Quantification of the absorption cross sections of surface-adsorbed nitric acid in the 335–365nm region by Brewster angle cavity ring-down spectroscopy

Quantifying HONO Production from Nitrate Photolysis in a Polluted Atmosphere

The photolysis of particulate nitrate (pNO3-) has been suggested to be an important source of nitrous acid (HONO) in the troposphere. However, determining the photolysis rate constant of pNO3- (jpNO3-) suffers from high uncertainty. Prior laboratory measurements of jpNO3- using aerosol filters have been complicated by the "shadow effect"─a phenomenon of light extinction within aerosol layers that potentially skews these measurements. We developed a method to correct the shadow effect on the photolysis rate constant of pNO3- for HONO production (jpNO3- → HONO) using aerosol filters with identical chemical compositions but different aerosol loadings. We applied the method to quantify jpNO3- → HONO over the North China Plain (NCP) during the winter haze period. After correcting for the shadow effect, the normalized average jpNO3- → HONO at 5 °C increased from 5.89 × 10-6 s-1 to 1.72 × 10-5 s-1. The jpNO3- → HONO decreased with increasing pH and nitrate proportions in PM2.5 and had no correlation with nitrate concentrations. A parametrization for jpNO3- → HONO was developed for model simulation of HONO production in NCP and similar environments.

Factors Influencing the Formation of Nitrous Acid from Photolysis of Particulate Nitrate

Enhanced photolysis of particulate nitrate (pNO3) to form photolabile species, such as gas-phase nitrous acid (HONO), has been proposed as a potential mechanism to recycle nitrogen oxides (NOx) in the remote boundary layer ("renoxification"). This article presents a series of laboratory experiments aimed at investigating the parameters that control the photolysis of pNO3 and the efficiency of HONO production. Filters on which artificial or ambient particles had been sampled were exposed to the light of a solar simulator, and the formation of HONO was monitored under controlled laboratory conditions. The results indicate that the photolysis of pNO3 is enhanced, compared to the photolysis of gas-phase HNO3, at low pNO3 levels, with the enhancement factor reducing at higher pNO3 levels. The presence of cations (Na+) and halides (Cl-) and photosensitive organic compounds (imidazole) also enhance pNO3 photolysis, but other organic compounds such as oxalate and succinic acid have the opposite effect. The precise role of humidity in pNO3 photolysis remains unclear. While the efficiency of photolysis is enhanced in deliquescent particles compared to dry particles, some of the experimental results suggest that this may not be the case for supersaturated particles. These experiments suggest that both the composition and the humidity of particles control the enhancement of particulate nitrate photolysis, potentially explaining the variability in results among previous laboratory and field studies. HONO observations in the remote marine boundary layer can be explained by a simple box-model that includes the photolysis of pNO3, in line with the results presented here, although more experimental work is needed in order to derive a comprehensive parametrization of this process.

Extensive field evidence for the release of HONO from the photolysis of nitrate aerosols

Particulate nitrate ([Formula: see text]) has long been considered a permanent sink for NO_x (NO and NO₂), removing a gaseous pollutant that is central to air quality and that influences the global self-cleansing capacity of the atmosphere. Evidence is emerging that photolysis of [Formula: see text] can recycle HONO and NO_x back to the gas phase with potentially important implications for tropospheric ozone and OH budgets; however, there are substantial discrepancies in "renoxification" photolysis rate constants. Using aircraft and ground-based HONO observations in the remote Atlantic troposphere, we show evidence for renoxification occurring on mixed marine aerosols with an efficiency that increases with relative humidity and decreases with the concentration of [Formula: see text], thus largely reconciling the very large discrepancies in renoxification photolysis rate constants found across multiple laboratory and field studies. Active release of HONO from aerosol has important implications for atmospheric oxidants such as OH and O₃ in both polluted and clean environments.

