Unveiling the intricate dynamics of PM2.5 sulfate aerosols in the urban boundary layer: A pioneering two-year vertical profiling and machine learning-enhanced analysis in global Mega-City

Abstract

Sulfate (SO₄²⁻) aerosols, a predominant chemical constituent of fine particulate matter (PM_2.5), wield profound influences on urban atmospheric environments, climate dynamics, and public health. While advancements have been made in understanding the formation mechanisms of ground-level SO₄²⁻, the scarcity of long-term, continuous vertical measurements has limited our understanding of SO₄²⁻ production across the entire boundary layer. This study bridges this gap by integrating two-year vertical profiles of aerosol components, derived from ground-based remote sensing data, with an advanced, interpretable machine learning model. We quantified the contributions of meteorological parameters and chemical species to SO₄²⁻ concentrations within 150–1500 m altitudes. The contributions of meteorological parameters in the boundary layer (54.87 %-65.29 %) exceed those of chemical species (34.71 %-45.13 %), with relative humidity and temperature as the main driving factors. Regional transport driven by southwest winds also increases SO₄²⁻ concentrations. Hydrogen peroxide (H₂O₂) serves as the primary oxidizing agent, and the acid-buffering capability of ammonia must not be disregarded. A substantial rise in the proportion of summertime SO₄²⁻ in PM_2.5 has been observed, with its proportion in the upper boundary layer reaching 30.29 %. This phenomenon mainly results from intensified photochemical reactions in the afternoon, whereby SO₂ oxidation facilitated by ozone, H₂O₂, and nitrogen dioxide promotes SO₄²⁻ production in the upper boundary layer. Our findings highlight that ground-based remote sensing retrieval can interpret the long-term continuous vertical distribution of aerosol components, thus providing a new perspective on elucidating the complex formation mechanisms of aerosol components within the boundary layer.