Zhaofeng Tan

Typhoons are major synoptic systems linked to ozone (O3) pollution in the Pearl River Delta, South China, particularly in autumn and summer. Although previous studies have identified various mechanisms of typhoon-induced O3 pollution, analyses of meteorological conditions or overall process contributions alone do not fully explain how typhoons affect O3 pollution through specific atmospheric processes, particularly transport and chemistry. Here, we combine O3 mass budget analysis, source apportionment, precursor-transport sensitivity simulations and chemical diagnostics based on WRF-CMAQ models to investigate the responses of these processes to typhoons in autumn and summer. Typhoons enhance O3 transport through both vertical exchange and horizontal transport, with increased transport of O3 derived from upwind emissions in both seasons and background O3 mainly in autumn. Transported precursors also contribute considerably to local O3 pollution (>10 µg m-3), which is further enhanced under typhoon influence, primarily driven by increased contributions from acyl peroxy nitrates (APNs) and NOx. Local photochemistry exhibits more complex responses: typhoons accelerate O3 production in autumn but suppress it in summer, reduce HOx cycling efficiency while enhancing O3 production efficiency, alter VOC contributions to OH reactivity, and generally shift O3 production toward more NOx-limited conditions. These findings reveal that typhoons influence regional O3 pollution through multiple transport pathways, transported precursor contributions and different aspects of photochemistry, providing further process-level evidence for understanding the mechanisms underlying typhoon-induced O3 pollution.

Nitryl chloride (ClNO ₂ ), a key precursor to chlorine radicals that accelerates atmospheric oxidation, has chemistry and spatial distribution, particularly its vertical distribution, that remain insufficiently constrained. Here we presented wintertime observations of ClNO ₂ across three Chinese megacities: Beijing (2024), Shanghai (2020), and Guangzhou (2025). Diurnal average peak concentrations reached 68.8, 112.6, and 77.6 ppt, respectively, with a maximum of 931.0 ppt recorded at 03:26 on November 10 in Beijing. Median nocturnal NO ₃ ‐to‐ClNO ₂ production efficiency ( ϵ ) were 0.3%, 0.5%, and 0.2% in Beijing, Shanghai, and Guangzhou, but variability was much larger in Beijing (SD 10.0%), reflecting perturbation from changing air masses. The inter‐city contrast in ϵ is driven by temperature, particulate chloride,and N ₂ O ₅ partitioning: the warmer coastal cities operate in a chloride‐supply‐limited regime in which ϵ increases with Cl ⁻ , whereas cold, chloride‐rich Beijing is chloride‐saturated. In Beijing, post‐sunrise ClNO ₂ increases occurred on 13 of 34 days. For the 8 November case, the BL‐RL (boundary layer‐residual layer) model revealed that the surge resulted from entrainment of ClNO ₂ stored in the RL with an inferred 720.2 ppt that subsequently decayed by photolysis as the boundary layer deepened. This morning BL‐RL interaction, observed in Beijing but not detected in our Shanghai and Guangzhou records, indicates that vertical exchange of reactive chlorine is a critical factor in the wintertime chlorine chemistry of North China Plain, amplified by regional residential coal combustion and elevated particulate chloride.

Isoprene is the most abundant biogenic volatile organic compound (BVOC) and has far-reaching impacts on secondary organic aerosol (SOA) formation globally. Its atmospheric oxidation produces diverse isomeric radicals that drive subsequent chain propagation and mechanistic branching. However, high-throughput experimental characterization of these isomeric-resolved radicals remains unavailable, leaving critical gaps in the underlying molecular mechanisms. Here we establish a radical-omics approach for isomer-specific identification and detection of hundreds of radical species generated during VOCs oxidation. Applied to OH-initiated isoprene oxidation, this method enables experimental quantification of four OH-added allylic radicals and determination of their branching ratios. We further found hydrogen-abstraction to be an unexpectedly important pathway, contributing up to 8.78 ± 3.96% of total branching. Incorporating the updated mechanism into a global chemical transport model shows that this pathway contributes up to 13.5% of isoprene-derived low-volatility SOA over tropical rainforests. These results provide an experimental foundation for radical screening and targeted mechanistic validation, revealing hidden pathways in complex atmospheric conditions.

