Toward Building a Physical Proxy for Gas-Phase Sulfuric Acid Concentration Based on Its Budget Analysis in Polluted Yangtze River Delta, East China

Investigation on the budget of peroxyacetyl nitrate (PAN) in the Yangtze River Delta: Unravelling local photochemistry and regional impact

Peroxyacetyl nitrate (PAN) is a significant indicator of atmospheric photochemical pollution, which can influence the regional distribution of ozone (O3) and hydroxyl radical (OH) through long-range transport. However, investigations of PAN incorporating comprehensive measurement and explicit modeling analysis are limited, hindering complete understandings of its temporal behavior, sources, and impacts on photochemistry. Here we conducted a 1-year continuous observation of PAN and relative atmospheric species in Nanjing located in Yangtze River Delta (YRD). The annual mean concentration of PAN was 0.62 ± 0.49 ppbv and showed a bimodal monthly variation, peaking in April-June and November-January, respectively. This pattern is different from the typical pattern of photochemistry, suggesting important contributions of other non-photochemical processes. We further analyzed the PAN budget using an observation-based model, by which, PAN from local photochemical production and regional source could be decoupled. Our results revealed that local photochemical production of PAN is the sole contributor to PAN in summer, whereas about half of the total PAN concentration is attributed to regional source in winter. Although the formation of PAN can suppress the atmospheric oxidation capacity by consuming the peroxyacetyl radical and nitrogen dioxide (NO2), our analyses suggested this effect is minor at our station (-3.2 ± 1.1 % in summer and - 7.2 ± 2.8 % in winter for O3 formation). However, it has the potential to enhance O3 and OH formation by 14.16 % and 5.93 %, if transported to cleaner environments with air pollutants halved. Overall, our study highlights the importance of both local photochemistry and regional process in PAN budget and provides a useful evaluation on the impact of PAN on atmospheric oxidation capacity.

Modeling the Formation of Organic Compounds across Full Volatility Ranges and Their Contribution to Nanoparticle Growth in a Polluted Atmosphere

Nanoparticle growth influences atmospheric particles' climatic effects, and it is largely driven by low-volatility organic vapors. However, the magnitude and mechanism of organics' contribution to nanoparticle growth in polluted environments remain unclear because current observations and models cannot capture organics across full volatility ranges or track their formation chemistry. Here, we develop a mechanistic model that characterizes the full volatility spectrum of organic vapors and their contributions to nanoparticle growth by coupling advanced organic oxidation modeling and kinetic gas-particle partitioning. The model is applied to Nanjing, a typical polluted city, and it effectively captures the volatility distribution of low-volatility organics (with saturation vapor concentrations <0.3 μg/m3), thus accurately reproducing growth rates (GRs), with a 4.91% normalized mean bias. Simulations indicate that as particles grow from 4 to 40 nm, the relative fractions of GRs attributable to organics increase from 59 to 86%, with the remaining contribution from H2SO4 and its clusters. Aromatics contribute much to condensable organic vapors (∼37%), especially low-volatility vapors (∼61%), thus contributing the most to GRs (32-46%) as 4-40 nm particles grow. Alkanes also contribute 19-35% of GRs, while biogenic volatile organic compounds contribute minimally (<13%). Our model helps assess the climatic impacts of particles and predict future changes.

An overlooked oxidation mechanism of toluene: computational predictions and experimental validations

Secondary organic aerosols (SOAs) influence the Earth's climate and threaten human health. Aromatic hydrocarbons (AHs) are major precursors for SOA formation in the urban atmosphere. However, the revealed oxidation mechanism dramatically underestimates the contribution of AHs to SOA formation, strongly suggesting the importance of seeking additional oxidation pathways for SOA formation. Using toluene, the most abundant AHs, as a model system and the combination of quantum chemical method and field observations based on advanced mass spectrometry, we herein demonstrate that the second-generation oxidation of AHs can form novel epoxides (TEPOX) with high yield. Such TEPOX can further react with H2SO4 or HNO3 in the aerosol phase to form less-volatile compounds including novel non-aromatic and ring-retaining organosulfates or organonitrates through reactive uptakes, providing new candidates of AH-derived organosulfates or organonitrates for future ambient observation. With the newly revealed mechanism, the chemistry-aerosol box modeling revealed that the SOA yield of toluene oxidation can reach up to 0.35, much higher than 0.088 based on the original mechanism under the conditions of pH = 2 and 0.1 ppbv NO. This study opens a route for the formation of reactive uptake SOA precursors from AHs and significantly fills the current knowledge gap for SOA formation in the urban atmosphere.

