Pengfei Liu

Chemically resolved gas- and particle-phase measurements capable of constraining new particle formation (NPF) remain scarce in South Asia. Here, we investigate the precursors driving NPF in a coastal megacity and examine why only a subset of nucleation events evolves into sustained particle growth. We present the first comprehensive 20-day field study in a Bay of Bengal coastal megacity, integrating aerosol microphysical, chemical, gas-phase, and VOC measurements with positive matrix factorization (PMF). Photochemically driven formation of 10–30 nm particles occurred between 07:00 and 11:00 IST, whereas sustained growth into the accumulation mode occurred only 2 of 20 days, with growth rates of 4.53 and 3.43 nm h⁻¹. The particle formation rate, condensation sink (CS), and coagulation sink did not differ between growth and non-growth days, indicating that neither formation intensity nor scavenging distinguished the two regimes. Instead, sustained growth occurred when northerly continental air masses prevailed, whereas growth ceased when a strong sea breeze shifted transport to marine air influenced by power-plant emissions. PMF resolved six factors, including an NPF factor dominated by 10–30 nm particles that was associated with biogenic VOCs. Its invariance between weekdays and weekends, and negative correlation with vehicular traffic factor, indicates that NPF was driven primarily by photo-oxidation of biogenic precursors, rather than by anthropogenic activity. These findings suggest that, in coastal cities, precursor availability and air-mass origin may exert a stronger influence on particle growth than CS, although long-term observations are needed to establish its broader applicability.

Chemically resolved gas- and particle-phase measurements capable of constraining new particle formation (NPF) remain scarce in South Asia. Here, we investigate the precursors driving NPF in a coastal megacity and examine why only a subset of nucleation events evolves into sustained particle growth. We present the first comprehensive 20-day field study in a Bay of Bengal coastal megacity, integrating aerosol microphysical, chemical, gas-phase, and VOC measurements with positive matrix factorization (PMF). Photochemically driven formation of 10–30 nm particles occurred between 07:00 and 11:00 IST, whereas sustained growth into the accumulation mode occurred only 2 of 20 days, with growth rates of 4.53 and 3.43 nm h⁻¹. The particle formation rate, condensation sink (CS), and coagulation sink did not differ between growth and non-growth days, indicating that neither formation intensity nor scavenging distinguished the two regimes. Instead, sustained growth occurred when northerly continental air masses prevailed, whereas growth ceased when a strong sea breeze shifted transport to marine air influenced by power-plant emissions. PMF resolved six factors, including an NPF factor dominated by 10–30 nm particles that was associated with biogenic VOCs. Its invariance between weekdays and weekends, and negative correlation with vehicular traffic factor, indicates that NPF was driven primarily by photo-oxidation of biogenic precursors, rather than by anthropogenic activity. These findings suggest that, in coastal cities, precursor availability and air-mass origin may exert a stronger influence on particle growth than CS, although long-term observations are needed to establish its broader applicability.

Dansgaard-Oeschger (D-O) events refer to a series of rapid climate fluctuations in the Northern Hemisphere during the last glacial period. These fluctuations are recorded in Greenland ice cores by large, abrupt changes in both water isotopes and aerosol deposition, but the processes driving these changes remain unclear. Here we present new high-resolution, multi-proxy measurements from the Greenland GISP2 ice core spanning eight D-O events. In addition to multiple dust and sea-salt tracers, we provide the first high-resolution biomass burning proxy records from refractory black carbon (rBC) concentration and particle size during these transitions. Bayesian ramp fitting analysis shows that water-isotope and aerosol changes generally occurred within a narrow temporal window with no robust and reproducible sequence. Using a simplified aerosol transport and deposition framework, we find that changes in atmospheric transport and removal processes can explain much of the variability shared among aerosols from distinct source regions. Residual variability shows stronger relationships within than across source classes and, particularly for dust, differs between cold and warm climate states. These results suggest that rapid large-scale atmospheric and hydrological reorganization drove the common aerosol response, while changes in individual source systems contributed to the remaining variability. These new multi-proxy records provide valuable constraints for evaluating climate-model representations of abrupt climate transitions.

