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.
Hydroxyl (OH) and peroxy radicals (HO2 and RO2) were measured in the Pearl River Delta, which is one of the most polluted areas in China, in autumn 2014. The radical observations were complemented by ...measurements of OH reactivity (inverse OH lifetime) and a comprehensive set of trace gases including carbon monoxide (CO), nitrogen oxides (NOx=NO, NO2) and volatile organic compounds (VOCs). OH reactivity was in the range from 15 to 80 s−1, of which about 50 % was unexplained by the measured OH reactants. In the 3 weeks of the campaign, maximum median radical concentrations were 4.5×106 cm−3 for OH at noon and 3×108 and 2.0×108 cm−3 for HO2 and RO2, respectively, in the early afternoon. The completeness of the daytime radical measurements made it possible to carry out experimental budget analyses for all radicals (OH, HO2, and RO2) and their sum (ROx). The maximum loss rates for OH, HO2, and RO2 reached values between 10 and 15 ppbv h−1 during the daytime. The largest fraction of this can be attributed to radical interconversion reactions while the real loss rate of ROx remained below 3 ppbv h−1. Within experimental uncertainties, the destruction rates of HO2 and the sum of OH, HO2, and RO2 are balanced by their respective production rates. In case of RO2, the budget could be closed by attributing the missing OH reactivity to unmeasured VOCs. Thus, the presumption of the existence of unmeasured VOCs is supported by RO2 measurements. Although the closure of the RO2 budget is greatly improved by the additional unmeasured VOCs, a significant imbalance in the afternoon remains, indicating a missing RO2 sink. In case of OH, the destruction in the morning is compensated by the quantified OH sources from photolysis (HONO and O3), ozonolysis of alkenes, and OH recycling (HO2+NO). In the afternoon, however, the OH budget indicates a missing OH source of 4 to 6 ppbv h−1. The diurnal variation of the missing OH source shows a similar pattern to that of the missing RO2 sink so that both largely compensate each other in the ROx budget. These observations suggest the existence of a chemical mechanism that converts RO2 to OH without the involvement of NO, increasing the RO2 loss rate during the daytime from 5.3 to 7.4 ppbv h−1 on average. The photochemical net ozone production rate calculated from the reaction of HO2 and RO2 with NO yields a daily integrated amount of 102 ppbv ozone, with daily integrated ROx primary sources being 22 ppbv in this campaign. The produced ozone can be attributed to the oxidation of measured (18 %) and unmeasured (60 %) hydrocarbons, formaldehyde (14 %), and CO (8 %). An even larger integrated net ozone production of 140 ppbv would be calculated from the oxidation rate of VOCs with OH if HO2 and all RO2 radicals react with NO. However, the unknown RO2 loss (evident in the RO2 budget) causes 30 ppbv less ozone production than would be expected from the VOC oxidation rate.
Nitryl chloride (ClNO2) is a dominant source of chlorine radical in polluted environment, and can significantly affect the atmospheric oxidative chemistry. However, the abundance of ClNO2 and its ...exact role are not fully understood under different environmental conditions. During the summer of 2014, we deployed a chemical ionization mass spectrometer to measure ClNO2 and dinitrogen pentoxide (N2O5) at a rural site in the polluted North China Plain. Elevated mixing ratios of ClNO2 (> 350 pptv) were observed at most of the nights with low levels of N2O5 (< 200 pptv). The highest ClNO2 mixing ratio of 2070 pptv (1 min average) was observed in a plume from a megacity (Tianjin), and was characterized with a faster N2O5 heterogeneous loss rate and ClNO2 production rate compared to average conditions. The abundant ClNO2 concentration kept increasing even after sunrise, and reached a peak 4 h later. Such highly sustained ClNO2 peaks after sunrise are discrepant from the previously observed typical diurnal pattern. Meteorological and chemical analysis shows that the sustained ClNO2 morning peaks are caused by significant ClNO2 production in the residual layer at night followed by downward mixing after breakup of the nocturnal inversion layer in the morning. We estimated that ∼ 1.7–4.0 ppbv of ClNO2 would exist in the residual layer in order to maintain the observed morning ClNO2 peaks at the surface site. Observation-based box model analysis show that photolysis of ClNO2 produced chlorine radical with a rate up to 1.12 ppbv h−1, accounting for 10–30 % of primary ROx production in the morning hours. The perturbation in total radical production leads to an increase of integrated daytime net ozone production by 3 % (4.3 ppbv) on average, and with a larger increase of 13 % (11 ppbv) in megacity outflow that was characterized with higher ClNO2 and a relatively lower oxygenated hydrocarbon (OVOC) to non-methane hydrocarbon (NMHC) ratio.
Amplified Trace Gas Removal in the Troposphere Hofzumahaus, Andreas; Rohrer, Franz; Lu, Keding ...
Science (American Association for the Advancement of Science),
06/2009, Volume:
324, Issue:
5935
Journal Article
Peer reviewed
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 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.
