Formation of indoor nitrous acid (HONO) by light-induced NO<Subscript>2</Subscript> heterogeneous reactions with white wall paint

Gaseous nitrogen dioxide (NO₂) represents an oxidant that is present in relatively high concentrations in various indoor settings. Remarkably increased NO₂ levels up to 1.5 ppm are associated with homes using gas stoves. The heterogeneous reactions of NO₂ with adsorbed water on surfaces lead to the generation of nitrous acid (HONO). Here, we present a HONO source induced by heterogeneous reactions of NO₂ with selected indoor paint surfaces in the presence of light (300 nm?<?λ?<?400 nm). We demonstrate that the formation of HONO is much more pronounced at elevated relative humidity. In the presence of light (5.5 W m^?2), an increase of HONO production rate of up to 8.6?·?10⁹ molecules cm^?2 s^?1 was observed at [NO₂]?=?60 ppb and 50 % relative humidity (RH). At higher light intensity of 10.6 (W m^?2), the HONO production rate increased to 2.1?·?10¹⁰ molecules cm^?2 s^?1. A high NO₂ to HONO conversion yield of up to 84 % was observed. This result strongly suggests that a light-driven process of indoor HONO production is operational. This work highlights the potential of paint surfaces to generate HONO within indoor environments by light-induced NO₂ heterogeneous reactions.     

Fast nitrogen oxide photochemistry in Summit,Greenland snow

During the 1999 summer field season at Summit, Greenland, we conducted several series of experiments to follow up on our 1998 discovery that NO_x is released from the sunlit snowpack. The 1999 experiments included measurements of HONO in addition to NO and NO₂, and were designed to confirm, for Greenland snow, that the processes producing reactive nitrogen oxides in the snow are largely photochemical. Long duration experiments (up to 48 h) in a flow-through chamber and in the natural snowpack revealed sun-synchronous diurnal variations of all three reactive nitrogen oxides. In a second set of experiments we alternately shaded or exposed snow (again in the natural snowpack and in the chamber) to ambient sunlight for short periods to reduce any temperature changes during variations in light intensity. All three N oxides increased (decreased) very rapidly when sunlit (shaded). In all experiments NO₂ was approximately 3-fold more abundant than NO and HONO (which were at similar levels). Higher concentrations of NO₃⁻ in the snow resulted in higher mixing ratios of HONO, NO and NO₂ in the snow pore air, consistent with our hypothesis that photolysis of NO₃⁻ is the source of the reactive N oxides.     

Transformations,Lifetimes, and Sources of NO2, HONO,and HNO3 in Indoor Environments

Recent research has demonstrated that nitrogen oxides are transformed to nitrogen acids in indoor environments, and that significant concentrations of nitrous acid are present in indoor air. The purpose of the study reported in this paper has been to investigate the sources, chemical transformations and lifetimes of nitrogen oxides and nitrogen acids under the conditions existing in buildings. An unoccupied single family residence was instrumented for monitoring of NO, NO₂, NO_y, MONO, HNO₃, CO, temperature, relative humidity, and air exchange rate. For some experiments, NO₂ and HONO were injected into the house to determine their removal rates and lifetimes. Other experiments investigated the emissions and transformations of nitrogen species from unvented natural gas appliances. We determined that HONO is formed by both direct emissions from combustion processes and reaction of NO₂ with surfaces present indoors. Equilibrium considerations influence the relative contributions of these two sources to the indoor burden of HONO. We determined that the lifetimes of trace nitrogen species varied in the order NO ~ HONO > NO₂ >HNO₃. The lifetimes with respect to reactive processes are on the order of hours for NO and HONO, about an hour for NO₂, and 30 minutes or less for HNO3. The rapid removal of NO₂ and long lifetime of HONO suggest that HONO may represent a significant fraction of the oxidized nitrogen burden in indoor air.     