The hydroxyl radical (OH) plays a central role in atmospheric chemistry, however, its accurate measurement by laser induced fluorescence with gas expansion (LIF-FAGE) is unavoidably compromised by wavelength drift of the excitation laser. To overcome this limitation, a real time reference system for active wavelength locking has been developed and systematically characterized in this work. Stable, and high concentration OH radicals were generated through thermolysis of ambient air on a heated filament within a low pressure cell. The excitation source was a 308 nm laser produced by frequency doubling the output of a DCM-ethanol dye laser pumped by an Nd:YAG laser. The induced fluorescence was monitored in real time using a non-gated photomultiplier tube (PMT). The wavelength locking program, implemented with a closed loop feedback mechanism, dynamically adjusted the laser wavelength to the optimal OH excitation line. Through comprehensive characterization of key parameters, including laser power, filament operating conditions (current, voltage), and cell environment (pressure/inlet flow rate, inlet gas relative humidity), an optimal operational window of the reference system has been identified. A 12 h continuous measurement demonstrated high system stability in OH generation and detection, the observed fluorescence intensity showed a low drift rate of 0.2 % h⁻¹ during the first nine hours. The good robustness of the reference system, and its integrated wavelength locking program, enable long-term and accurate ambient OH radical quantification in LIF-FAGE measurements.

An upgraded Peking University Laser-Induced Fluorescence (PKU-LIF) system for synchronized measurements of atmospheric OH, HO₂, and RO₂ radicals will be presented. The system features five integrated modules: (1) A Laser Source based on a frequency-doubled dye laser delivering 308 nm radiation at 8 kHz, with 90% power allocated to ambient measurements and 10% to reference monitoring; (2) An Ambient Radical Measurement Module comprising two detection cells, OH-HOx and HOx-ROx, where OH-HOx is used for the measurement of OH and HO2, and HOx-ROx cell combined with a convertor is used for the efficient conversion and detection of RO2. (3) A Calibration Module employing 185 nm Hg lamp photolysis to generate equal OH/HO₂ concentrations, with CO or CH₄ addition enabling quantitative conversion to HO₂ or CH₃O₂ for sensitivity determination; (4) A Chemical Modulation Module featuring a specially designed Teflon cylinder that ensures vertical airflow alignment and provides 96% OH scavenging efficiency through propane/N₂ injection; (5) A Reference Module with real-time wavelength stabilization using thermocatalytically generated OH, demonstrating exceptional long-term stability (0.2%/hour drift). Recently, the upgraded PKU-LIF system has been applied to the EXACT (Ensembled eXperiments of Atmospheric oxidation Capacity in the Troposphere) campaign to investigate the seasonal evolution of atmospheric radical chemistry and atmospheric oxidation capacity.

