and aerosol particle growth via two main mechanisms (<em>i</em>) in high HNO₃ concentrations (~ 1.0 &times; 10¹² molecules∙cm^-3), such as the North China Plain and New Delhi areas, HNO₃-involved pathways contribute up to 10 % of the total nucleation flux; (<em>ii</em>) NO₃^- exhibits a markedly enhanced capacity to promote aerosol hygroscopic growth by facilitating vapor uptake and stabilizing molecular clusters, thereby driving aerosol nucleation and growth. These findings offer molecular-level insights into the formation pathways of HNO₃ and the growth of aerosol particles in polluted boundary layer.