Jonathan D. Raff

Chlorine activation indoors and outdoors via surface-mediated reactions of nitrogen oxides with hydrogen chloride.

Gaseous HCl generated from a variety of sources is ubiquitous in both outdoor and indoor air. Oxides of nitrogen (NOy) are also globally distributed, because NO formed in combustion processes is oxidized to NO2, HNO3, N2O5 and a variety of other nitrogen oxides during transport. Deposition of HCl and NOy onto surfaces is commonly regarded as providing permanent removal mechanisms. However, we show here a new surface-mediated coupling of nitrogen oxide and halogen activation cycles in which uptake of gaseous NO2 or N2O5 on solid substrates generates adsorbed intermediates that react with HCl to generate gaseous nitrosyl chloride (ClNO) and nitryl chloride (ClNO2), respectively. These are potentially harmful gases that photolyze to form highly reactive chlorine atoms. The reactions are shown both experimentally and theoretically to be enhanced by water, a surprising result given the availability of competing hydrolysis reaction pathways. Airshed modeling incorporating HCl generated from sea salt shows that in coastal urban regions, this heterogeneous chemistry increases surface-level ozone, a criteria air pollutant, greenhouse gas and source of atmospheric oxidants. In addition, it may contribute to recently measured high levels of ClNO2 in the polluted coastal marine boundary layer. This work also suggests the potential for chlorine atom chemistry to occur indoors where significant concentrations of oxides of nitrogen and HCl coexist.

Catalytic Role for Water in the Atmospheric Production of ClNO

High level ab initio calculations of clusters comprised of water, HCl, and ON-ONO(2) are used to study nitrosyl chloride (ClNO) formation in gas phase water clusters, which are also mimics for thin water films present at environmental interfaces. Two pathways are considered, direct formation from the reaction of gaseous HCl with ON-ONO(2) and an indirect pathway involving the hydrolysis of ON-ONO(2) to form HONO, followed by the reaction of HONO with HCl to form ClNO. Surprisingly, direct formation of ClNO is found to be the dominant channel in the presence of water despite the possibility of a competing hydrolysis of ON-ONO(2) to form HONO. A single water molecule effectively catalyzes the ON-ONO(2) + HCl reaction, and in the presence of two or more water molecules the reaction to form ClNO becomes spontaneous. Direct formation of ClNO is fast at room and ice temperatures, indicating the possible significance of this pathway for chlorine activation chemistry in both the polar and midlatitude troposphere, in volcanic plumes and indoors. The reaction enthalpies, activation energies, and rate constants for all studied reactions are reported. The results are discussed in light of recent experiments.

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.

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.

Photooxidation of Ammonia on TiO2 as a Source of NO and NO2 under Atmospheric Conditions

Ammonia is the most abundant reduced nitrogen species in the atmosphere and an important precursor in the industrial-scale production of nitric acid. A coated-wall flow tube coupled to a chemiluminescence NOx analyzer was used to study the kinetics of NH3 uptake and NOx formation from photochemistry initiated on irradiated (λ > 290 nm) TiO2 surfaces under atmospherically relevant conditions. The speciation of NH3 on TiO2 surfaces in the presence of surface-adsorbed water was determined using diffuse reflection infrared Fourier transform spectroscopy. The uptake kinetics exhibit an inverse dependence on NH3 concentration as expected for reactions proceeding via a Langmuir-Hinshelwood mechanism. The mechanism of NOx formation is shown to be humidity dependent: Water-catalyzed reactions promote NOx formation up to a relative humidity of 50%. Less NOx is formed above 50%, where increasing amounts of adsorbed water may hinder access to reactive sites, promote formation of unreactive NH4(+), and reduce oxidant levels due to higher OH radical recombination rates. A theoretical study of the reaction between the NH2 photoproduct and O2 in the presence of H2O supports the experimental conclusion that NOx formation is catalyzed by water. Calculations at the MP2 and CCSD(T) level on the bare NH2 + O2 reaction and the reaction of NH2 + O2 in small water clusters were carried out. Solvation of NH2OO and NHOOH intermediates likely facilitates isomerization via proton transfer along water wires, such that the steps leading ultimately to NO are exothermic. These results show that photooxidation of low levels of NH3 on TiO2 surfaces represents a source of atmospheric NOx, which is a precursor to ozone. The proposed mechanism may be broadly applicable to dissociative chemisorption of NH3 on other metal oxide surfaces encountered in rural and urban environments and employed in pollution control applications (selective catalytic oxidation/reduction) and during some industrial processes.

Complexes of HNO3 and NO3− with NO2 and N2O4, and their potential role in atmospheric HONO formation

Calculations were performed to determine the structures, energetics, and spectroscopy of the atmospherically relevant complexes (HNO(3)).(NO(2)), (HNO(3)).(N(2)O(4)), (NO(3)(-)).(NO(2)), and (NO(3)(-)).(N(2)O(4)). The binding energies indicate that three of the four complexes are quite stable, with the most stable (NO(3)(-)).(N(2)O(4)) possessing binding energy of almost -14 kcal mol(-1). Vibrational frequencies were calculated for use in detecting the complexes by infrared and Raman spectroscopy. An ATR-FTIR experiment showed features at 1632 and 1602 cm(-1) that are attributed to NO(2) complexed to NO(3)(-) and HNO(3), respectively. The electronic states of (HNO(3)).(N(2)O(4)) and (NO(3)(-)).(N(2)O(4)) were investigated using an excited state method and it was determined that both complexes possess one low-lying excited state that is accessible through absorption of visible radiation. Evidence for the existence of (NO(3)(-)).(N(2)O(4)) was obtained from UV/vis absorption spectra of N(2)O(4) in concentrated HNO(3), which show a band at 320 nm that is blue shifted by 20 nm relative to what is observed for N(2)O(4) dissolved in organic solvents. Finally, hydrogen transfer reactions within the (HNO(3)).(NO(2)) and (HNO(3)).(N(2)O(4)) complexes leading to the formation of HONO, were investigated. In both systems the calculated potential profiles rule out a thermal mechanism, but indicate the reaction could take place following the absorption of visible radiation. We propose that these complexes are potentially important in the thermal and photochemical production of HONO observed in previous laboratory and field studies.

