Fast Photochemistry in Wintertime Haze: Consequences for Pollution Mitigation Strategies.

In contrast to summer smog, the contribution of photochemistry to the formation of winter haze in northern mid-to-high latitude is generally assumed to be minor due to reduced solar UV and water vapor concentrations. Our comprehensive observations of atmospheric radicals and relevant parameters during several haze events in winter 2016 Beijing, however, reveal surprisingly high hydroxyl radical oxidation rates up to 15 ppbv/h, which is comparable to the high values reported in summer photochemical smog and is two to three times larger than those determined in previous observations during winter in Birmingham (Heard et al. Geophys. Res. Lett. 2004, 31, (18)), Tokyo (Kanaya et al. J. Geophys. Res.: Atmos. 2007, 112, (D21)), and New York (Ren et al. Atmos. Environ. 2006, 40, 252-263). The active photochemistry facilitates the production of secondary pollutants. It is mainly initiated by the photolysis of nitrous acid and ozonolysis of olefins and maintained by an extremely efficiently radical cycling process driven by nitric oxide. This boosted radical recycling generates fast photochemical ozone production rates that are again comparable to those during summer photochemical smog. The formation of ozone, however, is currently masked by its efficient chemical removal by nitrogen oxides contributing to the high level of wintertime particles. The future emission regulations, such as the reduction of nitrogen oxide emissions, therefore are facing the challenge of reducing haze and avoiding an increase in ozone pollution at the same time. Efficient control strategies to mitigate winter haze in Beijing may require measures similar as implemented to avoid photochemical smog in summer.

ATMOSPHERIC SCIENCE. Response to Comment on "Missing gas-phase source of HONO inferred from Zeppelin measurements in the troposphere".

Ye et al. have determined a maximum nitrous acid (HONO) yield of 3% for the reaction HO2·H2O + NO2, which is much lower than the yield used in our work. This finding, however, does not affect our main result that HONO in the investigated Po Valley region is mainly from a gas-phase source that consumes nitrogen oxides.

Reversible addition of the OH radical to p-cymene in the gas phase: multiple adduct formation. Part 2.

A flash photolysis-resonance fluorescence (FP-RF) system was used to study the p-cymene (PC) + OH reaction at temperatures between 299 and 349 K in helium. Triexponential functions were fitted to groups of observed OH decay curves according to a model considering a reversible addition to form two adducts as thermolabile reservoirs of OH. Compared to Part 1 of this paper, consideration of a second adduct strongly improved the fits to our measurements, and the rate constants for the major pathways were optimized between 299 and 349 K. The Arrhenius expression for the rate constant of the sum of OH addition and H-atom abstraction pathways was found to be kOH = 1.9 × 10(-12) exp[(610 ± 210) K/T] cm(3) s(-1). Rate constants of unimolecular decomposition reactions of the adducts were similar to other aromatic compounds with the following Arrhenius expressions: 1 × 10(12) exp[(-7600 ± 800) K/T] s(-1) for adduct 1 and 4 × 10(11) exp[(-8000 ± 300) K/T] s(-1) for adduct 2. Adduct yields increased and decreased with temperature for adduct 1 and 2, respectively, but were similar (∼0.4) around room temperature. Equilibrium constants yielded values for reaction enthalpies and entropies of adduct formations. While for one adduct reasonable agreement was obtained with theoretical predictions, there were significant deviations for the other adduct. This indicates the presence of more than two adduct isomers that were not accounted for in the reaction model. Quantum chemical calculations (DFT M06-2X/6-31G(d,p)) and RRKM kinetics were employed with the aim of clarifying the mechanism of the OH addition to PC. These calculations show that formation of adducts with OH in ortho positions to the isopropyl and methyl substituents is predominant (55% and 24%) to those with OH in ipso positions (21% and 3%). A large fraction (>90%) of the ipso-C3H7 adduct is predicted to react by dealkylation forming p-cresol (in the absence of oxygen) and isopropyl radicals. These theoretical results agree well with the interpretation of the experimental results showing that the two ortho adducts (which appeared as OH reservoirs in the experiment) have been observed.

Parameterization of thermal properties of aging secondary organic aerosol produced by photo-oxidation of selected terpene mixtures.

