Production Of HNO3 Research Articles

Observations of NO<sub>x</sub>, ΣPNs, ΣANs, and HNO<sub>3</sub> at a Rural Site in the California Sierra Nevada Mountains: summertime diurnal cycles

Abstract. Observations of NO, NO2, total peroxy nitrates (ΣPNs), total alkyl nitrates (ΣANs), HNO3, CO, O3, and meteorological parameters were obtained from October 2000 through February 2002 at 1315 m a.s.l., 38.9° N, 120.6° W on Sierra Pacific Industries land, adjacent to the University of California Blodgett Forest Research Station (UC-BFRS). We describe the data set with emphasis on the diurnal cycles during summertime 2001. We show that transport of the Sacramento urban plume is a primary factor responsible for diurnal variation in total reactive nitrogen mixing ratios as well as in NOx, ΣPNs and ΣANs, all of which exhibit a late afternoon/early evening peak. In contrast, HNO3 has a peak just after local noon indicating that HNO3 is in near steady state during the day with production due to photochemistry and removal by deposition and mixing with the background free troposphere. Boundary layer dynamics influence mixing ratios of all species in the early morning. Analysis of the morning feature suggests that higher mixing ratios of NOx and HNO3 persist in the residual layer than in the nocturnal boundary layer indicating the presence of nocturnal sinks of both species. Nighttime observations also indicate large HNO3 and ΣANs production through oxidation of alkenes by NO3.

Reactive uptake coefficients for N2O5 determined from aircraft measurements during the Second Texas Air Quality Study: Comparison to current model parameterizations

This paper presents determinations of reactive uptake coefficients for N2O5, γ(N2O5), on aerosols from nighttime aircraft measurements of ozone, nitrogen oxides, and aerosol surface area on the NOAA P‐3 during Second Texas Air Quality Study (TexAQS II). Determinations based on both the steady state approximation for NO3 and N2O5 and a plume modeling approach yielded γ(N2O5) substantially smaller than current parameterizations used for atmospheric modeling and generally in the range 0.5–6 × 10−3. Dependence of γ(N2O5) on variables such as relative humidity and aerosol composition was not apparent in the determinations, although there was considerable scatter in the data. Determinations were also inconsistent with current parameterizations of the rate coefficient for homogenous hydrolysis of N2O5 by water vapor, which may be as much as a factor of 10 too large. Nocturnal halogen activation via conversion of N2O5 to ClNO2 on chloride aerosol was not determinable from these data, although limits based on laboratory parameterizations and maximum nonrefractory aerosol chloride content showed that this chemistry could have been comparable to direct production of HNO3 in some cases.

Measurements of HNO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> using ion drift-chemical ionization mass spectrometry during the MILAGRO/MCMA-2006 campaign

Abstract. An ion drift-chemical ionization mass spectrometer (ID-CIMS) was deployed in Mexico City between 7 and 31 March to measure gas-phase nitric acid (HNO3) and dinitrogen pentoxide (N2O5 during the Mexico City Metropolitan Area (MCMA)-2006 field campaign. The observation site was located at the Instituto Mexicano del Petróleo in the northern part of Mexico City urban area with major emissions of pollutants from residential, vehicular and industrial sources. Diurnally, HNO3 was less than 200 parts per trillion (ppt) during the night and early morning. The concentration of HNO3 increased steadily from around 09:00 a.m. central standard time (CST), reached a peak value of 0.5 to 3 parts per billion (ppb) in the early afternoon, and then declined sharply to less than half of the peak value near 05:00 p.m. CST. An inter-comparison between the ID-CIMS and an ion chromatograph/mass spectrometer (ICMS) showed a good agreement between the two HNO3 measurements (R2=0.75). The HNO3 mixing ratio was found to anti-correlate with submicron-sized aerosol nitrate, suggesting that the gas-particle partitioning process was a major factor in determining the gaseous HNO3 concentration. Losses by irreversible reactions with mineral dust and via dry deposition also could be important at this site. Most of the times during the MCMA 2006 field campaign, N2O5 was found to be below the detection limit (about 30 ppt for a 10 s integration time) of the ID-CIMS, because of high NO mixing ratio at the surface (>100 ppb) during the night. An exception occurred on 26 March 2006, when about 40 ppt N2O5 was observed during the late afternoon and early evening hours under cloudy conditions before the build-up of NO at the surface site. The results revealed that during the MCMA-2006 field campaign HNO3 was primarily produced from the reaction of OH with NO2 and regulated by gas/particle transfer and dry deposition. The production of HNO3 from N2O5 hydrolysis during the nighttime was small because of high NO and low O3 concentrations near the surface.

