Surface-Catalyzed Chlorine and Nitrogen Activation: Mechanisms for the Heterogeneous Formation of ClNO, NO, NO2, HONO, and N2O from HNO3 and HCl on Aluminum Oxide Particle Surfaces

Reduction and Photoreduction of NO2 in Humic Acid Films as a Source of HONO, ClNO, N2O, NO X , and Organic Nitrogen

Atmospheric nitrous acid (HONO), a trace atmospheric gas, is often underestimated in global atmospheric models due to the poor understanding of its daytime sources and sinks. HONO is known to accumulate during nighttime and undergo rapid photodissociation during the day to form NO and highly reactive OH radical, making it important to have accurate atmospheric HONO estimations. Despite its rapid photolysis, recent field observations have found quasi-steady-state concentrations of HONO at midday, suggesting photolytic HONO formation pathways to replenish daytime atmospheric HONO. Recent studies suggest that the presence of complex organic photosensitizers in atmospheric aerosols converts atmospheric NO₂ into HONO. To better understand the effect of environmental photosensitizers in daytime mechanisms of HONO formation, we present here laboratory studies on the heterogeneous photolytic reduction of NO₂ by humic acid films, a proxy for organic chromophoric compounds. The effect of pH and Cl^- in the photosensitized formation of HONO and other nitrogen-containing gases is also investigated. A dual Fourier transform infrared (FTIR) system is utilized to simultaneously perform in situ analysis of condensed-phase reactants and gas-phase products. We find that the rate of HONO formation is faster at lower pHs. Nitrogen incorporation in the complex organic chromophore is observed, suggesting a competing pathway that results in suppressed daytime formation of nitrogenous gases. Significantly, the presence of chloride ions also leads to the organic-mediated photolytic formation of nitrosyl chloride (ClNO), a known precursor of HONO. Overall, this work shows that organic acid photosensitizers can reduce adsorbed NO₂ to form HONO, ClNO, and NO while simultaneously incorporating nitrogen into the organic chromophores present in aerosol.

Propionamide participating in H2SO4-based new particle formation: a theory study

Propionamide (PA), an important pollutant emitted into the atmosphere from a variety of sources, is abundant in many areas worldwide, and could be involved in new particle formation (NPF). In this study, the enhancement of the H₂SO₄ (SA)-based NPF by PA was evaluated through investigating the formation mechanism of (PA)_m(SA)_n (m = 0–3 and n = 0–3) clusters using computational chemistry and kinetics modeling. Our study proved that the formation of all the PA-containing clusters is thermodynamically favorable. Furthermore, the Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O group in PA plays an important role in the clusters with more PA than SA, and the basicity of bases exerts a greater influence with an increasing amount of SA. We demonstrate that although the enhancing potential of PA is lower than that of the strongest enhancers of SA-based NPF such as methylamine (MA) and dimethylamine (DMA), PA can enhance the SA-based NPF at the parts per billion (ppb) level, which is typical for concentrations of C₃-amides in, for example, urban Shanghai (China). The monomer evaporation is the dominant degradation pathway for the (PA)_m(SA)_n clusters, which differs from that of the SA–DMA system. The formation rate of PA-containing clusters is comparable to the rate coefficients for PA oxidation by hydroxyl (OH) radicals, indicating that participating in the SA-based NPF is a crucial sink for PA.

Propionamide (PA), an important pollutant emitted into the atmosphere from a variety of sources, is abundant in many areas worldwide, and could be involved in new particle formation (NPF).

Uptake of gas phase nitrous acid onto boundary layer soil surfaces

Nitrous acid (HONO) is an important OH radical source that is formed on both ground and aerosol surfaces in the well-mixed boundary layer. Large uncertainties remain in quantifying HONO sinks and determining the mechanism of HONO uptake onto surfaces. We report here the first laboratory determination of HONO uptake coefficients onto actual soil under atmospheric conditions using a coated-wall flow tube coupled to a highly sensitive chemical ionization mass spectrometer (CIMS). Uptake coefficients for HONO decrease with increasing RH from (2.5 ± 0.4) × 10(-4) at 0% RH to (1.1 ± 0.4) × 10(-5) at 80% RH. A kinetics model of competitive adsorption of HONO and water onto the particle surfaces fits the dependence of the HONO uptake coefficients on the initial HONO concentration and relative humidity. However, a multiphase resistor model based on the physical and chemical processes affecting HONO uptake is more flexible as it accounts for the pH dependence of HONO uptake and bulk diffusion in the soil matrix. Fourier transform infrared (FTIR) spectrometry and cavity-enhanced absorption spectroscopy (CEAS) studies indicate that NO and N2O (16% and 13% yield, respectively) rather than NO2 are the predominant gas phase products, while NO2(-) and NO3(-) were detected on the surface post-exposure. Results are compared to uptake coefficients inferred from models and field measurements, and the atmospheric implications are discussed.