The gas-phase reaction of OH radicals with toluene and toluene-d 8 and reactions of the resulting OH adducts with O2 were studied in N2/O2 mixtures at atmospheric pressure and room temperature. OH ...production was performed by pulsed 248 nm photolysis of H2O2. Cw UV-laser long-path absorption at 308 nm was used for time-resolved detection of OH and OH−toluene adducts. The reaction OH + toluene was studied in N2 and O2 and similar rate constants of (5.70 ± 0.19) × 10-12 cm3 s-1 (N2) and (5.60 ± 0.14) × 10-12 cm3 s-1 (O2) were obtained at 100 kPa. For toluene-d 8 the corresponding rate constants are (5.34 ± 0.34) × 10-12 cm3 s-1 (N2) and (5.47 ± 0.14) × 10-12 cm3 s-1 (O2). Absorption cross-sections of (1.1 ± 0.2) × 10-17 cm2 were determined for both the OH−toluene and OH−toluene-d 8 adduct at 308 nm. Rate constants of (4.7 ± 1.4) × 10-11 cm3 s-1 and (1.8 ± 0.5) × 10-10 cm3 s-1 were determined for the OH−toluene adduct self-reaction and the OH−toluene adduct + HO2 reaction, respectively. Upper limits ≤8 × 10-15 cm3 s-1 were estimated for any reactions of the OH−toluene adduct with H2O2 or toluene. Adduct kinetics in the presence of O2 is consistent with reversible formation of peroxy radicals with an estimated rate constant of (3 ± 2) × 10-15 cm3 s-1 and an equilibrium constant of (3.25 ± 0.33) × 10-19 cm3 (toluene-d 8: (3.1 ± 1.2) × 10-19 cm3). The effective adduct loss from the equilibrium can be explained by (i) an additional, irreversible reaction of the adduct with O2 with a rate constant of (6.0 ± 0.5) × 10-16 cm3 s-1 (toluene-d 8: (4.7 ± 1.2) × 10-16 cm3 s-1), or (ii) a unimolecular reaction of the peroxy radical, with a rate constant of (1.85 ± 0.15) × 103 s-1 (toluene-d 8: (1.5 ± 0.4) × 103 s-1). Consequences for the atmospheric degradation of toluene will be discussed.
In 2014, a large, comprehensive field campaign was conducted in the densely populated North China Plain. The measurement site was located in a botanic garden close to the small town Wangdu, without ...major industry but influenced by regional transportation of air pollution. The loss rate coefficient of atmospheric hydroxyl radicals (OH) was quantified by direct measurements of the OH reactivity. Values ranged between 10 and 20 s−1 for most of the daytime. Highest values were reached in the late night with maximum values of around 40 s−1. OH reactants mainly originated from anthropogenic activities as indicated (1) by a good correlation between measured OH reactivity and carbon monoxide (linear correlation coefficient R2 = 0.33) and (2) by a high contribution of nitrogen oxide species to the OH reactivity (up to 30 % in the morning). Total OH reactivity was measured by a laser flash photolysis–laser-induced fluorescence instrument (LP-LIF). Measured values can be explained well by measured trace gas concentrations including organic compounds, oxygenated organic compounds, CO and nitrogen oxides. Significant, unexplained OH reactivity was only observed during nights, when biomass burning of agricultural waste occurred on surrounding fields. OH reactivity measurements also allow investigating the chemical OH budget. During this campaign, the OH destruction rate calculated from measured OH reactivity and measured OH concentration was balanced by the sum of OH production from ozone and nitrous acid photolysis and OH regeneration from hydroperoxy radicals within the uncertainty of measurements. However, a tendency for higher OH destruction compared to OH production at lower concentrations of nitric oxide is also observed, consistent with previous findings in field campaigns in China.
The HD(CP)2 Observational Prototype Experiment (HOPE) was performed as a major 2-month field experiment in Jülich, Germany, in April and May 2013, followed by a smaller campaign in Melpitz, Germany, ...in September 2013. HOPE has been designed to provide an observational dataset for a critical evaluation of the new German community atmospheric icosahedral non-hydrostatic (ICON) model at the scale of the model simulations and further to provide information on land-surface–atmospheric boundary layer exchange, cloud and precipitation processes, as well as sub-grid variability and microphysical properties that are subject to parameterizations. HOPE focuses on the onset of clouds and precipitation in the convective atmospheric boundary layer. This paper summarizes the instrument set-ups, the intensive observation periods, and example results from both campaigns. HOPE-Jülich instrumentation included a radio sounding station, 4 Doppler lidars, 4 Raman lidars (3 of them provide temperature, 3 of them water vapour, and all of them particle backscatter data), 1 water vapour differential absorption lidar, 3 cloud radars, 5 microwave radiometers, 3 rain radars, 6 sky imagers, 99 pyranometers, and 5 sun photometers operated at different sites, some of them in synergy. The HOPE-Melpitz campaign combined ground-based remote sensing of aerosols and clouds with helicopter- and balloon-based in situ observations in the atmospheric column and at the surface. HOPE provided an unprecedented collection of atmospheric dynamical, thermodynamical, and micro- and macrophysical properties of aerosols, clouds, and precipitation with high spatial and temporal resolution within a cube of approximately 10 × 10 × 10 km3. HOPE data will significantly contribute to our understanding of boundary layer dynamics and the formation of clouds and precipitation. The datasets have been made available through a dedicated data portal. First applications of HOPE data for model evaluation have shown a general agreement between observed and modelled boundary layer height, turbulence characteristics, and cloud coverage, but they also point to significant differences that deserve further investigations from both the observational and the modelling perspective.
Nitrous acid and OH were measured concurrently with a number of other atmospheric components and relevant photolysis frequencies during two campaigns at the Meteorological Observatory ...Hohenpeissenberg (980 m a.s.l.) in summer 2002 and 2004. On most of the 26 measurement days the HNO2 concentration surprisingly showed a broad maximum around noon (on average 100 pptv) and much lower concentrations during the night (∼30 pptv). The results indicate a strong unknown daytime source of HNO2 with a production rate on the order of 2–4 × 106 cm−3s−1. The data demonstrate an important contribution of HNO2 to local HOx levels over the entire day, comparable with the photolysis of O3 and HCHO. On average during the 2004 campaign, 42% of integrated photolytic HOx formation is attributable to HNO2 photolysis.
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