Influence of local photochemistry on isotopes of nitrate in Greenland snow

To explore the seasonality in δ15N and δ18O of nitrate in Greenland snow, we describe a simple box model of local photochemistry. Isotope ratios of HNO3 are controlled by the nitrogen isotope fractionation between NO and NO2, the ratio of NO2 to NO, and seasonal variations in HNO3 production. The observed seasonal range in δ15N requires either a large net fractionation (∼70‰) associated with NOx cycling or a seasonal change in the δ15N of NOx sources. The observed range in δ18O of nitrate is smaller than that calculated from HNO3 production pathways, suggesting that seasonal transport may also be required to explain the seasonality in nitrate δ18O.

About the increase of HNO3 in the stratopause region during the Halloween 2003 solar proton event

In this paper, we explain the HNO3 observations made with the MIPAS/Envisat instrument in the northern polar region at the time of the October‐November 2003 solar proton event. Increases of 0.5–5.5 ppbv are seen at altitudes 35–58 km after the onset of the event. Results from the Sodankylä Ion and Neutral Chemistry model are in good agreement with the MIPAS observations, except at around 58 km where the modeled mixing ratios are about a factor of two larger. According to the model results, HNO3 production at altitudes above 35 km is almost entirely due to ion‐ion recombination between NO3− and H+ cluster ions. At 35 km and below, there is contribution also from N2O5 reacting with ions as well as from NO2 reacting with OH.

Application of Calcined Layered Double Hydroxides as Catalysts for Abatement of N2O Emissions

The results of catalytic decomposition of N2O over mixed oxide catalysts obtained by calcination of layered double hydroxides (LDHs) are summarized. Mixed oxides were prepared by thermal treatment (500 °C) of coprecipitated LDH precursors with general chemical composition of MII1-xMIIIx(OH)2(CO3)x/2·yH2O, where MII was Ni, Co, Cu and/or Mg, MIII was Mn, Fe and/or Al, and the MII/MIII molar ratio was adjusted to 2. The influence of chemical composition of the MII-MIII mixed oxide catalysts on their activity and stability in N2O decomposition was examined. The highest N2O conversion was reached over Ni-Al (4:2) and Co-Mn-Al (4:1:1) catalysts. Their suitability for practical application was proved in simulated process stream in the presence of O2, NO, NO2 and H2O. It was found that N2O conversion decreased with increasing amount of oxygen in the feed. The presence of NO in the feed caused a slight decrease in N2O conversion. A strong decrease in the reaction rate was observed over the Ni-Al catalyst in the presence of NO2 while no N2O conversion decrease was observed over the Co-Mn-Al catalyst. Water vapor inhibited the N2O decomposition over all tested catalysts. The obtained kinetic data for N2O decomposition in a simulated process stream over the Co-Mn-Al catalyst were used for a preliminary reactor design. The packed bed volume necessary for N2O emission abatement in a HNO3 production plant was calculated as 35 m3 for waste gas flow rate of 30 000 m3 h-1.

