Meteorological Effects on the Vertical Variability of Aerosol Chemical Composition at a Rural Background Site

Vertically resolved aerosol observations are crucial for understanding boundary-layer pollution processes and improving air-quality assessments. This study presents concurrent dual-height measurements of submicron aerosol chemical composition at a rural background site using a compact Time-of-Flight Aerosol Mass Spectrometer at 230 m and a ToF-Aerosol Chemical Speciation Monitor at 4 m, complemented by Sunset OCEC analyzers. Measurements from December 2019 to May 2020 revealed systematically higher concentrations at 4 m than at 230 m for all major components: organics (Org), sulfate (SO₄^2-), nitrate (NO₃^-), ammonium (NH₄⁺), and equivalent black carbon (eBC), with median 230 m/4 m ratios between 0.55 and 0.82. Carbonaceous species exhibited the steepest vertical gradients and strongest seasonal contrasts, peaking during winter, reflecting near-surface accumulation of combustion-origin pollutants. In contrast, secondary inorganic species showed weaker gradients and more vertical homogeneity, consistent with their regional origin and sensitivity to thermal stability, humidity, and meteorological conditions. Seasonal differences in the 230 m/4 m ratios were significant for Org and SO₄^2-, whereas NO₃^-, NH₄⁺, and eBC showed no significant seasonal differences. Diurnal variability revealed pronounced height-dependent behavior with carbonaceous species remaining more strongly enriched near the surface and several secondary species showing weaker vertical contrasts.. Air-mass cluster and synoptic analyses indicated that large-scale processes shaped vertical aerosol structure, while local meteorology determined near-surface aerosol features. Specifically, temperature inversion and low mixing-layer heights (MLH < 230 m) favored near-surface accumulation, with concentrations at 4 m two to three times higher than those at 230 m, highlighting the limitations of relying solely on ground-based measurements for air quality assessments.