Simultaneous DOAS and mist-chamber IC measurements of HONO in Houston,TX

Nitrous acid is an important component of nighttime N-oxide chemistry, and provides a significant source of both OH and NO in polluted urban air masses shortly after sunrise. Several recent studies have called for new sources of HONO to account for daytime levels much higher than are consistent with current understanding. However, measurement of HONO is problematic, with most in-situ techniques reporting higher values than simultaneous optical measurements by long-path DOAS, especially during daytime. The discrepancy has been attributed to positive interference in the in-situ techniques, negative interference in DOAS retrievals, the difficulty of comparing the different air masses sampled by the methods, or combinations of these.During August and September 2006, HONO mixing ratios from collocated long-path DOAS and automated mist-chamber/ion chromatograph (MC/IC) systems ranged from several ppbv during morning rush hour to daytime minima near 100 pptv. Agreement between the two techniques was excellent across this entire range during many days, showing that both instruments accurately measured HONO during this campaign. A small bias towards higher LP-DOAS observations at night can be attributed to slow vertical mixing leading to pronounced HONO profiles. A positive daytime bias of the MC/IC instrument during several days in late August/early September was correlated with photochemically produced compounds such as ozone, HNO₃ and HCHO, but not with NO₂, NO_x, HO₂NO₂, or the NO₂ photolysis rate. While an interferant could not be identified organic nitrites appear a possible explanation for our observations.     

Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel

Simultaneous measurements of nitrous acid (HONO) and nitrogen dioxide (NO₂) using a differential optical absorption spectroscopy system, nitrogen oxide (NO) by an in situ chemiluminescence analyser and carbon dioxide (CO₂) by a gas chromatographic technique were carried out in the Wuppertal Kiesbergtunnel. At high traffic density HONO concentrations of up to 45 ppbV were observed. However, at low traffic density unexpectedly high HONO concentrations of up to 10 ppbV were measured caused by heterogeneous HONO formation on the tunnel walls. In addition to the tunnel campaigns, emission measurements of HONO, NO₂, NO and CO₂ from different single vehicles (a truck, a diesel and a gasoline passenger car) were also performed. For the correction of the HONO emission data, the heterogeneous HONO formation on the tunnel walls was quantified by two different approaches (a) in different NO₂ emission experiments in the tunnel without traffic and (b) on tunnel wall residue in the laboratory. The HONO concentration corrected for heterogeneous formation on the tunnel walls, in relation to the CO₂ concentration can be used to estimate the amount of HONO, which is directly emitted from the vehicle fleet. From the measured data, emission ratios (e.g. HONO/NO_x) and emission indices (e.g. mg HONO kg⁻¹ fuel) were calculated. The calculated emission index of 88±18 mg HONO kg⁻¹ fuel allows an estimation of the HONO emission rates from traffic into the atmosphere. Furthermore, the heterogeneous formation of HONO from NO₂ on freshly emitted exhaust particles is discussed.     

An observational study of the HONO–NO2 coupling at an urban site in Guangzhou City,South China

The temporal behavior of HONO and NO₂ was investigated at an urban site in Guangzhou city, China, by means of a DOAS system during the Pearl River Delta 2006 intensive campaign from 10 to 24 July 2006. Within the whole measurement period, unexpected high HONO mixing ratios up to 2 ppb were observed even during the day. A nocturnal maximum concentration of about 8.43 ± 0.4 ppb was detected on the night of 24 July 2006. Combining the data simultaneously observed by different instruments, the coupling of HONO–NO₂ and the possible formation sources of HONO are discussed. During the measurement period, concentration ratios of HONO to NO₂ ranged from (0.03 ± 0.1) to (0.37 ± 0.09), which is significantly higher than previously reported values (0.01–0.1). Surprisingly, in most cases a strong daytime correlation between HONO and NO₂ was found, contrary to previous observations in China. Aerosol was found to have a minor impact on HONO formation during the whole measurement period. Using a pseudo steady state approach for interpreting the nocturnal conversion of NO₂ to HONO suggests a non-negligible role of the relative humidity for the heterogeneous HONO formation from NO₂.     