Validation of the Aura Microwave Limb Sounder HNO3 measurements

We assess the quality of the version 2.2 (v2.2) HNO3 measurements from the Microwave Limb Sounder (MLS) on the Earth Observing System Aura satellite. The MLS HNO3 product has been greatly improved over that in the previous version (v1.5), with smoother profiles, much more realistic behavior at the lowest retrieval levels, and correction of a high bias caused by an error in one of the spectroscopy files used in v1.5 processing. The v2.2 HNO3 data are scientifically useful over the range 215 to 3.2 hPa, with single‐profile precision of ∼0.7 ppbv throughout. Vertical resolution is 3–4 km in the upper troposphere and lower stratosphere, degrading to ∼5 km in the middle and upper stratosphere. The impact of various sources of systematic uncertainty has been quantified through a comprehensive set of retrieval simulations. In aggregate, systematic uncertainties are estimated to induce in the v2.2 HNO3 measurements biases that vary with altitude between ±0.5 and ±2 ppbv and multiplicative errors of ±5–15% throughout the stratosphere, rising to ∼±30% at 215 hPa. Consistent with this uncertainty analysis, comparisons with correlative data sets show that relative to HNO3 measurements from ground‐based, balloon‐borne, and satellite instruments operating in both the infrared and microwave regions of the spectrum, MLS v2.2 HNO3 mixing ratios are uniformly low by 10–30% throughout most of the stratosphere. Comparisons with in situ measurements made from the DC‐8 and WB‐57 aircraft in the upper troposphere and lowermost stratosphere indicate that the MLS HNO3 values are low in this region as well, but are useful for scientific studies (with appropriate averaging).

Space‐based constraints on the production of nitric oxide by lightning

We interpret observations of trace‐gases from three satellite platforms to provide top‐down constraints on the production of NO by lightning. The space‐based observations are tropospheric NO2 columns from SCIAMACHY, tropospheric O3 columns from OMI and MLS, and upper tropospheric HNO3 from ACE‐FTS. A global chemical transport model (GEOS‐Chem) is used to identify locations and time periods in which lightning would be expected to dominate the trace gas observations. The satellite observations are sampled at those locations and time periods. All three observations exhibit a maximum in the tropical Atlantic region and a minimum in the tropical Pacific. This wave‐1 pattern is driven by injection of lightning NO into the upper troposphere over the tropical continents, followed by photochemical production of NO2, HNO3, and O3 during transport. Lightning produces a broad enhancement over the tropical Atlantic and Africa of 2–6 × 1014 molecules NO2 cm−2, 4 × 1017 molecules O3 cm−2 (15 Dobson Units), and 125 pptv of upper tropospheric HNO3. The lightning background is 25–50% weaker over the tropical Pacific. A global source of 6 ± 2 Tg N yr−1 from lightning in the model best represents the satellite observations of tropospheric NO2, O3, and HNO3.

The heterogeneous chemical kinetics of N<sub>2</sub>O<sub>5</sub> on CaCO<sub>3</sub> and other atmospheric mineral dust surrogates

Abstract. Uptake experiments of N2O5 on several mineral dust powder samples were carried out under continuous molecular flow conditions at 298±2 K. At [N2O5]0=(4.0±1.0)×1011 cm−3 we have found γss values ranging from (3.5±1.1)×10−2 for CaCO3 to (0.20±0.05) for Saharan Dust with γss decreasing as [N2O5]0 increased. The uptake coefficients reported in this work are to be regarded as upper limiting values owing to the fact that they are based on the geometric (projected) surface area of the mineral dust sample. We have observed delayed production of HNO3 upon uptake of N2O5 for every investigated sample owing to hydrolysis of N2O5 with surface-adsorbed H2O. Arizona Test Dust and Kaolinite turned out to be the samples that generated the largest amount of gas phase HNO3 with respect to N2O5 taken up. In contrast, the yield of HNO3 for Saharan Dust and CaCO3 is lower. On CaCO3 the disappearance of N2O5 was also accompanied by the formation of CO2. For CaCO3 sample masses ranging from 0.33 to 2.0 g, the yield of CO2 was approximately 42–50% with respect to the total number of N2O5 molecules taken up. The reaction of N2O5 with mineral dust and the subsequent production of gas phase HNO3 lead to a decrease in [NOx] which may have a significant effect on global ozone.