Combined Flux Chamber and Genomics Approach Links Nitrous Acid Emissions to Ammonia Oxidizing Bacteria and Archaea in Urban and Agricultural Soil.

Nitrous acid (HONO) is a photochemical source of hydroxyl radical and nitric oxide in the atmosphere that stems from abiotic and biogenic processes, including the activity of ammonia-oxidizing soil microbes. HONO fluxes were measured from agricultural and urban soil in mesocosm studies aimed at characterizing biogenic sources and linking them to indigenous microbial consortia. Fluxes of HONO from agricultural and urban soil were suppressed by addition of a nitrification inhibitor and enhanced by amendment with ammonium (NH4(+)), with peaks at 19 and 8 ng m(-2) s(-1), respectively. In addition, both agricultural and urban soils were observed to convert (15)NH4(+) to HO(15)NO. Genomic surveys of soil samples revealed that 1.5-6% of total expressed 16S rRNA sequences detected belonged to known ammonia oxidizing bacteria and archaea. Peak fluxes of HONO were directly related to the abundance of ammonia-oxidizer sequences, which in turn depended on soil pH. Peak HONO fluxes under fertilized conditions are comparable in magnitude to fluxes reported during field campaigns. The results suggest that biogenic HONO emissions will be important in soil environments that exhibit high nitrification rates (e.g., agricultural soil) although the widespread occurrence of ammonia oxidizers implies that biogenic HONO emissions are also possible in the urban and remote environment.

Hydroxyl Radical Quantum Yields from Isopropyl Nitrite Photolysis in Air

Alkyl nitrites photolyze in air to yield alkoxy radicals and NO which, through secondary reactions, generate OH radicals. This photochemistry is important in the atmosphere and in laboratory studies where nitrites are often used as a source of OH. The overall quantum yield for hydroxyl radical formation from irradiation of isopropyl nitrite (i-C(3)H(7)ONO) between 300 and 425 nm in 1 atm air at 296 ± 2 K is reported for the first time. The OH radical was scavenged by reaction with CF(3)CF═CF(2) and the formation of CF(3)CFO and CF(2)O monitored as a function of time using Fourier transform infrared spectrometry. The quantum yield was found to be 0.54 ± 0.07 (2σ) and is independent of whether or not NO was added (up to 3 × 10(14) molecules cm(-3)) prior to photolysis to increase NO concentrations above those due to the photolysis of the nitrite. Ultraviolet-visible and infrared cross sections of i-C(3)H(7)ONO are also reported. These data on the OH quantum yields as well as the UV-visible and infrared cross sections for isopropyl nitrite are critical for quantitatively interpreting the results of laboratory studies where i-C(3)H(7)ONO is employed as an OH source as well as for assessing the role of alkyl nitrites in the chemistry of the troposphere.

The Role of Iron-Bearing Minerals in NO2 to HONO Conversion on Soil Surfaces.

Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO2 on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO2 reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO2 by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO2 hydrolysis mechanism. The magnitude of NO2-to-HONO conversion depends on the amount of Fe(2+) present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO2 and Fe(2+)(aq) present in thin films of water coating the surface, whereas in the range of pH 5-8 HONO stems from reaction of NO2 with structural iron or surface complexed Fe(2+) followed by protonation of nitrite via surface Fe-OH2(+) groups. Reduction of NO2 on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO2] ratios observed during dust storms in urban areas.

Evidence for Quinone Redox Chemistry Mediating Daytime and Nighttime NO2-to-HONO Conversion on Soil Surfaces

Humic acid (HA) is thought to promote NO2 conversion to nitrous acid (HONO) on soil surfaces during the day. However, it has proven difficult to identify the reactive sites in natural HA substrates. The mechanism of NO2 reduction on soil surrogates composed of HA and clay minerals was studied by use of a coated-wall flow reactor and cavity-enhanced spectroscopy. Conversion of NO2 to HONO in the dark was found to be significant and correlated to the abundance of C-O moieties in HA determined from the X-ray photoelectron spectra of the C 1s region. Twice as much HONO was formed when NO2 reacted with HA that was photoreduced by irradiation with UV-visible light compared to the dark reaction; photochemical reactivity was correlated to the abundance of C═O moieties rather than C-O groups. Bulk electrolysis was used to generate HA in a defined reduction state. Electrochemically reduced HA enhanced NO2-to-HONO conversion by a factor of 2 relative to non-reduced HA. Our findings suggest that hydroquinones and benzoquinones, which are interchangeable via redox equilibria, contribute to both thermal and photochemical HONO formation. This conclusion is supported by experiments that studied NO2 reactivity on mineral surfaces coated with the model quinone, juglone. Results provide further evidence that redox-active sites on soil surfaces drive ground-level NO2-to-nitrite conversion in the atmospheric boundary layer throughout the day, while amphoteric mineral surfaces promote the release of nitrite formed as gaseous HONO.