First detection of a key intermediate in the oxidation of fuel + NO systems: HONO

Computational studies on the clusters of HONO•(H2O)n(n = 1-7): structures and enthalpy of formation

CONTEXT

Nitrous acid (HONO) is often associated with many air pollution events, such as the ozone hole, acid rain, and human health. Herein, we performed the theoretical studies on the structures and enthalpy of formation for the hydrated clusters HONO∙(H2O)n(n = 1-7). Two different isomers of HONO including cis-HONO and trans-HONO were studied. Minima structures of trans-HONO∙(H2O)n(n = 1-7) and cis-HONO∙(H2O)n(n = 1-7) clusters containing forty-eight and twenty-one were found, respectively. The hydrogen-bonded interactions between HONO and water molecules in HONO∙(H2O)n(n = 1-7) clusters were analyzed. Enthalpies of the formation of the most stable isomers of trans-HONO∙(H2O)n(n = 1-7) and cis-HONO∙(H2O)n(n = 1-7) clusters are predicted theoretically. These results can provide a new understanding of the atmospheric circulation of HONO.

METHODS

Geometric structures and vibrational frequencies of the HONO∙(H2O)n(n = 1-7) clusters were investigated by using the QCISD(T)/6-311 + G(3df,2p)//M06-2X/6-311 + G(3df,2p) method. Enthalpies of formation of the global minimal isomers of the HONO∙(H2O)n(n = 1-7) clusters were calculated at the CBS-QB3 level of theory. Atoms in molecules (AIM) theory was applied to the analysis of hydrogen-bonded interactions among the HONO∙(H2O)n(n = 1-7) clusters.

Revisiting the estimation indicator for HONO emissions from light-duty vehicles

Gaseous nitrous acid (HONO), playing a crucial role in the generation of hydroxyl (OH) radicals and thus secondary pollution, lacks a source. Vehicular emission is a significant HONO source and is usually estimated by a traditional estimation indicator (RHONO/NOx = 0.8 %). Nevertheless, with more direct measurements for vehicular HONO emissions, RHONO/NOx values have been reported to vary over a wide range. In this study, we conducted the driving tests with a chassis dynamometer for ten light-duty gasoline vehicles. HONO emission factors have realized a significant reduction with the updating of emission standards, with emission factors of 0.40 mg/km, 0.13 mg/km, and 0.06 mg/km for China IV, China V, and China VI vehicles, respectively. Besides precursors, water content and exhaust temperature were found to be possible decisive factors for initiating HONO generation. Furthermore, by coupling NOx emissions and combustion efficiency, we modified the estimation indicator for vehicular HONO emissions and a better estimation effect has been verified. Additionally, we established a dynamic inventory of vehicular HONO emissions in Jinniu District in Chengdu and further found the traditional estimation indicator would overestimate HONO emissions by around 17 %. Our findings would help to advance a deeper understanding of vehicular HONO emissions and the modified estimation indicator would be beneficial in minimizing the uncertainties of the HONO budget in the troposphere.

Line positions and effective line strengths of trans-HONO near 1280 cm-1

The measurement of the line positions and effective line strengths of the ν3 fundamental band of trans-nitrous acid (trans-HONO) near 1280 cm-1 (7.8 µm) by tunable laser absorption spectroscopy (TLAS) utilizing a room temperature continuous-wave quantum cascade laser (cw-QCL) was reported. The effective line strengths of 30 well-resolved trans-HONO absorption lines in the range of 1279.8-1282.2 cm-1 were determined using the HONO line strength at 1280.3841 cm-1 as a scale. The maximum measurement uncertainty of 7.64% in the line strengths is mainly determined by the uncertainty of the referenced line strength, while the measurement precision of the line positions is better than 5.56 * 10-3 cm-1. The line positions and strengths of the trans-HONO absorption lines obtained in this work provide a reference for continuous gas monitoring and analysis of the sources and sinks of atmospheric HONO.

Direct Observation of HONO Emissions from Real-World Residential Natural Gas Heating in China

Atmospheric nitrous acid (HONO) is an important precursor of atmospheric hydroxyl radicals. Vehicle emissions and heterogeneous reactions have been identified as major sources of urban HONO. Here, we report on HONO emissions from residential natural gas (RNG) for water and space heating in urban areas based on in situ measurements. The observed HONO emission factors (EFs) of RNG heating vary between 6.03 and 608 mg·m^-3 NG, which are highly dependent on the thermal load. The highest HONO EFs are observed at a high thermal load via the thermal NO homogeneous reaction. The average HONO EFs of RNG water heating in winter are 1.8 times higher than that in summer due to the increased thermal load caused by the lower inlet water temperatures in winter. The power-based HONO EFs of the traditional RNG heaters are 1085 times and 1.7 times higher than those of gasoline and diesel vehicles that meet the latest emission standards, respectively. It is estimated that the HONO emissions from RNG heaters in a typical Chinese city are gradually close to emissions from on-road vehicles when temperatures decline. These findings highlight that RNG heating is a non-negligible source of urban HONO emissions in China. With the continuous acceleration of coal-to-gas projects and the continuous tightening of NOx emission standards for vehicle exhaust, HONO emissions from RNG heaters will become more prominent in urban areas. Hence, it is urgently needed to upgrade traditional RNG heaters with efficient emission reduction technologies such as frequency-converted blowers, secondary condensers, and low-NOx combustors.

High Gaseous Nitrous Acid (HONO) Emissions from Light-Duty Diesel Vehicles

Nitrous acid (HONO) plays an important role in the budget of hydroxyl radical (^•OH) in the atmosphere. Vehicular emissions are a crucial primary source of atmospheric HONO, yet remain poorly investigated, especially for diesel trucks. In this study, we developed a novel portable online vehicular HONO exhaust measurement system featuring an innovative dilution technique. Using this system coupled with a chassis dynamometer, we for the first time investigated the HONO emission characteristics of 17 light-duty diesel trucks (LDDTs) and 16 light-duty gasoline vehicles in China. Emissions of HONO from LDDTs were found to be significantly higher than previous studies and gasoline vehicles tested in this study. The HONO emission factors of LDDTs decrease significantly with stringent control standards: 1.85 ± 1.17, 0.59 ± 0.25, and 0.15 ± 0.14 g/kg for China III, China IV, and China V, respectively. In addition, we found poor correlations between HONO and NOx emissions, which indicate that using the ratio of HONO to NOx emissions to infer HONO emissions might lead to high uncertainty of HONO source budget in previous studies. Lastly, the HONO emissions are found to be influenced by driving conditions, highlighting the importance of conducting on-road measurements of HONO emissions under real-world driving conditions. More direct measurements of the HONO emissions are needed to improve the understanding of the HONO emissions from mobile and other primary sources.

Threshold photoionization shows no sign of nitryl hydride in methane oxidation with nitric oxide

No nitryl hydride was detected in partial oxidation of nitric oxide doped methane, despite recent theoretical reaction rates suggesting otherwise.

An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia

A wide-range experimental and theoretical investigation of ammonia gas-phase oxidation is performed, and a predictive, detailed kinetic model is developed.