Formation of HNO3 and NO3− in the anthropogenically‐influenced eastern Mediterranean marine boundary layer

To investigate the seasonal behaviour of nitrate radicals (NO3) as a source of nitric acid (HNO3) and particulate nitrate (NO3−) in the eastern Mediterranean atmosphere, we conducted NO3 measurements on the island of Crete for more than two years. Ancillary measurements of chemical compounds and meteorological parameters were used to analyse the observations. Nighttime production of HNO3 plus NO3−, as initiated by NO3 radicals, accounts for about 50–65% of the total production rate depending on season. The remainder is produced by the hydroxyl radical (OH) reaction with nitrogen dioxide during daytime. On a yearly mean basis, about 17% of the HNO3 plus NO3− formation results from the reaction of NO3 radicals with dimethylsulfide of marine origin. This shows important interactions between biogenic and anthropogenic compounds contributing to nutrient deposition to the sea.

Glacial/interglacial changes in the isotopes of nitrate from the Greenland Ice Sheet Project 2 (GISP2) ice core

The 15N/14N and 18O/16O ratios of nitrate in the Greenland Ice Sheet Project 2 (GISP2) (Summit, Greenland) ice core are much higher in ice from the last glacial period than in the pre‐industrial Holocene, despite the lack of a significant glacial/interglacial change in nitrate concentration. While both the 15N/14N and 18O/16O records are anticorrelated with snow accumulation rate, neither is satisfactorily explained by accumulation changes or post‐depositional processes. The similarity in the glacial/interglacial change in 15N/14N from several different Greenland ice cores and the large amplitude of this change relative to observed seasonal variation raise the possibility that the isotopes of nitrate in ice cores indicate a large‐scale glacial/interglacial change in the isotopic composition of atmospheric NOx. The glacial/interglacial change in 18O/16O is best explained by a greater contribution of HNO3 production from hydrolysis of N2O5, which has implications for reconstruction of past atmospheric oxidant levels. Although isotope effects associated with NOx photochemistry and nitrate scavenging have not been fully characterized, the 15N/14N data may indicate glacial/interglacial changes in the relative contributions from different natural sources of NOx on a hemispheric or global scale.

Seasonal variations in N and O isotopes of nitrate in snow at Summit, Greenland: Implications for the study of nitrate in snow and ice cores

Nitrogen and oxygen isotopes of NO3− have been measured in snow and firn from Summit, Greenland. The 15N/14N and 18O/16O ratios of NO3− in recently fallen snow are similar to those of surface snow. Diurnal variation is observed in 15N/14N of NO3−, and possibly 18O/16O, suggesting fractionating loss of NO3− from snow during the day, which is subsequently recovered at night. A larger seasonal variation is observed, with higher 15N/14N and lower 18O/16O of NO3− in summer than winter, which cannot be explained by postdepositional fractionation. The generally high 18O/16O of NO3− in Greenland snow (δ18O versus VSMOW = 65.2 to 79.6‰) indicates that oxygen atoms from ozone have been incorporated into NOx that was subsequently deposited as HNO3. The lower mean δ18O of NO3− in summer snow relative to winter (68.9‰ in summer 2000 and 70.5‰ in summer 2001 versus 77.5‰ in winter 2000–01) is a result of summertime HNO3 production via NO2 reaction with hydroxyl radical (OH), which dilutes the high δ18O imparted on NO2 from ozone. The higher mean 15N/14N of NO3− observed in snow from spring (δ15N versus air N2 = +5.9‰ in 2000 and −1.4‰ in 2001) and summer (+0.1‰ in 2000 and −0.8‰ in 2001) than fall (−9.2‰ in 2000) and winter (−10.0‰ in 2000–01) is more difficult to explain with seasonal photochemistry, given current knowledge. The seasonal 15N/14N change may reflect NOx sources, with a greater fall and wintertime contribution from fossil fuel emissions relative to other inputs of NOx (i.e., biogenic soil emissions, biomass burning, and lightning).