HONO Production from Gypsum Surfaces Following Exposure to NO2 and HNO3: Roles of Relative Humidity and Light Source

Nitrous acid (HONO) is a toxic household pollutant and a major source of indoor OH radicals. The high surface-to-volume ratio and diverse lighting conditions make the indoor photochemistry of HONO complex. This study demonstrates surface uptake of NO₂ and gaseous HNO₃ followed by gas-phase HONO generation on gypsum surfaces, model system for drywall, under reaction conditions appropriate for an indoor air environment. Tens of parts per billion of steady-state HONO are detected under these experimental conditions. Mechanistic insight into this heterogeneous photochemistry is obtained by exploring the roles of material compositions, relative humidities, and light sources. NO₂ and HNO₃ are adsorbed onto drywall surfaces, which can generate HONO under illumination and under dark conditions. Photoenhanced HONO generation is observed for illumination with a solar simulator as well as with the common indoor light sources such as compact fluorescence light and incandescent light bulbs. Incandescent light sources release more HONO and NO₂ near the light source compared to the solar radiation. Overall, HONO production on the gypsum surface increases with the increase of RH up to 70% relative humidity; above that, the gaseous HONO level decreases due to surface loss. Heterogeneous hydrolysis of NO₂ is predicted to be the dominant HONO generation channel, where NO₂ is produced through the photolysis of surface-adsorbed nitrates. This hydrolysis reaction predominantly occurs in the first layer of surface-adsorbed water.

The Key Role of Sulfate in the Photochemical Renoxification on Real PM2.5

The active nitrogen species (HONO, NO, and NO₂) have important impacts on the atmospheric oxidative capacity and the transformation of many atmospheric species. In this study, a fast photochemical renoxification rate of adsorbed HNO₃/NO₃^- to active nitrogen species (HONO, NO, and NO₂) was detected on real urban PM_2.5, and sulfate was found to play a key role in this process. Different from the reported direct photolysis pathway, the photochemical reaction of HNO₃/NO₃^- on PM_2.5 is dominated by a photosensitizing mechanism. Acidic protons are proved to be essential for this pathway. The role of sulfate, because of the nonvolatility of its conjugated acid, is to conserve the necessary acidic protons when interacting with HNO₃ and thus maintain its photoreactivity. This work implies that sulfate will have important implications in atmospheric nitrogen cycling by accelerating the release of nitrogen oxides from photochemical renoxification of HNO₃/NO₃^- adsorbed on ambient particulates and thus can cause major environmental problems.

Matrix effect on surface-catalyzed photolysis of nitric acid

Photolysis rate constant of HNO₃ on the surface (HNO_3(s)) has been found to be enhanced by 1–4 orders of magnitude from that of gaseous HNO₃, with HONO and NO₂ as the main products. Such Re-NOx-ification pathway extends the apparent lifetime of reactive nitrogen species and modifies the atmospheric oxidative capacity along its long-rang transport. Despite of its importance, the detailed kinetics and mechanisms of HNO_3(s) photolysis are still not clear. Surface film of HNO₃ and organic compounds is ubiquitous in the environment and imposes matrix effect on HNO_3(s) photolysis. Here we studied photolysis of HNO₃ on Pyrex glass in a photochemical flow reactor over a wide range of HNO₃ surface density (D_HNO3) with or without the presence of model organic compounds. The photolysis rate constant of HNO_3(s) varied with D_HNO3 and surface-catalysis mechanism was proposed. Organic compounds further enhance the photolysis rate constant by up to one order of magnitude via both photosensitization and H-donating reaction. The H-donating reaction enhances as well the secondary HONO yield from reaction between the primary product NO₂ and adjacent H-donor, and thus increases the HONO/NO₂ production ratio. Finally, detailed mechanisms involving surface-catalyisis, photosensitization and H-donating reactions was integrated.

Comment on “Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces” by S. Laufs and J. Kleffmann, Phys. Chem. Chem. Phys., 2016, 18, 9616

Determination of HNO₃ coverage on silica provides insight into different HNO₃ surface photolysis rates without and with humidity.

Reply to the ‘Comment on “Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces”’ by M. N. Sullivan, L. T. Chu and L. Zhu, Phys. Chem. Chem. Phys., 2018, 20, DOI: 10.1039/C8CP04497J

Photolysis of HNO₃ on clean surfaces is no significant source of HONO and NO_x at atmospheric humidity.