Chemically resolved gas- and particle-phase measurements capable of constraining new particle formation (NPF) remain scarce in South Asia. Here, we investigate the precursors driving NPF in a coastal megacity and examine why only a subset of nucleation events evolves into sustained particle growth. We present the first comprehensive 20-day field study in a Bay of Bengal coastal megacity, integrating aerosol microphysical, chemical, gas-phase, and VOC measurements with positive matrix factorization (PMF). Photochemically driven formation of 10–30 nm particles occurred between 07:00 and 11:00 IST, whereas sustained growth into the accumulation mode occurred only 2 of 20 days, with growth rates of 4.53 and 3.43 nm h⁻¹. The particle formation rate, condensation sink (CS), and coagulation sink did not differ between growth and non-growth days, indicating that neither formation intensity nor scavenging distinguished the two regimes. Instead, sustained growth occurred when northerly continental air masses prevailed, whereas growth ceased when a strong sea breeze shifted transport to marine air influenced by power-plant emissions. PMF resolved six factors, including an NPF factor dominated by 10–30 nm particles that was associated with biogenic VOCs. Its invariance between weekdays and weekends, and negative correlation with vehicular traffic factor, indicates that NPF was driven primarily by photo-oxidation of biogenic precursors, rather than by anthropogenic activity. These findings suggest that, in coastal cities, precursor availability and air-mass origin may exert a stronger influence on particle growth than CS, although long-term observations are needed to establish its broader applicability.

Chemically resolved gas- and particle-phase measurements capable of constraining new particle formation (NPF) remain scarce in South Asia. Here, we investigate the precursors driving NPF in a coastal megacity and examine why only a subset of nucleation events evolves into sustained particle growth. We present the first comprehensive 20-day field study in a Bay of Bengal coastal megacity, integrating aerosol microphysical, chemical, gas-phase, and VOC measurements with positive matrix factorization (PMF). Photochemically driven formation of 10–30 nm particles occurred between 07:00 and 11:00 IST, whereas sustained growth into the accumulation mode occurred only 2 of 20 days, with growth rates of 4.53 and 3.43 nm h⁻¹. The particle formation rate, condensation sink (CS), and coagulation sink did not differ between growth and non-growth days, indicating that neither formation intensity nor scavenging distinguished the two regimes. Instead, sustained growth occurred when northerly continental air masses prevailed, whereas growth ceased when a strong sea breeze shifted transport to marine air influenced by power-plant emissions. PMF resolved six factors, including an NPF factor dominated by 10–30 nm particles that was associated with biogenic VOCs. Its invariance between weekdays and weekends, and negative correlation with vehicular traffic factor, indicates that NPF was driven primarily by photo-oxidation of biogenic precursors, rather than by anthropogenic activity. These findings suggest that, in coastal cities, precursor availability and air-mass origin may exert a stronger influence on particle growth than CS, although long-term observations are needed to establish its broader applicability.

Black carbon (BC) and iron oxide (FeO_x) aerosols represent important contributors to shortwave atmospheric heating and have been associated with adverse human health outcomes. Accurate measurement of BC and FeO_x is crucial for understanding their roles in climate forcing and air quality. However, observational data─particularly for anthropogenic FeO_x─remain limited. In this study, we use a modified Single Particle Soot Photometer to measure atmospheric concentrations of refractory BC (rBC) and FeO_x particles in urban (Atlanta, GA) and rural (Boone, NC) environments in the southeastern U.S. We identify likely emission sources and estimate light absorption properties to assess the relative contributions of rBC and FeO_x to atmospheric heating. Our findings indicate that rBC and FeO_x concentrations and absorption are substantially higher in the urban environment, reflecting significant anthropogenic contributions to pollutant emissions. Assuming external mixing, we estimate that FeO_x contributes 1.3% of total absorption by rBC at the urban site and approximately 0.6% at the rural site. These results highlight the spatial variability of light-absorbing aerosol components and the importance of localized assessments to better understand their impacts on air quality and climate.