Soluble reactive nitrogen oxides at South Pole during ISCAT 2000

Abstract The mist chamber/ion chromatography technique was used to measure soluble gaseous nitrite (NO2−) and nitrate (NO3−) within the lowermost atmosphere (2 cm up to 10 m above the snow) and in the firn air at South Pole during the last 2 weeks of December 2000. Collected NO2− and NO3− are attributed to nitrous (HONO) and nitric (HNO3) acids, respectively. Firn air mixing ratios of HONO were more than five times higher than those just above the snow. A single day of measurements showed HONO mixing ratios to be significantly lower at 10 m than 85 cm above the snow. These gradients suggest that HONO is produced in the snowpack and fluxes out into the overlying air. A strong and persistent local source of HONO is required to sustain mean mixing ratios of 30 ppt against very fast loss by photolysis under the intense 24 h sunlight at South Pole. For HNO3, measurements in the atmosphere were made with two independent techniques. A CIMS technique provided data at 1 min resolution, at a sampling height of 10 m. The mist chamber/ion chromatography technique integrated over nominally 30 min intervals, with most measurements made 85 cm above the snow. Mixing ratios of HNO3 were highest (mean 38 ppt) at 85 cm above the snow, compared to those both in the firn air and at 10 m above the snow. Comparing HNO3 mixing ratios between 85 cm and firn air suggests flux into the snow, while the comparisons between 85 cm and 10 m suggest fluxes in the opposite direction. We speculate that the maximum just above the snow surface reflects in situ production of HNO3, supported by fluxes of NOx and OH precursors (e.g., HONO, HCHO, HOOH) out of the snow. Comparison of NO3− concentrations in snow immediately upwind of the Atmospheric Research Observatory, 50 m away from the building and 15 km away from the station revealed no significant differences. This finding indicates that the very high NO mixing ratios observed during ISCAT 1998 and 2000 cannot be attributed to NO3− contamination of the snow surrounding the sampling location.

Reaction of OH with HO2NO2 (Peroxynitric Acid): Rate Coefficients between 218 and 335 K and Product Yields at 298 K

Rate coefficients (k3(T)) for the reaction of OH with HO2NO2 (peroxynitric acid, PNA) in the gas phase were measured in the temperature range of 218−335 K by producing OH via pulsed laser photolysis and detecting it via laser-induced fluorescence. The PNA concentration was measured in situ by UV and IR absorption. The H2O2, HNO3, and NO2 impurities present in the PNA sample were quantified by mass spectrometry and/or UV/IR absorption. The measured value of k3(298 K) is (3.4 ± 1.0) × 10-12 cm3 molecule-1 s-1. The temperature dependence of k3 is best described by the relation k3(T) = (8.8 ± 2.6) × 10-19T2 exp[(1130 ± 20)/T] cm3 molecule-1 s-1. The quoted errors for k3 are at the 2σ level and include estimated systematic errors, which contribute the most to this uncertainty. The measured values of k3(T) were independent of pressure between 10 and 100 Torr of helium. The branching ratios of the reaction OH + HO2NO2 → products, for the production of HO2 and HNO3 and of NO3 and H2O2, respectively, were determined to be <10% and <5%, respectively, at 298 K. Thus, it was deduced that the main pathway for reaction 3 produces H2O, O2, and NO2 at 298 K. Our measurements reduce the uncertainties but do not significantly alter the currently calculated impacts of HO2NO2 in the upper troposphere and lower stratosphere. In the course of this study, the rate coefficient for the reaction of OH with H2O2 was measured to be k4(T) = (2.9 ± 1.8) × 10-12 exp[−(110 ± 150)/T] cm3 molecule-1 s-1 in the temperature range of 273−356 K.