Impacts of a strong cold front on concentrations of HONO,HCHO, O3, and NO2 in the heavy traffic urban area of Beijing

Much rain and strong winds caused by a cold front occurred in Beijing during the period of Sep. 27 to Oct. 4, 2004 and led to sharp drops in maximum and mean concentrations of HONO, HCHO, O₃, and NO₂, i.e., the maximum concentrations were reduced by 5.9, 21.3, 45.6, and 44.4 ppb, respectively, and the mean concentrations were decreased by 4.0, 5.5, 30.3, and 32.3 ppb, respectively. For daily HO_x production rates HONO photolysis was the largest contributor and over 90% contributions were from photolysis of HONO and HCHO. Large number and area percentages of soot aggregate from PM10, and high correlations between concentrations of PM10 and chemical formation of HONO suggested that heterogeneous reactions of NO₂ on surfaces of soot aggregate could be a key source of HONO in the heavy traffic areas of Beijing during the night and should be considered in air quality simulations for such areas.     

A long-range air pollution transport model for eastern north America—II. Nitrogen oxides

A simple, five-species algorithm to model the chemistry of oxides of nitrogen during long-range transport has been developed and used in the Eastern North America Model of Air Pollution (ENAMAP). The algorithm models the reactions and the physical depositions of NO and NO₂ as the primary pollutants, HNO₃ and particulate nitrate as the reaction products, and peroxyacetyl nitrate (PAN). PAN is utilized as a temporary reservoir for NO_x, reacting during the day to form HNO₃, nitrate, and itself, then decomposing back to NO₂ at night. The NO and NO₂ are kept at equilibrium, and the NO₂ reacts to form PAN, HNO₃, and nitrate. The reaction rates and NO: NO₂ equilibrium constant vary diurnally. The model has been run for the months of January and August 1977, and has produced NO₂ concentration patterns that have characteristics consistent with the limited existing measurements of atmospheric nitrogen compounds.     

Measurements of reactive nitrogen compounds in the free troposphere

NO, NO₂ and total reactive nitrogen, NO_y, were measured using chemiluminescence throughout the troposphere. Flights were made from Frankfurt (West Germany) to Abu Dhabi (United Arab Emirates), and from Frankfurt to Sao Paulo (Brazil). NO_x (NO + NO₂) concentrations were determined with a FeSO₄ converter and no_y, with a 425°C Mo converter. NO_x levels over the Northern Hemisphere at 9 km altitude were about 200 ppt. On one flight over central Europe the aircraft encountered a thunderstorm system over an area 600km wide. NO_y, was markedly elevated in the area of these thunderstorms due, we believe, to the rapid transport of polluted boundary layer air into the upper troposphere. The long lifetime of these trace gases in the upper troposphere allows them to be transported to the remote atmosphere where photolysis and radical reactions could produce significant quantities of ozone. The NO_y concentration of 3.5 (^+0.7_−0.9) ppb was significantly higher than the concentration of about 190 ppt measured on a separate flight. These and NO_x data indicate that tropospheric reactive nitrogen species vary widely in concentration, and a ‘representative’ distribution may not exist.     

Chemical Transformations of Nitrogen Oxides While Sampling Combustion Products

The present study reviews the sampling environments and chemical transformations of nitrogen oxides that may occur within probes and sample lines while sampling combustion products. Experimental data are presented for NO_x transformations in silica and 316 stainless steel tubing when sampling simulated combustion products in the presence of oxygen, carbon monoxide, and hydrogen. A temperature range of 25° to 400°C is explored. In the absence of CO and H₂, 316 stainless steel is observed to promote the reduction of nitrogen dioxide to nitric oxide at temperatures in excess of 300°C, and silica is found to be passive to chemical transformation. In the presence of CO, reduction of N0₂ to NO is observed in 316 stainless steel at temperatures in excess of 100°C, and reduction of NO₂ to NO in silica is observed at 400°C. In the presence of H₂, NO₂ is reduced to NO in 316 stainless steel at 200°C and NO_x is removed at temperatures exceeding 200°C. In silica, the presence of H₂ promotes the reduction of NO₂ to NO at 300°C and the removal of NO_x above 300°C.     

Identification and measurement of nitrous acid in an indoor environment

We report here direct observation by differential optical absorption spectroscopy (DOAS) of the formation of ppb levels of gaseous nitrous acid (MONO) from the reaction of ppm levels of nitrogen dioxide (NO₂) with water vapor, in an indoor environment. The rate of formation of HONO displayed first order kinetics with respect to NO₂ with a rate of (0.25 ±0.04) ppb min⁻¹ per ppm of NO₂ present. Assuming a lifetime of l h with respect to both physical and chemical removal processes for HONO, this leads to an estimated steady state concentration of ~ 15 ppb of HONO per ppm of NO₂ present. This relatively high level of HONO associated with NO₂-air mixtures raises new questions concerning the health implications of elevated NO₂ concentrations in indoor environments e.g. HONO is a respirable nitrite known to convert secondary amines in vitro to carcinogenic nitrosamines.     