Formation and evolution of secondary organic aerosols (SOA) from biogenic VOCs influences the Earth's radiative balance. We have examined the photo-oxidation and aging of boreal terpene mixtures in the SAPHIR simulation chamber. Changes in thermal properties and chemical composition, deduced from mass spectrometric measurements, were providing information on the aging of biogenic SOA produced under ambient solar conditions. Effects of precursor mixture, concentration, and photochemical oxidation levels (OH exposure) were evaluated. OH exposure was found to be the major driver in the long term photochemical transformations, i.e., reaction times of several hours up to days, of SOA and its thermal properties, whereas the initial concentrations and terpenoid mixtures had only minor influence. The volatility distributions were parametrized using a sigmoidal function to determine TVFR0.5 (the temperature yielding a 50% particle volume fraction remaining) and the steepness of the volatility distribution. TVFR0.5 increased by 0.3±0.1% (ca. 1 K), while the steepness increased by 0.9±0.3% per hour of 1×10(6) cm(-3) OH exposure. Thus, aging reduces volatility and increases homogeneity of the vapor pressure distribution, presumably because highly volatile fractions become increasingly susceptible to gas phase oxidation, while less volatile fractions are less reactive with gas phase OH.

Missing gas-phase source of HONO inferred from Zeppelin measurements in the troposphere.

Gaseous nitrous acid (HONO) is an important precursor of tropospheric hydroxyl radicals (OH). OH is responsible for atmospheric self-cleansing and controls the concentrations of greenhouse gases like methane and ozone. Due to lack of measurements, vertical distributions of HONO and its sources in the troposphere remain unclear. Here, we present a set of observations of HONO and its budget made onboard a Zeppelin airship. In a sunlit layer separated from Earth's surface processes by temperature inversion, we found high HONO concentrations providing evidence for a strong gas-phase source of HONO consuming nitrogen oxides and potentially hydrogen oxide radicals. The observed properties of this production process suggest that the generally assumed impact of HONO on the abundance of OH in the troposphere is substantially overestimated.

Kinetics and mechanism of the reaction of OH with the trimethylbenzenes--experimental evidence for the formation of adduct isomers.

The reversible gas-phase addition of OH radicals to the trimethylbenzenes was investigated in pulsed experiments utilizing VUV flash-photolysis resonance-fluorescence of H(2)O in the temperature range of 275-340 K. Triexponential OH decays were observed in the presence of the trimethylbenzenes, indicating the participation of more than one adduct species. Analytical solutions for the system of differential equations with two adduct isomers were derived, and the OH decay curves were evaluated based on this reaction model. This led to significant improvements of fit qualities and notable changes in OH rate constants compared to a previous model with a single adduct species. The detailed analysis was confined to 1,3,5-trimethylbenzene where reversible formation of two OH-aromatic ortho- and ipso-adduct isomers is feasible in accordance with the extended reaction model. Only after inclusion of additional isomerization reactions, consistent thermochemical data were obtained from the fitted rate constants. Reaction enthalpies of -83 ± 7 kJ mol(-1) and -35 ± 22 kJ mol(-1) were derived for the formation of one adduct isomer and the isomerization into the other, respectively. Based on literature data, the more and less stable adducts were assigned to ipso- and ortho-adduct isomers, respectively. The potential isomerization precluded the determination of primary yields of adduct isomers but formation of the ipso-adduct in any case is a minor process. For the rate constants of the OH + 1,3,5-trimethylbenzene reaction an Arrhenius expression k(OH) = 1.32 × 10(-11) cm(3) s(-1) exp(450 ± 50 K/T) was obtained. Based on the same approach, the rate constants of the OH reactions with 1,2,3-trimethylbenzene and 1,2,4-trimethylbenzene were derived as k(OH) = 3.61 × 10(-12) cm(3) s(-1) exp(620 ± 80 K/T) and k(OH) = 2.73 × 10(-12) cm(3) s(-1) exp(730 ± 70 K/T), respectively.

Aging of biogenic secondary organic aerosol via gas-phase OH radical reactions.

The Multiple Chamber Aerosol Chemical Aging Study (MUCHACHAS) tested the hypothesis that hydroxyl radical (OH) aging significantly increases the concentration of first-generation biogenic secondary organic aerosol (SOA). OH is the dominant atmospheric oxidant, and MUCHACHAS employed environmental chambers of very different designs, using multiple OH sources to explore a range of chemical conditions and potential sources of systematic error. We isolated the effect of OH aging, confirming our hypothesis while observing corresponding changes in SOA properties. The mass increases are consistent with an existing gap between global SOA sources and those predicted in models, and can be described by a mechanism suitable for implementation in those models.

Prompt HO2 formation following the reaction of OH with aromatic compounds under atmospheric conditions.