A modeling study of the stratospheric NOx/NOy and NOx/HNO3 ratios: Single‐ versus dual‐channeled mode of OH, NO2 association

UARS observations bearing upon the NOx/NOy and NOx/HNO3 ratios were revisited within the framework of advances in laboratory studies that were spurred by the findings, around 1999, of considerable model underestimation of the observations. The objective was to study the two models of the OH, NO2 association reaction, one that considers the single HNO3 product channel and the other that considers dual HNO3 and HOONO products channels. Rate coefficients of the OH + HNO3 and the single‐channeled OH + NO2 → HNO3 reactions recommended in JPL00‐3 produce remarkably improved agreement between the modeled and observed NOx/HNO3 and NOx/NOy, compared to the case with the recommendations in JPL‐97. This agrees with the results of other previous studies, such as those using the Photochemistry of Ozone Loss in the Arctic Region in Summer and MkIV data. The consumption of OH, NO2 pair in the lower stratosphere via the three‐body association occurs at almost the same rate (within ±5%) whether the single‐ or the double‐channeled version of that process is used with rate coefficients from JPL00‐3. However, only 70–80% of this consumption yields HNO3 in the dual‐channeled version, compared to 100% in the case of the single‐channeled version. Consequently, the dual‐channeled model of HNO3 formation performs better than the single‐channeled model in predicting NOx/HNO3 and NOx/NOy ratios. At 46.4 mbar, for example, the mean percentage difference between the modeled and the observed NOx/HNO3 equals about −10%, if HOONO is included. The corresponding number is about −19% when HOONO is excluded. Because laboratory experiments on OH, NO2 association that directly detect HOONO isomer are still maturing, further studies are needed. Further studies are also needed to understand the intriguing tendency of the model‐measurement differences to maximize around 20 mbar.

First measurements and modeling of Δ17O in atmospheric nitrate

Atmospheric nitrate is anomalously enriched in both the 18O/16O and 17O/16O isotopic ratios, with Δ17O = 20–30.8‰ (Δ17O = δ17O − .52 × δ18O). We have modeled the seasonal variation in Δ17O by utilizing the transfer of Δ17O from ozone to HNO3 during NOx oxidation reactions and by evaluating the relative source strengths of heterogeneous and homogenous HNO3 production. The implications for new insights into the global nitrogen cycle, N2O production and paleoclimate studies are discussed.

Global and regional decreases in tropospheric oxidants from photochemical effects of aerosols

We evaluate the sensitivity of tropospheric OH, O3, and O3 precursors to photochemical effects of aerosols not usually included in global models: (1) aerosol scattering and absorption of ultraviolet radiation and (2) reactive uptake of HO2, NO2, and NO3. Our approach is to couple a global 3‐D model of tropospheric chemistry (GEOS‐CHEM) with aerosol fields from a global 3‐D aerosol model (GOCART). Reactive uptake by aerosols is computed using reaction probabilities from a recent review (γHO2 = 0.2, γNO2 = 10−4, γNO3 = 10−3). Aerosols decrease the O3 → O(1D) photolysis frequency by 5–20% at the surface throughout the Northern Hemisphere (largely due to mineral dust) and by a factor of 2 in biomass burning regions (largely due to black carbon). Aerosol uptake of HO2 accounts for 10–40% of total HOx radical (≡ OH + peroxy) loss in the boundary layer over polluted continental regions (largely due to sulfate and organic carbon) and for more than 70% over tropical biomass burning regions (largely due to organic carbon). Uptake of NO2 and NO3 accounts for 10–20% of total HNO3 production over biomass burning regions and less elsewhere. Annual mean OH concentrations decrease by 9% globally and by 5–35% in the boundary layer over the Northern Hemisphere. Simulated CO increases by 5–15 ppbv in the remote Northern Hemisphere, improving agreement with observations. Simulated boundary layer O3 decreases by 15–45 ppbv over India during the biomass burning season in March and by 5–9 ppbv over northern Europe in August, again improving comparison with observations. We find that particulate matter controls would increase surface O3 over Europe and other industrial regions.