Evaluation of Novel Routes for NOx Formation in Remote Regions

Photochemical cycling of nitrogen oxides (NO_x) produces tropospheric ozone (O₃), and NO_x is traditionally considered to be directly emitted. The inability of current global models to accurately calculate NO_x levels, and concurrently, difficulties in performing direct NO_x measurements in low-NO_x regimes (several pptv or several tens of pptv) globally introduce a large uncertainty in the modeling of O₃ formation. Here, we use the near-explicit Master Chemical Mechanism (MCM v3.2) within a 0D box-model framework, to describe the chemistry of NO_x and O₃ in the remote marine boundary layer at Cape Verde. We explore the impact of a recently discovered NO_x recycling route, namely photolysis of particulate nitrate, on the modeling of NO_x abundance and O₃ formation. The model is constrained to observations of long-lived species, meteorological parameters, and photolysis frequencies. Only a model with this novel NO_x recycling route reproduces levels of gaseous nitrous acid, NO, and NO₂ within the model and measurement uncertainty. O₃ formation from NO oxidation is several times more efficient than previously considered. This study highlights the need for the inclusion of particulate nitrate photolysis in future models for O₃ and for the photolysis rate of particulate nitrate to be quantified under variable atmospheric conditions.

Photolysis of Particulate Nitrate as a Source of HONO and NOx

Photolysis of nitric acid on the surface has been found recently to be greatly enhanced from that in the gas phase. Yet, photolysis of particulate nitrate (pNO₃) associated with atmospheric aerosols is still relatively unknown. Here, aerosol filter samples were collected both near the ground surface and throughout the troposphere on board the NSF/NACR C-130 aircraft. The photolysis rate constants of pNO₃ were determined from these samples by directly monitoring the production rates of nitrous acid (HONO) and nitrogen dioxide (NO₂) under UV light (>290 nm) irradiation. Scaled to the tropical noontime condition on the ground level (solar zenith angle = 0°), the normalized photolysis rate constants (j_pNO3^N) are in the range from 6.2 × 10^-6 s^-1 to 5.0 × 10^-4 s^-1 with a median of 8.3 × 10^-5 s^-1 and a mean (±1 SD) of (1.3 ± 1.2) × 10^-4 s^-1. Chemical compositions, specifically nitrate loading and organic matter, affect the rate of photolysis. Extrapolated to ambient pNO₃ loading conditions, e.g. ≤ 10 nmol m^-3, the mean j_pNO3^N value is over 1.8 × 10^-4 s^-1 in the suburban, rural, and remote environments. Photolysis of particulate nitrate is thus a source of HONO and NO₂ in the troposphere.

Introductory lecture: atmospheric chemistry in the Anthropocene

The term “Anthropocene” was coined by Professor Paul Crutzen in 2000 to describe an unprecedented era in which anthropogenic activities are impacting planet Earth on a global scale. Greatly increased emissions into the atmosphere, reflecting the advent of the Industrial Revolution, have caused significant changes in both the lower and upper atmosphere. Atmospheric reactions of the anthropogenic emissions and of those with biogenic compounds have significant impacts on human health, visibility, climate and weather. Two activities that have had particularly large impacts on the troposphere are fossil fuel combustion and agriculture, both associated with a burgeoning population. Emissions are also changing due to alterations in land use. This paper describes some of the tropospheric chemistry associated with the Anthropocene, with emphasis on areas having large uncertainties. These include heterogeneous chemistry such as those of oxides of nitrogen and the neonicotinoid pesticides, reactions at liquid interfaces, organic oxidations and particle formation, the role of sulfur compounds in the Anthropocene and biogenic–anthropogenic interactions. A clear and quantitative understanding of the connections between emissions, reactions, deposition and atmospheric composition is central to developing appropriate cost-effective strategies for minimizing the impacts of anthropogenic activities. The evolving nature of emissions in the Anthropocene places atmospheric chemistry at the fulcrum of determining human health and welfare in the future.