Adsorptive conversion of nitrogen dioxide from etching vent gases over activated carbon

Some metal etching operations emit limited flow rates of waste gases with reddish-brown NO₂ fume, which may cause visual and acidic-odor complaints, as well as negative health effects. In this study, tests were performed by passing caustic-treated waste gases vented from Al-etching operations through columns packed either with virgin or regenerated granular activated carbon (GAC) to test their adsorptive conversion performance of NO₂ in the gases. The gases contained 5–55 ppm NO₂ and acetic and nitric acids of below 3 ppm. Exhausted carbon was regenerated by scrubbing it with caustic solution and water, and dried for further adsorption tests. Results indicate that with an (empty bed residence time (EBRT) of 0.15 sec for the gas through the GAC-packed space, around 60% of the influent NO₂ of 54 ppm could be removed, and 47% of the removed NO₂ was converted by and desorbed from the carbon as NO. GAC used in the present study could be regenerated at least twice to restore its capacity for NO₂ adsorption. Within EBRTs of 0.076–0.18 sec, the adsorptive conversion capacity was linearly varied with EBRT. In practice, with an EBRT of 0.20 sec, a conversion capacity of 0.80 kg NO₂ (kg GAC)^?1 with an influent NO₂ of 40 ppm can be used as a basis for system design.

Implications: Some metal etching operations emit waste gases with reddish-brown (yellow when diluted) NO₂ fume which may cause visual and acidic-odor complaints, as well as negative health effects. This study provides a simple process for the adsorptive conversion of NO₂ in caustic-treated waste gases vented from metal-etching operations through a GAC column. With an EBRT of 0.20 sec, a conversion capacity of 0.80 kg NO₂ (kg GAC)^?1 with an influent NO₂ of 40 ppm can be used as a basis for system design. Saturated GAC can be regenerated at least twice by simply scrubbing it with aqueous caustic solution.     

Development of a Method for the Analysis of NO2 and NH3 by NO-Measuring Instruments

Adaptation of available instruments to the analysis of NO₂ and NH₃ became important with the introduction of the Federal Constant Volume Sampling procedure for exhaust analysis in 1970. Two instruments, the NO optical detector (NOOD) and the non-dispersive infrared analyzer (ND1R), are both fast and accurate. The use of a system of two converters in conjunction with either of these instruments permits the analysis of a gas stream for NO, NO₂, and NH₃. NO is measured on the gas stream before conversion; NO₂ is determined after the gas stream has passed over a C-Mo converter at 475°C; and NH₃ is determined after the gas has been conducted over a C-Cu converter.     

Methodology for the analysis of Peroxyacetyl nitrate (PAN) in the unpolluted atmosphere

A light weight electron capture, gas chromatograph has been laboratory- and field-tested to conduct surface and airborne PAN measurements in the unpolluted troposphere. A dynamic calibration system based on CH₃CHO/NO₂/Cl₂ photolysis studies by Gay et al. (1976) was constructed and successfully tested. PAN was cryogenically preconcentrated prior to analysis. A sensitivity of 5 parts per trillion (ppt) and an overall accuracy of ± 20 % is estimated. It is shown that gas phase coulometry (GPC) is unsuited for absolute PAN analysis—principally, because a significant fraction of PAN is destroyed prior to coulometric detection. The kinetics of this destruction process are nonlinear. PAN measurements at a marine Pacific site, and aboard an aircraft, show that PAN is always present at a concentration range of 10–100 ppt, although concentrations as high as 400 ppt were measured at an altitude of 4.6 km over the Pacific Ocean. Surface PAN measurements at a Pacific marine site indicate a distinct diurnal behavior, tentatively attributed to photochemistry involving alkenes, alkanes and NO_x. There was no evidence of PAN diurnal variation in the free troposphere, but the data are currently too sparse. Measurements in the global atmosphere are needed to accurately describe the distribution and the role of PAN in the chemistry of the natural atmosphere.     

