Gang Zhao

Humid atmospheric processing can alter the spectral response of black‐carbon‐containing aerosols and complicate brown‐carbon (BrC) attribution from the absorption Ångström exponent (AAE). We combined alternating ambient and thermodenuded multiwavelength absorption measurements with particle‐size observations and optical sensitivity calculations in the Sichuan Basin. During the Strong masking period, mean thermodenuded and ambient AAE were 2.02 and 1.53, respectively, while Ratio _AAE remained above unity during most of the campaign. The E _abs – Ratio _AAE slope was larger at relative humidity (RH) ≥ 80% than below 80% (1.780 vs. 1.169; p = 0.006), and the contrast persisted across alternative RH thresholds and after covariate adjustment, supporting an association with humid atmospheric processing. Strong masking coincided with an accumulation‐mode‐rich aerosol population. Multilayer calculations showed that particle geometry and coating absorption can produce stronger long‐ than short‐wavelength enhancement, flattening the ambient spectrum. Low ambient AAE therefore does not necessarily indicate weak BrC absorption.

Fogs over polluted regions often exhibit extremely low supersaturation, yet the controlling mechanisms remain poorly constrained. Existing frameworks attribute supersaturation adjustment mainly to competition among activated droplets, while the role of interstitial aerosol water has been largely overlooked due to observational limitations. Here, we develop a new approach based on an advanced optical instrument to achieve online, size‐resolved quantification of interstitial aerosol water uptake. By combining optical measurements with a machine learning–constrained framework, we retrieve dry aerosol volume and hygroscopic growth at saturation of interstitial aerosols, enabling fog water budget analysis. Observations on the North China Plain show that interstitial aerosol water contributes >50% of total fog water during formation and remains comparable (>25%) to droplet water throughout evolution. This competition helps suppress the supersaturation to <0.03%, limiting aerosol activation and droplet growth. These results identify interstitial aerosol water as a key regulator of fog supersaturation in polluted atmospheres.

Near-surface aerosol Ångström exponent (AE) profiles are needed for size-related air-quality assessment and to reduce uncertainties in multi-wavelength optical retrievals, yet pulsed lidars typically cannot resolve the lowest few hundred metres and surface in-situ optics lack vertical coverage. Using dual-wavelength (405/532 nm) charge-coupled device (CCD)–laser aerosol detection system (CLADS)—an imaging lidar with dual laser and dual CCD—we retrieve continuous nocturnal AE and extinction profiles from the ground through the boundary layer over seven nights in November 2018 at Gucheng (North China Plain) during the McFAN campaign. On each of the seven nights, multi-layer AE structure is resolved below ∼1.5 km. The central result is a recurrent elevated AE peak: while dual-wavelength extinction is highest at the surface, AE systematically maximizes in the ∼110–230 m band (composite median near ∼180 m), with a surface-to-peak contrast of ∼1.1–1.4 relative to the 10–50 m layer (∼1.0–1.3 if the two lowest bins, 10 and 20 m, are excluded and the surface reference is taken as 30–50 m). Aerosol loading and size-sensitive spectral slope are therefore vertically decoupled. Near-surface CLADS extinction at 405 and 532 nm covaries strongly with co-located dry nephelometry (pooled R ≈0.88–0.92). By resolving the lower-boundary-layer blind zone, dual-wavelength CLADS indicates that, in these cases, surface-based AE measurements can under-represent the fine-particle-dominated layer aloft, which is relevant to satellite/lidar retrieval constraints and boundary-layer radiative calculations. Nocturnal stable stratification is a plausible but untested explanation, because no concurrent thermodynamic profiles were available.

