Nan Wang

Bottom-up wildfire inventories commonly assign fixed emission factors (EFs) to vegetation classes, introducing uncertainty because EFs vary with fuel conditions. Here, we conducted laboratory burns of live-origin and litter-derived dead Pinus massoniana needles across a broad range of dry-basis moisture contents (MC) to quantify stable-spread-stage EFs for CO2, CO, and CH4 and develop fuel-condition-resolved parameterizations. Burning was smoldering-dominated, with sustained flaming observed only for litter-derived dead needles at 0.89 % MC. Increasing MC decreased EF(CO2) and increased EF(CO) for live-origin needles, whereas the opposite responses occurred for smoldering-dominated litter-derived dead needles. The moisture-response regressions for EF(CO2) and EF(CO) explained 60.3–81.5 % of the variability in mean EFs, while EF(CH4) showed no detectable moisture dependence and was represented by fuel-state-specific means of 15.7 ± 4.3 and 8.0 ± 2.1 g kg-1 for live and dead fuels, respectively. Applying these parameterizations in a bottom-up wildfire emission framework showed that similar total gaseous carbon emissions can mask substantial shifts in species-level carbon partitioning. Over 2012–2021, cumulative gaseous carbon emissions differed by only 4.3 % from a fixed-EF baseline, whereas CO2-C fell by 14.0 % and CO-C and CH4-C rose by 154.6 % and 344.1 %, respectively. These shifts translated into an approximately 65.63 % greater 20-year global warming potential-based near-term climate impact. These findings highlight the importance of fuel-condition-resolved EFs for better representing wildfire emission composition and its atmospheric and climate implications.

Photochemical mechanisms are core components of air quality models, yet differences among them introduce substantial uncertainty in ozone (O₃) simulations. Here, we systematically evaluate three widely used mechanisms (CB06, SAPRC07, and RACM2) using the CMAQ model during an O₃ pollution episode in Chengdu, China. To ensure consistency, a customized volatile organic compounds emission inventory was developed for RACM2. The results show pronounced inter-mechanistic differences in simulated O₃, radical chemistry, and precursor sensitivities under distinct NO_x conditions. SAPRC07 and RACM2 produced higher and comparable O₃ levels (~95 ppbv) with better performance in urban areas, whereas CB06 simulated lower O₃ (~80 ppbv) and performed better under low-NOₓ conditions. These differences are linked to variations in radical concentrations and reaction pathways, with RACM2 yielding the highest OH levels, SAPRC07 better capturing HO₂, and CB06 simulating lower RO₂. Despite these discrepancies, all mechanisms consistently reproduced the dominant O₃ formation pathway, with RO₂+NO contributing over 50 % of net production. However, radical sources and termination pathways differed substantially between urban and suburban environments, reflecting regime-dependent chemistry. Sensitivity analysis further shows that anthropogenic aromatics and alkenes dominate urban O₃ formation, whereas biogenic emissions dominate in suburban areas. Notably, O₃ sensitivity to biogenic emissions is higher in urban high-NO_x conditions, highlighting an amplified biogenic contribution. These findings demonstrate that mechanism choice fundamentally affects O₃ formation and precursor sensitivities, emphasizing the need for multi-mechanism approaches to improve model reliability and support effective emission control strategies.

Photochemical mechanisms are core components of air quality models, yet differences among them introduce substantial uncertainty in ozone (O₃) simulations. Here, we systematically evaluate three widely used mechanisms (CB06, SAPRC07, and RACM2) using the CMAQ model during an O₃ pollution episode in Chengdu, China. To ensure consistency, a customized volatile organic compounds emission inventory was developed for RACM2. The results show pronounced inter-mechanistic differences in simulated O₃, radical chemistry, and precursor sensitivities under distinct NO_x conditions. SAPRC07 and RACM2 produced higher and comparable O₃ levels (~95 ppbv) with better performance in urban areas, whereas CB06 simulated lower O₃ (~80 ppbv) and performed better under low-NOₓ conditions. These differences are linked to variations in radical concentrations and reaction pathways, with RACM2 yielding the highest OH levels, SAPRC07 better capturing HO₂, and CB06 simulating lower RO₂. Despite these discrepancies, all mechanisms consistently reproduced the dominant O₃ formation pathway, with RO₂+NO contributing over 50 % of net production. However, radical sources and termination pathways differed substantially between urban and suburban environments, reflecting regime-dependent chemistry. Sensitivity analysis further shows that anthropogenic aromatics and alkenes dominate urban O₃ formation, whereas biogenic emissions dominate in suburban areas. Notably, O₃ sensitivity to biogenic emissions is higher in urban high-NO_x conditions, highlighting an amplified biogenic contribution. These findings demonstrate that mechanism choice fundamentally affects O₃ formation and precursor sensitivities, emphasizing the need for multi-mechanism approaches to improve model reliability and support effective emission control strategies.