Behaviour and variability of local and regional oxidant levels (OX = O3 + NO2) measured in a polluted area in central-southern of Iberian Peninsula

The purpose of this work is to contribute to the understanding of the photochemical air pollution in central-southern of the Iberian Peninsula, analysing the behaviour and variability of oxidant levels (OX?=?O₃?+?NO₂), measured in a polluted area with the highest concentration of heavy industry in central Spain. A detailed air pollution database was observed from two monitoring stations. The data period used was 2008 and 2009, around 210,000 data, selected for its pollution and meteorological statistics, which are very representative of the region. Data were collected every 15 min, however hourly values were used to analyse the seasonal and daily ozone, NO, NO₂ and OX cycles. The variation of OX concentrations with NO _x is investigated, for the first time, in the centre of the Iberian Peninsula. The concentration of OX was calculated using the sum of a NO _x -independent ‘regional’ contribution (i.e. the O₃ background), and a linearly NO _x -dependent ‘local’ contribution. Monthly dependence of regional and local OX concentration was observed to determine when the maximum values may be expected. The variation of OX concentrations with levels of NO _x was also measured, in order to pinpoint the atmospheric sources of OX in the polluted areas. The ratios [NO₂]/[OX] and [NO₂]/[NO _x ] vs. [NO _x ] were analysed to find the fraction of OX in the form of NO₂, and the possible source of the local NO _x -dependent contribution, respectively. The progressive increase of the ratio [NO₂]/[OX] with [NO _x ] observed shows a greater proportion of OX in the form of NO₂ as the level of NO _x increases. The higher measured values in the ratio [NO₂]/[NO _x ] should not be attributed to NO _x emissions by vehicles; they could be explained by industrial emission, termolecular reactions or formaldehyde and HONO directly emitted by vehicles exhausts. We also estimate the rate of NO₂ photolysis, J _NO2?=?0.18–0.64 min^?1, a key atmospheric reaction that influence O₃ production and then the regional air quality. The first surface plot study of annual variation of the daily mean oxidant levels, obtained for this polluted area may be used to improve the atmospheric photochemical dynamic in this region of the Iberian Peninsula where there are undeniable air quality problems.     

Seasonal dependence of the oxidation capacity of the city of Santiago de Chile

The oxidation capacity of the highly polluted urban area of Santiago de Chile has been evaluated during a winter measurement campaign from May 25 to June 07, 2005, with the results compared and contrasted with those previously evaluated during a summer campaign from March 8 to 20, 2005. The OH radical budget was evaluated in both campaigns employing a simple quasi-photostationary state model (PSS) constrained with simultaneous measurements of HONO, HCHO, O₃, NO, NO₂, j(O¹D), j(NO₂), 13 alkenes and meteorological parameters. In addition, a zero dimensional photochemical box model based on the Master Chemical Mechanism (MCMv3.1) has been used for the analysis of the radical budgets and concentrations of OH, HO₂ and RO₂. Besides the above parameters, the MCM model has been constrained by the measured CO and other volatile organic compounds (VOCs) including alkanes and aromatics. Total production and destruction rates of OH and HO₂ in winter are about two times lower than that during summer. Simulated OH levels by both PSS and MCM models are similar during the daytime for both winter and summer indicating that the primary OH sources and sinks included in the simple PSS model are predominant. On a 24 h basis, HONO photolysis was shown to be the most important primary OH radical source comprising 81% and 52% of the OH initiation rate during winter and summer, respectively followed by alkene ozonolysis (12.5% and 29%), photolysis of HCHO (6.1% and 15%), and photolysis of O₃ (<1% and 4%), respectively. During both winter and summer, there was a balance between the OH secondary production (HO₂ + NO) and destruction (OH + VOCs) showing that initiation sources of RO₂ and HO₂ are no net OH initiation sources. This result was found to be fulfilled also for all other studies investigated. Seasonal impacts on the radical budgets are also discussed.     