Absorption of Near UV Light by HNO3/NO3(-) on Sapphire Surfaces

We have determined absorption of the near UV light (290-345 nm) by nitric acid (HNO3) deposition on sapphire window surfaces as a function of the HNO3 pressure, by using Brewster angle cavity ring-down spectroscopy. Apparent monolayer HNO3 surface absorption cross sections have been obtained; they range between (1.7 ± 1.1) × 10(-19) and (0.29 ± 0.03) × 10(-19) cm(2)/molecule. When nitric acid cross section values on sapphire surfaces were divided by those on fused silica surfaces for which only molecular HNO3 adsorption was reported, a new absorption band appeared in the 320-345 nm region. The shape of this absorption band is similar to that reported for surface nitrate (NO3(-)) at quartz/water interfaces, but is red-shifted by about 10 nm. Our study suggests that a small percentage (<7%) of adsorbed HNO3 formed by HNO3 deposition on sapphire surfaces is dissociated into surface nitrate on the time scale of about 5-7 min. Background transmission changes in the 320-350 nm region after exposing clean sapphire surfaces with many repeated HNO3 deposition/evacuation cycles are consistent with surface nitrate formation. We obtained nitrate surface absorption cross section data over 320-350 nm range. We also modeled photolysis rates of HNO3/NO3(-) on urban grimes. Atmospheric implications of the results are discussed.

Rapid cycling of reactive nitrogen in the marine boundary layer

Nitrogen oxides are essential for the formation of secondary atmospheric aerosols and of atmospheric oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atmosphere. Nitric acid, a major oxidation product of nitrogen oxides, has traditionally been considered to be a permanent sink of nitrogen oxides. However, model studies predict higher ratios of nitric acid to nitrogen oxides in the troposphere than are observed. A 'renoxification' process that recycles nitric acid into nitrogen oxides has been proposed to reconcile observations with model studies, but the mechanisms responsible for this process remain uncertain. Here we present data from an aircraft measurement campaign over the North Atlantic Ocean and find evidence for rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. Laboratory experiments further demonstrate the photolysis of particulate nitrate collected on filters at a rate more than two orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Box model calculations based on the Master Chemical Mechanism suggest that particulate nitrate photolysis mainly sustains the observed levels of nitrous acid and nitrogen oxides at midday under typical marine boundary layer conditions. Given that oceans account for more than 70 per cent of Earth's surface, we propose that particulate nitrate photolysis could be a substantial tropospheric nitrogen oxide source. Recycling of nitrogen oxides in remote oceanic regions with minimal direct nitrogen oxide emissions could increase the formation of tropospheric oxidants and secondary atmospheric aerosols on a global scale.

Photolysis of Nitric Acid and Nitrate on Natural and Artificial Surfaces

Photolysis of nitric acid and nitrate (HNO3/nitrate) was investigated on the surfaces of natural and artificial materials, including plant leaves, metal sheets, and construction materials. The surfaces were conditioned in the outdoor air prior to experiments to receive natural depositions of ambient HNO3/nitrate and other atmospheric constituents. The photolysis rate constant (JHNO3(s)) of the surface HNO3/nitrate was measured based on the production rates of nitrous acid (HONO) and nitrogen oxides (NOx). The JHNO3(s) values, from 6.0 × 10(-6) s(-1) to 3.7 × 10(-4) s(-1), are 1 to 3 orders of magnitude higher than that of gaseous HNO3. The HONO was the major product from photolysis of HNO3/nitrate on most plant leaves, whereas NOx was the major product on most artificial surfaces. The JHNO3(s) values decreased with HNO3/nitrate surface density and could be described by a simple analytical equation. Within a typical range of HNO3/nitrate surface density in the low-NOx forested areas, photolysis of HNO3/nitrate on the forest canopy can be a significant source for HONO and NOx for the overlying atmosphere.

Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces

HONO formation by photolysis of HNO₃ on clean surfaces is no significant source of HONO and NO_x in the atmosphere.