The emission and chemistry of reactive nitrogen species in the plume of an Athena II solid‐fuel rocket motor

In situ measurements of NO, NOy, HNO3, ClO, CO2, H2O and particles were made in the plume of an Athena II solid‐fuel rocket motor (SRM) with instruments onboard the NASA WB‐57F high‐altitude aircraft. By normalization to CO2, the NOy emission index was calculated to be 2.7±0.6 g NO2 (kg fuel)−1. HNO3 is a significant and evolving component plume NOy. The principal source of HNO3 is thought to be the heterogeneous reaction between ClONO2 and HCl on emitted alumina particles. These measurements provide the first experimental evidence for the production of HNO3 and the activation of chlorine on SRM‐emitted alumina particles in the lower stratosphere.

Heterogeneous reactions of HNO3with flame soot generated under different combustion conditions. Reaction mechanism and kinetics

The reaction of HNO3 with decane (C10H22) soot generated in the laboratory has been studied in a Knudsen flow reactor. Two different types of soot were produced: Soot originating from a rich flame of decane and air (“grey” soot) and soot generated from a leaner flame of decane and air (“black” soot). Both HNO3 uptake and product release have been observed. Whereas soot from a rich decane flame leads to HONO as the main product, NO and small amounts of NO2 are observed from soot generated in a lean flame. A reaction mechanism is proposed in which HNO3 is reduced to HONO, which is released into the gas phase on grey soot, but undergoes decomposition reactions on black soot to NO and NO2, the latter of which also reacts on black soot to produce additional NO. Experiments on black soot show that at [HNO3] > 1011 molecule cm−3 a higher order process is induced which is associated with an additional production of NO and NO2. Further studies of the reaction of gaseous NO with adsorbed HNO3 reveal a slow reaction leading to HONO and NO2. Both initial uptake (γ0) of HNO3 as well as steady state uptake probabilities (γss) have been obtained: γ0 = (4.6 ± 2.3) × 10−3 and γss = (5.2 ± 1.3) × 10−4 for reaction on grey soot and γ0 = (2.0 ± 0.1) × 10−2 and γss = (4.6 ± 1.6) × 10−3 for black soot, using the geometric surface area over which the samples are spread.

Gas Phase Reactions of HONO with NO2, O3, and HCl: Ab Initio and TST Study

Gas-phase reactions of HONO with species of atmospheric interest, NO2, O3, and HCl, have been investigated by means of ab initio molecular-orbital and transition-state theory calculations. For the HONO + NO2 reaction, the most favorable pathway is found to be the abstraction of OH in HONO by NO2, leading to the formation of HNO3 and NO products. The activation energies computed at the G2M (RCC,MP2) level of theory for the abstraction from trans- and cis-HONO are predicted to be 31.6 and 33.4 kcal/mol, respectively. At 300 K, the calculated rate constant is smaller than 1.2 × 10-14 cm3 mol-1 s-1. For the HONO + O3 reaction, the barrier height for the production of HNO3 + O2 is predicted to be 15.0 kcal/mol with the rate constant at 300 K, 5.7 × 10-2 cm3 mol-1 s-1. For the trans- and cis-HONO + HCl → H2O + ClNO reactions, our G2M calculations predict the corresponding barrier heights of 12.9 and 14.4 kcal/mol with predicted rate constants, 1.3 × 102 and 0.13 × 102 cm3 mol-1 s-1 at 300 K. The results of our ...