Radical precursors and related species from traffic as observed and modeled at an urban highway junction

Nitrous acid (HONO) and formaldehyde (HCHO) are important precursors for radicals and are believed to favor ozone formation significantly. Traffic emission data for both compounds are scarce and mostly outdated. A better knowledge of today's HCHO and HONO emissions related to traffic is needed to refine air quality models. Here the authors report results from continuous ambient air measurements taken at a highway junction in Houston, Texas, from July 15 to October 15, 2009. The observational data were compared with emission estimates from currently available mobile emission models (MOBILE6; MOVES [MOtor Vehicle Emission Simulator]). Observations indicated a molar carbon monoxide (CO) versus nitrogen oxides (NO_x) ratio of 6.01 ± 0.15 (r ² = 0.91), which is in agreement with other field studies. Both MOBILE6 and MOVES overestimate this emission ratio by 92% and 24%, respectively. For HCHO/CO, an overall slope of 3.14 ± 0.14 g HCHO/kg CO was observed. Whereas MOBILE6 largely underestimates this ratio by 77%, MOVES calculates somewhat higher HCHO/CO ratios (1.87) than MOBILE6, but is still significantly lower than the observed ratio. MOVES shows high HCHO/CO ratios during the early morning hours due to heavy-duty diesel off-network emissions. The differences of the modeled CO/NO_x and HCHO/CO ratios are largely due to higher NO_x and HCHO emissions in MOVES (30% and 57%, respectively, increased from MOBILE6 for 2009), as CO emissions were about the same in both models. The observed HONO/NO_x emission ratio is around 0.017 ± 0.0009 kg HONO/kg NO_x which is twice as high as in MOVES. The observed NO₂/NO_x emission ratio is around 0.16 ± 0.01 kg NO₂/kg NO_x, which is a bit more than 50% higher than in MOVES. MOVES overestimates the CO/CO₂ emission ratio by a factor of 3 compared with the observations, which is 0.0033 ± 0.0002 kg CO/kg CO₂. This as well as CO/NO_x overestimation is coming from light-duty gasoline vehicles.

Implications: Nitrous acid (HONO) and formaldehyde (HCHO) are important precursors for radicals that ultimately contribute to ozone formation. There still exist uncertainties in emission sources of HONO and HCHO and thus regional air quality modeling still tend to underestimate concentrations of free radicals in the atmosphere. This paper demonstrates that the latest U.S. Environmental Protection Agency (EPA) traffic emission model MOVES still shows significant deviations from observed emission ratios, in particular underestimation of HCHO/CO and HONO/NO_x ratios. Improving the performance of MOVES may improve regional air quality modeling.     

The Clean Air Act Amendments and Motor Vehicle Inspection/Maintenance

Seventeen days of detailed measurements of NO, NO₂, O₃, HNO₃ and the frequency of NO₂ photolysis—j(NO₂)—were carried out in Claremont, CA, in September 1980. Under conditions when the rate of change of NO concentration is small, there must be a balance between formation and loss processes. In the classical photostationary state this balance is between NO₂ photolysis: NO₂ + hv → NO + O and reaction with ozone: NO + O₃ → NO₂ + O₂. The results show that the latter reaction with ozone is inadequate to balance the formation step; a significant contribution is required from another NO oxidation process, possibly peroxy radical oxidation. If so, the inferred concentration of peroxy radicals shows a diurnal variation, peaking around solar noon.     

Heterogeneous NOx chemistry in the polluted PBL

The significance of heterogeneous mechanisms in controlling gas-phase NO_x (NO, NO₂) mixing ratios in polluted urban air, especially during nighttime, is not well established. Several recent studies have suggested that carbon soot can provide an effective surface for mediating the inter conversion among several NO_y members. However, a number of such reactions reported in the literature have widely varying reaction probabilities and often conflicting pathways. We evaluated several of these reactions and choose the NO₂ conversion to HONO on the surface of soot particles for further analysis with a box photochemical model. These calculations show that the conversion of NO₂ to HONO on particle surfaces produces a large, measurable signal (up to several parts per billion) in nighttime HONO mixing ratios. Inclusion of this reaction was also shown to have significant impacts on ozone, OH and HO₂ in the polluted planetary boundary layer (PBL). The sensitivity of these results to the different reaction rate probabilities (γ) and particle surface areas was also examined. Results are then evaluated to find the combination of γ and surface areas that would mostly likely occur in the PBL within the limitations of the model.