Formation of nitrous acid (HONO) in the gas phase has been observed for the first time in a flow tube photoreactor upon irradiation (λ = 300–500 nm) of 2-nitrophenol and methyl substituted derivatives using a selective and sensitive instrument (LOPAP) for the detection of HONO. Formation of HONO by heterogeneous NO₂ photochemistry has been excluded, since production of NO₂ under the experimental conditions is negligible. Variation of the surface to volume ratio and the nitrophenol concentration showed that the photolysis occurred in the gas phase indicating that HONO formation is initiated by intramolecular hydrogen transfer from the phenolic OH group to the nitro group. From the measured linear dependence of the HONO formation rate on the reactant’s concentration and photolysis light intensity, a non-negligible new HONO source is proposed for the urban atmosphere during the day. Unexpectedly high HONO mixing ratios have been observed recently in several field campaigns during the day. It is proposed that the photolysis of aromatic compounds containing the ortho-nitrophenol entity could help to explain, at least in part, this high contribution of HONO to the oxidation capacity of the urban atmosphere.

Reliable measurements of atmospheric trace gases are necessary for both, a better understanding of the chemical processes occurring in the atmosphere, and for the validation of model predictions. Nitrogen dioxide (NO₂) is a toxic gas and is thus a regulated air pollutant. Besides, it is of major importance for the oxidation capacity of the atmosphere and plays a pivotal role in the formation of ozone and acid precipitation. Detection of NO₂ is a difficult task since many of the different commercial techniques used are affected by interferences. The chemiluminescence instruments that are used for indirect NO₂ detection in monitoring networks and smog chambers use either molybdenum or photolytic converters and are affected by either positive (NO_y) or negative interferences (radical formation in the photolytic converter). Erroneous conclusions on NO₂ can be drawn if these interferences are not taken into consideration. In the present study, NO₂ measurements in the urban atmosphere, in a road traffic tunnel and in a smog-chamber using different commercial techniques, i.e. chemiluminescence instruments with molybdenum or photolytic converters, a Luminol based instrument and a new NO₂-LOPAP, were compared with spectroscopic techniques, i.e. DOAS and FTIR. Interferences of the different instruments observed during atmospheric measurements were partly characterised in more detail in the smog chamber experiments. Whereas all the commercial instruments showed strong interferences, excellent agreement was obtained between a new NO₂-LOPAP instrument and the FTIR technique for the measurements performed in the smog chamber.

Methyl-2-nitrophenols can be emitted directly to the atmosphere or can be formed in situ via the oxidation of aromatic hydrocarbons. Nitrophenols possess phytotoxic properties and recent studies indicate their photooxidation is effective in producing secondary organic aerosols. Therefore, investigations on the major photooxidation pathways of these compounds with respect to assessing their environmental impacts and effects on human health are highly relevant. Presented here are determinations of the rate coefficients for the reactions of OH radicals with four methyl-2-nitrophenol isomers using a relative kinetic technique. The experiments were performed in a 1080 l photoreactor at (760 ± 10) Torr total pressure of synthetic air at (296 ± 3) K. The following rate coefficients (in units of cm³ molecule⁻¹ s⁻¹) have been obtained: 3-methyl-2-nitrophenol, (3.69 ± 0.70) × 10⁻¹²; 4-methyl-2-nitrophenol, (3.59 ± 1.17) × 10⁻¹²; 5-methyl-2-nitrophenol, (6.72 ± 2.14) × 10⁻¹²; 6-methyl-2-nitrophenol, (2.70 ± 0.57) × 10⁻¹². Photolysis of the methyl-2-nitrophenols with the superactinic fluorescent lamps (320 < λ < 480 nm, λ_max = 360 nm) used in the experiments was observed. Photolysis frequencies measured for the methyl-2-nitrophenols in the photoreactor have been determined and scaled to atmospheric conditions. The results suggest that photolysis rather than the reaction with OH radicals will be the dominant gas phase atmospheric loss process for methyl-2-nitrophenols.

The reaction CH₃C(O)O₂ + HO₂ → CH₃C(O)OOH + O₂ (Reaction R5a), CH₃C(O)OH + O₃ (Reaction R5b), CH₃ + CO₂ + OH + O₂ (Reaction R5c) was studied in a series of experiments conducted at 1000 mbar and (293 ± 2) K in the HIRAC simulation chamber. For the first time, products, (CH₃C(O)OOH, CH₃C(O)OH, O₃ and OH) from all three branching pathways of the reaction have been detected directly and simultaneously. Measurements of radical precursors (CH₃OH, CH₃CHO), HO₂ and some secondary products HCHO and HCOOH further constrained the system. Fitting a comprehensive model to the experimental data, obtained over a range of conditions, determined the branching ratios α_(R5a) = 0.37 ± 0.10, α_(R5b) = 0.12 ± 0.04 and α_(R5c) = 0.51 ± 0.12 (errors at 2σ level). Improved measurement/model agreement was achieved using k_(R5) = (2.4 ± 0.4) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, which is within the large uncertainty of the current IUPAC and JPL recommended rate coefficients for the title reaction. The rate coefficient and branching ratios are in good agreement with a recent study performed by Groß et al. (2014b); taken together, these two studies show that the rate of OH regeneration through Reaction (R5) is more rapid than previously thought. GEOS-Chem has been used to assess the implications of the revised rate coefficients and branching ratios; the modelling shows an enhancement of up to 5 % in OH concentrations in tropical rainforest areas and increases of up to 10 % at altitudes of 6–8 km above the equator, compared to calculations based on the IUPAC recommended rate coefficient and yield. The enhanced rate of acetylperoxy consumption significantly reduces PAN in remote regions (up to 30 %) with commensurate reductions in background NO_x.