Decadal variability of PM_2.5-associated nitrate in the megacity of Beijing, China: Revelations achieved through continuous real-time in-situ observation

Atmospheric nitrate (NO₃⁻), a key component of fine particulate matter (PM_2.5), significantly impacts air quality and climate. This study investigates decadal variability (2013−2022) of PM_2.5-associated NO₃⁻ in Beijing using continuous real-time measurements, meteorological normalization, and source identification. Results reveal an annual NO₃⁻ decline of 4.8 %, driven by stringent clean air policies like “coal-to-gas” initiatives, with autumn and winter reductions (−8.6 %/yr and − 8.4 %/yr, respectively) outpacing other seasons due to targeted emission controls. Despite progress, NO₃⁻ remains elevated (11.7 ± 15.8 μg/m³), contributing 33.0 % to PM_2.5 by 2022, underscoring persistent secondary formation under high oxidation capacity. Diurnal patterns show high nocturnal and morning values (shallow boundary layers and traffic emissions) and minima at 16:00–18:00 (volatilization and deep boundary layers), while absent weekend effects reflect unregulated traffic and sustained residential activity. Meteorological normalization attributes 78.4 % of NO₃⁻ decline to emission controls, emphasizing policy efficacy, yet chemical feedbacks (enhanced NOₓ-to-HNO₃ conversion) and transboundary transport challenge further mitigation. The interplay between local emissions and regional transport significantly shaped the variability of PM_2.5-associated NO₃⁻ in Beijing from 2013 to 2022. The nonparametric wind regression analyses identify northwest and southern hotspots, with shifting source regions post-2018 highlighting growing contributions from southern industrial zones. Based on the potential source contribution function and concentration weighted trajectory methods, it can be found that dominant source regions of NO₃⁻ and its precursor (NOₓ) were located south of Beijing. Findings advocate interaction of policy, chemistry, and meteorology in shaping PM_2.5 composition, integrated NH₃-NOₓ controls and regional coordination, offering critical insights for optimizing air quality strategies in rapidly urbanizing regions globally.