The secondary formation of HO(2) radicals following OH + aromatic hydrocarbon reactions in synthetic air under normal pressure and temperature was investigated in the absence of NO after pulsed production of OH radicals. OH and HO(x) (=OH + HO(2)) decay curves were recorded using laser-induced fluorescence after gas-expansion. The prompt HO(2) yields (HO(2) formed without preceding NO reactions) were determined by comparison to results obtained with CO as a reference compound. This approach was recently introduced and applied to the OH + benzene reaction and was extended here for a number of monocyclic aromatic hydrocarbons. The measured HO(2) formation yields are as follows: toluene, 0.42 ± 0.11; ethylbenzene, 0.53 ± 0.10; o-xylene, 0.41 ± 0.08; m-xylene, 0.27 ± 0.06; p-xylene, 0.40 ± 0.09; 1,2,3-trimethylbenzene, 0.31 ± 0.06; 1,2,4-trimethylbenzene, 0.37 ± 0.09; 1,3,5-trimethylbenzene, 0.29 ± 0.08; hexamethylbenzene, 0.32 ± 0.08; phenol, 0.89 ± 0.29; o-cresol, 0.87 ± 0.29; 2,5-dimethylphenol, 0.72 ± 0.12; 2,4,6-trimethylphenol, 0.45 ± 0.13. For the alkylbenzenes HO(2) is the proposed coproduct of phenols, epoxides, and possibly oxepins formed in secondary reactions with O(2). In most product studies the only quantified coproducts were phenols whereas only a few studies reported yields of epoxides. Oxepins have not been observed so far. Together with the yields of phenols from other studies, the HO(2) yields determined in this work set an upper limit to the combined yields of epoxides and oxepins that was found to be significant (≤0.3) for all investigated alkylbenzenes except m-xylene. For the hydroxybenzenes the currently proposed HO(2) coproducts are dihydroxybenzenes. For phenol and o-cresol the determined HO(2) yields are matching the previously reported dihydroxybenzene yields, indicating that these are the only HO(2) forming reaction channels. For 2,5-dimethylphenol and 2,4,6-trimethylphenol no complementary product studies are available.

HO2 formation from the OH + benzene reaction in the presence of O2.

In this study we investigated the secondary formation of HO(2) following the benzene + OH reaction in N(2) with variable O(2) content at atmospheric pressure and room temperature in the absence of NO. After pulsed formation of OH, HO(x) (= OH + HO(2)) and OH decay curves were measured by means of a laser-induced fluorescence technique (LIF). In synthetic air the total HO(2) yield was determined to be 0.69 ± 0.10 by comparison to results obtained with CO as a reference compound. HO(2) is expected to be a direct product of the reaction of the intermediately formed OH-benzene adduct with O(2). The HO(2) yield is slightly greater than the currently recommended yield of the proposed HO(2) co-product phenol (∼53%). This hints towards other, minor HO(2) forming channels in the absence of NO, e.g. the formation of epoxide species that was proposed in the literature. For other test compounds upper limits of HO(2) yields of 0.10 (isoprene) and 0.05 (cyclohexane) were obtained, respectively. In further experiments at low O(2) concentrations (0.06-0.14% in N(2)) rate constants of (2.4 ± 1.1) × 10(-16) cm(3) s(-1) and (5.6 ± 1.1) × 10(-12) cm(3) s(-1) were estimated for the OH-benzene adduct reactions with O(2) and O(3), respectively. The rate constant of the unimolecular dissociation of the adduct back to benzene + OH was determined to be (3.9 ± 1.3) s(-1). The HO(2) yield at low O(2) was similar to that found in synthetic air, independent of O(2) and O(3) concentrations indicating comparable HO(2) yields for the adduct + O(2) and adduct + O(3) reactions.

Amplified trace gas removal in the troposphere.

The degradation of trace gases and pollutants in the troposphere is dominated by their reaction with hydroxyl radicals (OH). The importance of OH rests on its high reactivity, its ubiquitous photochemical production in the sunlit atmosphere, and most importantly on its regeneration in the oxidation chain of the trace gases. In the current understanding, the recycling of OH proceeds through HO2 reacting with NO, thereby forming ozone. A recent field campaign in the Pearl River Delta, China, quantified tropospheric OH and HO2 concentrations and turnover rates by direct measurements. We report that concentrations of OH were three to five times greater than expected, and we propose the existence of a pathway for the regeneration of OH independent of NO, which amplifies the degradation of pollutants without producing ozone.

The photolysis of ortho-nitrophenols: a new gas phase source of HONO.

Formation of nitrous acid (HONO) in the gas phase has been observed for the first time in a flow tube photoreactor upon irradiation (lambda = 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.