The photolysis frequency of nitrogen dioxide (NO₂), J(NO₂), links aerosol radiative effects with near-surface photochemistry and ozone formation. Its response to aerosol optical properties is difficult to quantify because scattering, absorption, hygroscopic growth, black carbon coating properties, aerosol vertical distribution, and surface reflection are coupled. We developed a framework combining Mie calculations, the Constrained Parameter (CP) method, and the Tropospheric Ultraviolet and Visible (TUV) model to quantify sensitivities and interactions controlling J(NO₂) using inputs representative of northern China. Two pathways were examined: direct perturbation of aerosol optical depth (AOD), single-scattering albedo (SSA), the asymmetry factor (g), and surface albedo in TUV, and a microphysical pathway in which AOD, SSA, and g were derived from aerosol microphysical parameters using Mie calculations. Among direct radiative inputs, SSA and surface albedo showed the largest CP sensitivities, approximately 12.0×10^-3 and 5.3×10^-3 dJ/(dx/x), respectively, indicating stronger control by scattering–absorption partitioning and the lower radiative boundary than by column extinction alone. In the microphysical analysis, coating thickness, black carbon density, black carbon absorption enhancement (BCAE), the imaginary part of the black carbon refractive index, hygroscopicity, and surface albedo were key factors. Interaction and fixed-factor analyses showed that sensitivity rankings were state dependent. The strongest interaction occurred between coating thickness and BCAE. Better constraints on SSA, surface albedo, coating thickness, BCAE, hygroscopicity, and aerosol vertical distribution are needed to improve photolysis-frequency calculations and assessments of coupled fine particulate matter and ozone pollution.

The photolysis frequency of nitrogen dioxide (NO2), J(NO2), links aerosol radiative effects with near-surface photochemistry and ozone formation. Its response to aerosol optical properties is difficult to quantify because scattering, absorption, hygroscopic growth, black carbon coating properties, aerosol vertical distribution, and surface reflection are coupled. We developed a framework combining Mie calculations, the Constrained Parameter (CP) method, and the Tropospheric Ultraviolet and Visible (TUV) model to quantify sensitivities and interactions controlling J(NO2) using inputs representative of northern China. Two pathways were examined: direct perturbation of aerosol optical depth (AOD), single-scattering albedo (SSA), the asymmetry factor (g), and surface albedo in TUV, and a microphysical pathway in which AOD, SSA, and g were derived from aerosol microphysical parameters using Mie calculations. Among direct radiative inputs, SSA and surface albedo showed the largest CP sensitivities, approximately 12.0×10-3 and 5.3×10-3 dJ/(dx/x), respectively, indicating stronger control by scattering–absorption partitioning and the lower radiative boundary than by column extinction alone. In the microphysical analysis, coating thickness, black carbon density, black carbon absorption enhancement (BCAE), the imaginary part of the black carbon refractive index, hygroscopicity, and surface albedo were key factors. Interaction and fixed-factor analyses showed that sensitivity rankings were state dependent. The strongest interaction occurred between coating thickness and BCAE. Better constraints on SSA, surface albedo, coating thickness, BCAE, hygroscopicity, and aerosol vertical distribution are needed to improve photolysis-frequency calculations and assessments of coupled fine particulate matter and ozone pollution.

New particle formation (NPF) strongly influences aerosol number size distributions and cloud condensation nuclei (CCN), yet its characteristics and climatic relevance in high-altitude urban environments over the Tibetan Plateau (TP) remain poorly constrained. We conducted in situ observations at Tibet University (TU) in Lhasa during the spring and autumn observation periods. NPF event frequencies were 86.2 % during the autumn observation period and 62.1 % during the spring observation period. Observation days were classified as NPF, Undefined, or Non-Event days based on PNSD evolution, supported by size-segregated particle number concentrations and particle growth rates. Observation-period comparisons indicate clear differences in the factors associated with NPF: NPF during the autumn observation period may have been favored by greater sulfur-precursor availability, whereas no single dominant controlling factor could be identified during the spring observation period. Estimated CCN concentrations were higher on NPF days (mean ± standard deviation: (2.3±1.5)×103 cm ⁻³ ; median: 1.9×10 ³ cm ⁻³ ). Corresponding mean ± standard deviation and median values were (0.9±0.4)×103 and 0.81×10 ³ cm ⁻³ on Undefined days and (1.3±0.7)×103 and 1.1×10 ³ cm ⁻³ on Non-Event days, respectively. The impact of NPF on CCN depends not only on nucleation, but also on the continued growth and survival of newly formed particles. This study provides new insights into urban NPF characteristics and its potential contribution to CCN on the TP, improving our understanding of aerosol–climate interactions in high-altitude urban environments.