ROx radicals (including OH, HO2 and RO2) are central to atmospheric chemistry, governing the removal of trace gases and the formation of secondary pollutants such as ozone and secondary organic aerosols. Despite decades of research since OH radical was recognized as the core atmospheric oxidant, the chemical production and destruction processes of ROx radicals under varied NOx levels remain insufficiently constrained, limiting a mechanistic comprehension of atmospheric oxidation capacity. To address this, the Ensembled eXperiment of Atmospheric oxidation Capacity in the Troposphere (EXACT) campaign was conducted over one year across urban, regional, and background sites in the North China Plain. OH, HO2 and RO2 were measured online using an updated Peking University Laser Induced Fluorescence (PKU-LIF) system during one representative month per season (autumn, winter, spring, summer).Preliminary results indicate that the daily maximum OH concentrations reached up to 1.5×107 molecules cm-3 in summer, which is five times higher than in winter (~3×106 molecules cm-3). HO2 and RO2 concentrations typically peaked at (2-3)×109 molecules cm-3 in summer across different sites, approximately an order of magnitude higher than (1-2)×108 molecules cm-3 in winter, with spring and autumn exhibiting intermediate levels. Total OH reactivity (kOH) showed distinct spatiotemporal patterns, with daily peak values ranging from 10 s-1 at the background site to over 30 s-1 at the regional site, reflecting the complex mixture of anthropogenic and biogenic VOCs. The seasonal and diurnal variability of ROx concentrations highlights distinct patterns influenced by local environmental conditions and photochemical activity. Based on the comprehensive observational datasets, we perform a detailed experimental budget analysis for individual radicals and their sum (ROx). The research quantifies the contributions of critical radical sources and sinks, and identifies the dominant chemical pathways driving ROx levels under varying NOx conditions. Our findings advance the mechanistic understanding of radical chemistry and provide observational constraints that refine current chemical mechanisms for simulating atmospheric oxidation capacity and secondary pollution formation.

Atmospheric radicals critically govern tropospheric oxidation processes, yet chemical models systematically underestimate OH and HO2 under high- and low-NOₓ conditions, limiting accurate prediction of haze and ozone in polluted regions like China. To address these gaps, the Ensembled eXperiment of Atmospheric oxidation Capacity in the Troposphere (EXACT) was conducted across the North China Plain, deploying advanced instrumentation at urban (Beijing), rural (Luancheng), and remote (Shangdianzi) sites over four seasons from autumn 2024 to summer 2025.Preliminary results reveal strong seasonal variability: peak OH concentrations reached ~2×107 cm-3 in spring and summer but dropped to ~2×106 cm-3 in winter. Significant nighttime HO2 and RO2 were observed in rural and remote areas, indicating active dark chemistry. OH-j(O1D) correlations strengthened from winter (R2≈0.5) to summer (R2≈0.8), reflecting increasing photochemical dominance. HONO photolysis dominated ROx production in winter, while O3 and carbonyl photolysis became more important in warmer seasons. Chlorine chemistry also contributed significantly to ROₓ, with distinct diurnal ClNO2 patterns suggesting multiple source mechanisms.Compared to earlier campaigns, EXACT shows elevated OH turnover rates since 2020, offering a partial explanation for slowed PM2.5 decline and rising ozone. Building on these findings, the EXACT-Plus campaign will expand into central China’s Gan-E-Xiang region, where complex terrain, high humidity, and unique chlorine sources, such as kilns, fireworks, and biomass burning, may drive unexplored oxidation pathways. This work advances understanding of radical-driven pollution and supports improved model development and air quality management.

The hydroxyl radical (OH) plays a central role in atmospheric chemistry, however, its accurate measurement by laser induced fluorescence with gas expansion (LIF-FAGE) is unavoidably compromised by wavelength drift of the excitation laser. To overcome this limitation, a real time reference system for active wavelength locking has been developed and systematically characterized in this work. Stable, and high concentration OH radicals were generated through thermocatalytic decomposition of ambient air on a heated filament within a low pressure cell. The excitation source was a 308 nm laser produced by frequency doubling the output of a DCM-ethanol dye laser pumped by an Nd:YAG laser. The induced fluorescence was monitored in real time using a non-gated photomultiplier tube (PMT). The wavelength locking program, implemented with a closed loop feedback mechanism, dynamically adjusted the laser wavelength to the optimal OH excitation line. Through comprehensive characterization of the key parameters, including laser power, filament operating conditions (current, voltage), and cell environment (pressure/inlet flow rate, inlet gas relative humidity), an optimal operational window of the reference system has been identified. A 12-hour continuous measurement demonstrated high system stability in OH generation and detection, the observed fluorescence intensity showed a low drift rate of 0.2% per hour during the first nine hours. The good robustness of the reference system, and its integrated wavelength locking program, enable long term and accurate ambient OH radical quantification in LIF-FAGE measurements.