In Situ Individual Particle Observations Revealing the Enhanced Photolysis of Nitrate Associated With Soot‐Containing Particles in the Atmosphere

Abstract The photolysis of particulate nitrate critically regulates atmospheric oxidative capacity, yet the influence of coexist soot is unexplored. Here, single‐particle aerosol mass spectrometry was used to probe the coexistence and atmospheric behavior of soot and nitrate. The nitrate number fraction, defined here as the fraction of individual particles containing a detectable nitrate ion signal (m/z −62 [NO 3 ] − ) in the single‐particle mass spectra, showed seasonal lows in summer/autumn and an afternoon diurnal dip, contrasting with radiation and temperature profiles. The reduction in nitrate was more pronounced in soot‐containing particles compared with soot‐free particles. Regression models revealed that the nitrate associated with soot‐containing particles during summer and autumn could be well explained ( R 2 ≈ 0.6) by radiation‐related parameters (accounting for >30% of the variance) and, critically, by the relative soot content (∼20%), whereas these factors excluding soot content explained only ∼40% of the nitrate variation in soot‐free particles. The sunny‐versus‐cloudy‐day contrast further corroborated the radiation dependence of the enhanced nitrate loss. These field observations led us to hypothesize that soot intrinsically enhances the photolysis of particulate nitrate. This hypothesis was confirmed by laboratory simulations, which showed that introducing diesel soot into an irradiated dilute sodium nitrate solution (mimicking soot‐containing cloud droplets) enhanced NO 2 − production by a factor of 1.5–3.0 in a soot‐content‐dependent manner. Collectively, these findings provide the first ambient observational evidence of soot's unique role in the photochemical evolution of nitrate, with significant implications for quantitative assessments and atmospheric modeling of nitrate photolysis.