The temperature sensitivity of fine particulate matter (PM2.5) critically influences air quality and human health under a warming climate, yet models struggle to accurately reproduce observed sensitivities. This study improves the representation of PM2.5-temperature relationships in the chemical transport model GEOS-Chem through targeted improvements and analyses of the underlying drivers based on simulations across the contiguous US (2000–2022). Our simulations reveal that chemical production processes, particularly isoprene secondary organic aerosol (SOA) and sulfate formation, determine the magnitude of PM2.5 sensitivity in the eastern US. In the western US, primary emissions drive the increasing PM2.5-temperature sensitivity. Transport processes contribute to interannual variability in PM2.5 sensitivity across all regions. We quantified the contributions from individual temperature-sensitive processes for the first time. Sulfate concentration plays a pivotal role in modulating the sensitivity of isoprene SOA due to its direct influence on isoprene SOA formation. Furthermore, the increased SO2 emissions on warm days dictates both the magnitude and variability of sulfate sensitivity in the eastern and central US. In the western US, however, sulfate sensitivity is primarily controlled by the temperature response of hydroxyl radicals (⚫OH). These findings highlight the impact of anthropogenic emission reductions on declining PM2.5–temperature sensitivity in the eastern US, improve our understanding of climate-driven air quality changes, and underscore the importance of accurately representing temperature-dependent processes in future air quality projections.

Severe wintertime fog frequently affects Delhi National Capital Rregion (NCR), causing extreme visibility reduction and exacerbating air quality and transportation disruptions. Fog formation over the Indo-Gangetic Plain is commonly linked to high aerosol loading, elevated humidity, and favourable synoptic conditions; however, the influence of rapid urban expansion and land-use driven surface processes driven by micro-meteorology remains unexplored. Urban built-up surface possesses distinct radiative and thermal properties compared to surrounding agricultural land, potentially modifying nocturnal cooling, near-surface microclimate and thus vertical structure of fog.This study investigates the influence of urban expansion over the last three decades on the surface energy balance and its feedback on different stages of urban fog genesis to delineate the aerosol induced effects across the extensive urban built-up landmass of the Delhi–NCR region. High-resolution (9–3–1 km) numerical simulations are conducted using the WRF-ARW/Chem model, employing historical (1992) and latest (2024) urban land-use datasets to isolate the impact of urban expansion and to examine the coupled effects of aerosols, radiation and microphysics. The model reasonably captures the spatial and temporal evolution of observed fog events.Results show that urban built-up areas in Delhi–NCR have expanded by over 100% in recent decades, primarily replacing irrigated cropland and vegetation, thereby altering surface radiative-thermal properties and intensifying the urban heat island effect resulting in distinct impact and effect of surface cooling after sunset. Consequently, during dense fog events, fog onset over urban areas is delayed and dissipation occurs earlier than over surrounding agricultural regions, leading to reduced liquid water content across the fog life cycle, both spatially and vertically. In contrast, radiative fog event exhibits an increase in LWC during the fog evolution, with a pronounced enhancement in the lower fog layers and a simultaneous reduction in the upper fog layers. This vertical redistribution of LWC is consistently reproduced in urban sensitivity simulations. Furthermore, WRF-Chem simulations reveal stronger LWC increases (>0.5 g kg-1) during fog initiation and continues to enhance in the upper fog layers throughout the event, while LWC decreases (< -0.4 g kg-1) in the lower layers during fog development and dissipation.Overall, the results demonstrate that urban expansion influences fog initiation and dissipation but also its vertical structure and microphysical characteristics through combined thermal and chemical feedback. The results highlight an underexplored pathway linking urbanization and aerosol feedback, surface thermal dynamics, and atmospheric chemistry in fog genesis.