Phase State Regulates Photochemical HONO Production from NaNO 3 /Dicarboxylic Acid Mixtures

Significant discrepancies persist between field observations and model simulations regarding the strength of marine-derived HONO sources, underscoring the urgency to resolve unidentified HONO sources. In this study, sodium dodecyl sulfate (SDS) was chosen as a proxy for marine surfactants to investigate its impact on aqueous nitrate photolysis for the first time. Remarkable increases in HONO and NO 2 production rates by factors of 3.3 and 5.6, respectively, along with a 1.9-fold rise in NO 2 − concentration, were observed at a very low SDS concentration of 0.01 mM, strongly illustrating the promoting effect on nitrate photolysis. Furthermore, at an SDS concentration of 2 mM, intriguingly aligned with the critical micelle concentration, there was an additional 41.7% increase in HONO production rates. Vertically resolved Raman measurements indicated that SDS anions at the aqueous-air interface attracted NO 3 − closer to the aqueous surfaces, increasing the amount of incompletely solvated surface nitrate. Importantly, the anionic surfactant exhibited a greater promoting effect on HONO production compared to other typical nitrate photochemistry systems with the addition of a marine dissolved organic matter proxy, halogen, photosensitizer, or OH scavenger. These findings offer new insights into marine-derived HONO sources and should be considered in model simulations concerning the budgets of NO x , OH, and O 3 .

Surface Nitrate Enrichment and Enhanced HONO Production from Ionic Surfactant Aggregation at the Aqueous-Air Interface

Li,

Ma,

et al. 2024

Formic Acid-Intensified Photoreduction of NOx on Iron Minerals Triggers Daytime HONO Formation through Active Hydrogen

Chen,

Li,

Liu

et al. 2024

Nitrous acid (HONO) is crucial in atmospheric chemistry as a precursor to morning peak hydroxyl radicals and significantly affects urban air quality by forming secondary pollutants, yet the mechanisms of its daytime formation is not fully understood. This study investigates the role of formic acid (HCOOH), a prevalent electron and proton donor, in the transformation of nitrogen oxides (NOx) and the formation of HONO on photoactive mineral dust. Exploiting hematite (Fe 2 O 3 ) as an environmental indicator, we demonstrate that HCOOH significantly promotes the photoreduction of NO 2 to HONO, while suppressing nitrate accumulation. This occurs through the formation of a surface �Fe−OOCH complex, where sunlight activates the C−H bond to generate and transfer active hydrogen, directly converting NO 2 to HONO. Additionally, HCOOH can trigger the photolysis of nitrates as predeposited on Fe 2 O 3 , further increasing HONO production. These findings show that HCOOH-mediated photochemical reactions on iron minerals may contribute to elevated atmospheric HONO levels, highlighting a crucial pathway with broad effects on atmospheric chemistry and public health.

Integrating Chemical Mechanisms and Feature Engineering in Machine Learning Models: A Novel Approach to Analyzing HONO Budget

Chen,

Zhou,

Wang

et al. 2024

Nitrous acid (HONO) serves as the primary source of OH radicals in the atmosphere, exerting significant impacts on atmospheric secondary pollution. The heterogeneous reactions of NO 2 on surfaces and photolysis of particulate nitrate or adsorbed nitric acid are important sources of atmospheric HONO, yet the corresponding kinetic parameters based on laboratory investigations and field observations exhibit considerable variations. In this study, we developed an explainable machine learning model to analyze the HONO budget using two years of summer urban supersite observations. By integrating chemical mechanisms and feature engineering into our machine learning model, we assessed the contributions of different sources to HONO and inferred the kinetic parameters for the primary HONO formation pathways, thereby partially addressing the limitations associated with predetermined rate coefficients. Our findings revealed that the primary source of daytime HONO in the summer was the photolysis of nitric acid adsorbed on both aerosol and ground surfaces, accounting for over 40% of its unknown sources. This was followed by the photoenhanced heterogeneous conversion of NO 2 and the photolysis of particulate nitrate. Additionally, we derived the corresponding kinetic parameters, analyzed their influencing factors, and confirmed that machine learning methods hold great potential for the study of the HONO budget.