Release of nitrous acid and nitrogen dioxide from nitrate photolysis in acidic aqueous solutions

Nitrate (NO3(-)) is an abundant component of aerosols, boundary layer surface films, and surface water. Photolysis of NO3(-) leads to NO2 and HONO, both of which play important roles in tropospheric ozone and OH production. Field and laboratory studies suggest that NO3¯ photochemistry is a more important source of HONO than once thought, although a mechanistic understanding of the variables controlling this process is lacking. We present results of cavity-enhanced absorption spectroscopy measurements of NO2 and HONO emitted during photodegradation of aqueous NO3(-) under acidic conditions. Nitrous acid is formed in higher quantities at pH 2-4 than expected based on consideration of primary photochemical channels alone. Both experimental and modeled results indicate that the additional HONO is not due to enhanced NO3(-) absorption cross sections or effective quantum yields, but rather to secondary reactions of NO2 in solution. We find that NO2 is more efficiently hydrolyzed in solution when it is generated in situ during NO3(-) photolysis than for the heterogeneous system where mass transfer of gaseous NO2 into bulk solution is prohibitively slow. The presence of nonchromophoric OH scavengers that are naturally present in the environment increases HONO production 4-fold, and therefore play an important role in enhancing daytime HONO formation from NO3(-) photochemistry.

Absorption cross sections of surface-adsorbed H2O in the 295-370 nm region and heterogeneous nucleation of H2O on fused silica surfaces

We have determined absorption cross sections of a monolayer of H2O adsorbed on the fused silica surfaces in the 295-370 nm region at 293 ± 1 K by using Brewster angle cavity ring-down spectroscopy. Absorption cross sections of surface-adsorbed H2O vary between (4.66 ± 0.83) × 10(-20) and (1.73 ± 0.52) × 10(-21) cm(2)/molecule over this wavelength range, where errors quoted represent experimental scatter (1σ). Our experimental study provides direct evidence that surface-adsorbed H2O is an absorber of the near UV solar radiation. We also varied the H2O pressure in the surface study cell over the 0.01-17 Torr range and obtained probe laser absorptions at 295, 340, and 350 nm by multilayer of adsorbed H2O molecules until the heterogeneous nucleation of water occurred on fused silica surfaces. The average absorption cross sections of multilayer adsorbed H2O are (2.17 ± 0.53) × 10(-20), (2.48 ± 0.67) × 10(-21), and (2.34 ± 0.59) × 10(-21) cm(2)/molecule at 295, 340, and 350 nm. The average absorption cross sections of transitional H2O layer are (6.06 ± 2.73) × 10(-20), (6.48 ± 3.85) × 10(-21), and (8.04 ± 4.92) × 10(-21) cm(2)/molecule at 295, 340, and 350 nm. The average thin water film absorption cross sections are (2.39 ± 0.50) × 10(-19), (3.21 ± 0.81) × 10(-20), and (3.37 ± 0.94) × 10(-20) cm(2)/molecule at 295 nm, 340 nm, and 350 nm. Atmospheric implications of the results are discussed.

Chemistry of NOx on TiO2 Surfaces Studied by Ambient Pressure XPS: Products, Effect of UV Irradiation, Water, and Coadsorbed K(.)

Self-cleaning surfaces containing TiO2 nanoparticles have been postulated to efficiently remove NOx from the atmosphere. However, UV irradiation of NOx adsorbed on TiO2 also was shown to form harmful gas-phase byproducts such as HONO and N2O that may limit their depolluting potential. Ambient pressure XPS was used to study surface and gas-phase species formed during adsorption of NO2 on TiO2 and subsequent UV irradiation at λ = 365 nm. It is shown here that NO3(-), adsorbed on TiO2 as a byproduct of NO2 disproportionation, was quantitatively converted to surface NO2 and other reduced nitrogenated species under UV irradiation in the absence of moisture. When water vapor was present, a faster NO3(-) conversion occurred, leading to a net loss of surface-bound nitrogenated species. Strongly adsorbed NO3(-) in the vicinity of coadsorbed K(+) cations was stable under UV light, leading to an efficient capture of nitrogenated compounds.