New particle formation (NPF) strongly influences aerosol number size distributions and cloud condensation nuclei (CCN), yet its characteristics and climatic relevance in high-altitude urban environments over the Tibetan Plateau (TP) remain poorly constrained. We conducted in situ observations at Tibet University in Lhasa during the monsoon and pre-monsoon seasons. NPF event frequencies were 86.2 % during the monsoon season and 55.1 % during the pre-monsoon period. Observation days were classified into four types (Class I, Class II, Undefined, and Non-Event) based on the evolution of particle number size distribution (PNSD) and modal development. Seasonal comparisons indicate clear differences in the factors associated with NPF: monsoon season NPF was mainly associated with sulfur precursor availability, whereas no single clear controlling factor was evident during the pre-monsoon period. NPF substantially enhanced CCN at TU, with mean concentrations of 3351 ± 1919 cm⁻³ on Class I days, 1540.2 ± 872 cm⁻³ on Class II days, 880.5 ± 374 cm⁻³ on Undefined days, and 1278.1 ± 667 cm⁻³ on Non-Event days. The impact of NPF on CCN depends not only on nucleation, but also on the continued growth and survival of newly formed particles. This study provides new insights into urban NPF characteristics and the contribution of NPF to CCN on the TP, improving our understanding of aerosol–climate interactions in high-altitude urban environments.

New particle formation (NPF) strongly influences aerosol number size distributions and cloud condensation nuclei (CCN), yet its characteristics and climatic relevance in high-altitude urban environments over the Tibetan Plateau (TP) remain poorly constrained. We conducted in situ observations at Tibet University in Lhasa during the monsoon and pre-monsoon seasons. NPF event frequencies were 86.2 % during the monsoon season and 55.1 % during the pre-monsoon period. Observation days were classified into four types (Class I, Class II, Undefined, and Non-Event) based on the evolution of particle number size distribution (PNSD) and modal development. Seasonal comparisons indicate clear differences in the factors associated with NPF: monsoon season NPF was mainly associated with sulfur precursor availability, whereas no single clear controlling factor was evident during the pre-monsoon period. NPF substantially enhanced CCN at TU, with mean concentrations of 3351 ± 1919 cm⁻³ on Class I days, 1540.2 ± 872 cm⁻³ on Class II days, 880.5 ± 374 cm⁻³ on Undefined days, and 1278.1 ± 667 cm⁻³ on Non-Event days. The impact of NPF on CCN depends not only on nucleation, but also on the continued growth and survival of newly formed particles. This study provides new insights into urban NPF characteristics and the contribution of NPF to CCN on the TP, improving our understanding of aerosol–climate interactions in high-altitude urban environments.

New particle formation (NPF) strongly influences aerosol number size distributions and cloud condensation nuclei (CCN), yet its characteristics and climatic relevance in high-altitude urban environments over the Tibetan Plateau (TP) remain poorly constrained. We conducted in situ observations at Tibet University in Lhasa during the monsoon and pre-monsoon seasons. NPF event frequencies were 86.2 % during the monsoon season and 55.1 % during the pre-monsoon period. Observation days were classified into four types (Class I, Class II, Undefined, and Non-Event) based on the evolution of particle number size distribution (PNSD) and modal development. Seasonal comparisons indicate clear differences in the factors associated with NPF: monsoon season NPF was mainly associated with sulfur precursor availability, whereas no single clear controlling factor was evident during the pre-monsoon period. NPF substantially enhanced CCN at TU, with mean concentrations of 3351 ± 1919 cm⁻³ on Class I days, 1540.2 ± 872 cm⁻³ on Class II days, 880.5 ± 374 cm⁻³ on Undefined days, and 1278.1 ± 667 cm⁻³ on Non-Event days. The impact of NPF on CCN depends not only on nucleation, but also on the continued growth and survival of newly formed particles. This study provides new insights into urban NPF characteristics and the contribution of NPF to CCN on the TP, improving our understanding of aerosol–climate interactions in high-altitude urban environments.