Escalating exposure to extreme heat and ozone (O3) pollution has emerged as a principal challenge for cities worldwide. While urban greening is widely advocated for mitigating extreme heat, its impacts on compound heat-O3 pollution remain poorly understood due to intricate urban meteorology-chemistry coupling. By integrating observations, reanalysis data, and urban-land-atmosphere modeling, we demonstrate that urban greenery provides a notable yet uneven cooling effect of −0.26°C to −1.08°C in Northern Hemisphere’s metropolises, shaped by greenery fraction and local climate. Furthermore, the nonlinearly weakened O3 photochemical production within a greenery-stabilized urban boundary layer is capable of decreasing O3 in populated city centers. Despite the dual positive effects in alleviating heat and O3 exposures meteorologically, greenery-boosted biogenic emissions could chemically raise urban O3 via supplementing precursors, totally by up to 7.6 µg m−3. These findings underscore the need to incorporate fully coupled urban meteorology-chemistry interactions in urban greening strategies to achieve climatic and environmental co-benefits.

Photochemical mechanisms are core components of air quality models, yet differences among them introduce substantial uncertainty in ozone (O₃) simulations. Here, we systematically evaluate three widely used mechanisms (CB06, SAPRC07, and RACM2) using the CMAQ model during an O₃ pollution episode in Chengdu, China. To ensure consistency, a customized volatile organic compounds emission inventory was developed for RACM2. The results show pronounced inter-mechanistic differences in simulated O₃, radical chemistry, and precursor sensitivities under distinct NO_x conditions. SAPRC07 and RACM2 produced higher and comparable O₃ levels (~95 ppbv) with better performance in urban areas, whereas CB06 simulated lower O₃ (~80 ppbv) and performed better under low-NOₓ conditions. These differences are linked to variations in radical concentrations and reaction pathways, with RACM2 yielding the highest OH levels, SAPRC07 better capturing HO₂, and CB06 simulating lower RO₂. Despite these discrepancies, all mechanisms consistently reproduced the dominant O₃ formation pathway, with RO₂+NO contributing over 50 % of net production. However, radical sources and termination pathways differed substantially between urban and suburban environments, reflecting regime-dependent chemistry. Sensitivity analysis further shows that anthropogenic aromatics and alkenes dominate urban O₃ formation, whereas biogenic emissions dominate in suburban areas. Notably, O₃ sensitivity to biogenic emissions is higher in urban high-NO_x conditions, highlighting an amplified biogenic contribution. These findings demonstrate that mechanism choice fundamentally affects O₃ formation and precursor sensitivities, emphasizing the need for multi-mechanism approaches to improve model reliability and support effective emission control strategies.

Climate warming intensifies heatwave-ozone (O3) extremes, yet the role of temperature-sensitive emissions remains poorly constrained. Using WRF-CMAQ simulations, we disentangled O3 enhancements driven by meteorological forcing and temperature-sensitive emissions from natural (biogenic VOCs, BVOCs; soil nitrogen) and anthropogenic (traffic and solvent evaporation) sources during a severe heatwave–ozone episode in the Sichuan Basin. Heatwave conditions boosted basin-wide BVOCs, transportation-evaporative, and solvent-use volatile organic compound (AVOCs) emissions by 53.6%, 26.7%, and 21.9%, respectively. The total heatwave-induced MDA8 O3 enhancement reached 40–60 μg m–3. While BVOCs remained dominant, temperature-enhanced AVOCs contributions surged by 68%, highlighting amplified urban anthropogenic influences. Meteorological and emission pathways showed pronounced spatial heterogeneity: meteorological stagnation dominating O3 increases in the western basin (∼30 μg m–3), whereas temperature-sensitive emissions added enhancements in the eastern urban regions (∼10 μg m–3). Under future warming, temperature-sensitive emissions are projected to increase O3 by 3.6–5.3 μg m–3, but carbon-neutrality pathways can offset these effects, producing a net decrease of up to −18.5 μg m–3. These findings reveal temperature-driven emissions represent a pathway that warrants greater consideration in linking climate warming to O3 pollution, highlighting the importance of integrating climate and emission controls to mitigate future air quality risks.

Photochemical mechanisms are core components of air quality models, yet differences among them introduce substantial uncertainty in ozone (O₃) simulations. Here, we systematically evaluate three widely used mechanisms (CB06, SAPRC07, and RACM2) using the CMAQ model during an O₃ pollution episode in Chengdu, China. To ensure consistency, a customized volatile organic compounds emission inventory was developed for RACM2. The results show pronounced inter-mechanistic differences in simulated O₃, radical chemistry, and precursor sensitivities under distinct NO_x conditions. SAPRC07 and RACM2 produced higher and comparable O₃ levels (~95 ppbv) with better performance in urban areas, whereas CB06 simulated lower O₃ (~80 ppbv) and performed better under low-NOₓ conditions. These differences are linked to variations in radical concentrations and reaction pathways, with RACM2 yielding the highest OH levels, SAPRC07 better capturing HO₂, and CB06 simulating lower RO₂. Despite these discrepancies, all mechanisms consistently reproduced the dominant O₃ formation pathway, with RO₂+NO contributing over 50 % of net production. However, radical sources and termination pathways differed substantially between urban and suburban environments, reflecting regime-dependent chemistry. Sensitivity analysis further shows that anthropogenic aromatics and alkenes dominate urban O₃ formation, whereas biogenic emissions dominate in suburban areas. Notably, O₃ sensitivity to biogenic emissions is higher in urban high-NO_x conditions, highlighting an amplified biogenic contribution. These findings demonstrate that mechanism choice fundamentally affects O₃ formation and precursor sensitivities, emphasizing the need for multi-mechanism approaches to improve model reliability and support effective emission control strategies.