NO at low concentration can enhance the formation of highly oxygenated biogenic molecules in the atmosphere

The interaction between nitrogen monoxide (NO) and organic peroxy radicals (RO₂) greatly impacts the formation of highly oxygenated organic molecules (HOM), the key precursors of secondary organic aerosols. It has been thought that HOM production can be significantly suppressed by NO even at low concentrations. Here, we perform dedicated experiments focusing on HOM formation from monoterpenes at low NO concentrations (0 - 82 pptv). We demonstrate that such low NO can enhance HOM production by modulating the RO₂ loss and favoring the formation of alkoxy radicals that can continue to autoxidize through isomerization. These insights suggest that HOM yields from typical boreal forest emissions can vary between 2.5%-6.5%, and HOM formation will not be completely inhibited even at high NO concentrations. Our findings challenge the notion that NO monotonically reduces HOM yields by extending the knowledge of RO₂-NO interactions to the low-NO regime. This represents a major advance towards an accurate assessment of HOM budgets, especially in low-NO environments, which prevails in the pre-industrial atmosphere, pristine areas, and the upper boundary layer.

Theoretical Study of the Gas-Phase Hydrolysis of Formaldehyde to Produce Methanediol and Its Implication to New Particle Formation

Aldehydes were speculated to be important precursor species in new particle formation (NPF). The direct involvement of formaldehyde (CH₂O) in sulfuric acid and water nucleation is negligible; however, whether its atmospheric hydrolysate, methanediol (CH₂(OH)₂), which contains two hydroxyl groups, participates in NPF is not known. This work investigates both CH₂O hydrolysis and NPF from sulfuric acid and CH₂(OH)₂ with quantum chemistry calculations and atmospheric cluster dynamics modeling. Kinetic calculation shows that reaction rates of the gas-phase hydrolysis of CH₂O catalyzed by sulfuric acid are 11-15 orders of magnitude faster than those of the naked path at 253-298 K. Based on structures and the calculated formation Gibbs free energies, the interaction between sulfuric acid/its dimer/its trimer and CH₂(OH)₂ is thermodynamically favorable, and CH₂(OH)₂ forms hydrogen bonds with sulfuric acid/its dimer/its trimer via two hydroxyl groups to stabilize clusters. Our further cluster kinetic calculations suggested that the particle formation rates of the system are higher than those of the binary system of sulfuric acid and water at ambient low sulfuric acid concentrations and low relative humidity. In addition, the formation rate is found to present a negative temperature dependence because evaporation rate constants contribute significantly to it. However, cluster growth is essentially limited by the weak formation of the largest clusters, which implies that other stabilizing vapors are required for stable cluster formation and growth.

Response of organic aerosol characteristics to emission reduction in Yangtze River Delta region

Organic aerosol (OA) is a major component of atmospheric particulate matter (PM) with complex composition and formation processes influenced by various factors. Emission reduction can alter both precursors and oxidants which further affects secondary OA formation. Here we provide an observational analysis of secondary OA (SOA) variation properties in Yangtze River Delta (YRD) of eastern China in response to large scale of emission reduction during Chinese New Year (CNY) holidays from 2015 to 2020, and the COVID-19 pandemic period from January to March, 2020. We found a 17% increase of SOA proportion during the COVID lockdown. The relative enrichment of SOA is also found during multi-year CNY holidays with dramatic reduction of anthropogenic emissions. Two types of oxygenated OA (OOA) influenced by mixed emissions and SOA formation were found to be the dominant components during the lockdown in YRD region. Our results highlight that these emission-reduction-induced changes in organic aerosol need to be considered in the future to optimize air pollution control measures.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at 10.1007/s11783-023-1714-0 and is accessible for authorized users.

Unexpected catalytic influence of atmospheric pollutants on the formation of environmentally persistent free radicals

Environmentally persistent free radicals (EPFRs) have been recognized as harmful and persistent environmental pollutants. In polluted regions, many acidic and basic atmospheric pollutants, which are present at high concentrations, may influence the extent of the formation of EPFRs. In the present paper, density functional theory (DFT) and ab-initio molecular dynamics (AIMD) calculations were performed to investigate the formation mechanisms of EPFRs with the influence of the acidic pollutants sulfuric acid (SA), nitric acid (NA), organic acid (OA), and the basic pollutants, ammonia (A), dimethylamine (DMA) on α-Al₂O₃ (0001) surface. Results indicate that both acidic and basic pollutants can enhance the formation of EPFRs by acting as "bridge" or "semi-bridge" roles by proceeding via a barrierless process. Acidic pollutants enhance the formation of EPFRs by first transferring its hydrogen atom to the α-Al₂O₃ surface and subsequently reacting with phenol to form an EPFR. In contrast, basic pollutants enhance the formation of EPFRs by first abstracting a hydrogen atom from phenol to form a phenoxy EPFR and eventually interacting with the α-Al₂O₃ surface. These new mechanistic insights will inform in understanding the abundant EPFRs in polluted regions with high mass concentrations of acidic and basic pollutants.