Amplified Trace Gas Removal in the Troposphere Hofzumahaus, Andreas; Rohrer, Franz; Lu, Keding ...
Science (American Association for the Advancement of Science),
06/2009, Letnik:
324, Številka:
5935
Journal Article
Recenzirano
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.
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.
Reaction rate coefficients and product yields in the gas phase reaction of O3 with the short‐chained alkenes ethene, propene, 1‐butene, isobutene, (E)‐butene, and (Z)‐butene were determined by ...Simulation of Atmospheric Photochemistry in a Large Reaction Chamber (SAPHIR). In a first set of experiments, reaction rate coefficients were acquired in an absolute reaction rate study from the measured concentration time profiles of ozone and the alkenes with side reactions being suppressed by adding a radical scavenger. The rate coefficients obtained agree well with literature data; for all but one alkene, the deviation was less than 10%. In a second set of experiments, OH yields were derived from the additional alkene turnover in the absence of a radical scavenger. In contrast to other studies, the OH yields determined in the dry chamber (propene, 0.10 ± 0.07; 1‐butene, 0.00 ± 0.08, isobutene, 0.30 ± 0.14; (Z)‐butene, 0.18 ± 0.09; and (E)‐butene, 0.70 ± 0.12) differed from the yields obtained under humid conditions (propene, 0.30 ± 0.08; 1‐butene, 0.30 ± 0.09; isobutene, 0.80 ± 0.10; (Z)‐butene, 0.40 ± 0.05; and (E)‐butene, 0.60 ± 0.12). The only exception was ethene ozonolysis, where no OH production was observed. HO2 yields (propene, 1.50 ± 0.75; 1‐butene, 1.60 ± 0.80; and isobutene, 2.00 ± 1.00) estimated from the additional ozone turnover compared to the experiments where radicals were not scavenged are reported here for the first time. Furthermore, the yields of the stable ozonolysis products CO, acetaldehyde, and formaldehyde were acquired by monitoring the concentration time profile of the respective compound.
At the atmosphere simulation chamber SAPHIR in Jülich both Laser-Induced Fluorescence Spectroscopy (LIF) and Long-Path Differential Optical Laser Absorption Spectroscopy (DOAS) are operational for ...the detection of OH radicals at tropospheric levels. The two different spectroscopic techniques were compared within the controlled environment of SAPHIR based on all simultaneous measurements acquired in 2003 (13 days). Hydroxyl radicals were scavenged by added CO during four of these days in order to experimentally check the calculated precisions at the detection limit. LIF measurements have a higher precision (σ= 0.88×10^sup 6^ cm^sup -3^) and better time resolution (Δt = 60 s), but the DOAS method (σ= 1.24×10^sup 6^ cm^sup -3^, Δt = 135 s) is regarded as primary standard for comparisons because of its good accuracy. A high correlation coefficient of r = 0.95 was found for the whole data set highlighting the advantage of using a simulation chamber. The data set consists of two groups. The first one includes 3 days, where the LIF measurements yield (1 - 2) ×10^sup 6^ cm^sup -3^ higher OH concentrations than observed by the DOAS instrument. The experimental conditions during these days are characterized by increased NO^sub x^ concentration and a small dynamic range in OH. Excellent agreement is found within the other group of 6 days. The regression to the combined data of this large group yields unity slope without a significant offset.PUBLICATION ABSTRACT
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.
An instrument based on 20 m open-path incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) was established at the Jülich SAPHIR chamber in spring 2011. The setup was optimized for ...the detection of HONO and NO2 in the near-UV region 352–386 nm, utilizing a bright hot-spot Xe-arc lamp and a UV-enhanced charge-coupled device (CCD) detector. A
2σ detection limit of 26 pptv for HONO and 76 pptv for NO2 was
achieved for an integration time of 1 min. Methacrolein (MACR) was also
detected at mixing ratios below 5 ppbv with an estimated 2σ
detection limit of 340 pptv for the same integration time. The IBBCEAS
instrument's performance for HONO and NO2 detection was compared to
that of extractive wet techniques, long-path absorption photometry (LOPAP),
and chemiluminescence spectrometry (CLS) NOx detection, respectively.
For the combined data sets an overall good agreement for both trend and
absolute mixing ratios was observed between IBBCEAS and these established
instruments at SAPHIR. Correlation coefficients r for HONO range from 0.930 to 0.994 and for NO2 from 0.937 to 0.992. For the single measurement
of MACR r=0.981 is found in comparison to proton-transfer-reaction
mass spectrometry (PTRMS).
For CareBeijing‐2006, two sites were established in urban and suburban regions of Beijing in summer 2006. Observations of O3 and its precursors together with meteorological parameters at both sites ...are presented. Gross ozone production rate P(O3) and sensitivity to nitric oxides (NOx) and volatile organic compounds (VOCs) were investigated using an observation‐based photochemical box model (OBM). P(O3) varied from nearly zero to 120 and 50 ppb h−1 for urban and suburban sites, respectively. These rates were greater than the accumulation rates of the observed oxidant (O3 + NO2) concentrations. The O3 episodes typically appeared under southerly wind conditions with high P(O3), especially at the urban site. Sensitivity studies with and without measured nitrous acid (HONO) as a model constraint suggested that the estimated P(O3) at both sites was strongly enhanced by radical production from HONO photolysis. Both NOx‐ and VOC‐sensitive chemistries existed over time scales from hours to days at the two sites. The variation in O3‐sensitive chemistry was relatively well explained by the ratio of the average daytime total VOC reactivity (kTVOC) to NO, with the transition chemistry corresponding to a kTVOC/NO value of 2–4 s−1 ppb−1. Pronounced diurnal variations in the O3 production regime were found. In the morning, conditions were always strongly VOC‐limited, while in the afternoon, conditions were variable for different days and different sites. The model‐calculated results were tested by measurements of H2O2, HNO3, total OH reactivity, and HOx radicals. The OBM was generally capable of correctly simulating the levels of P(O3), although it might tend to overpredict the VOC‐sensitive chemistry.
Besides isoprene, monoterpenes are the non-methane volatile organic compounds (VOCs) with the highest global emission rates. Due to their high reactivity towards OH, monoterpenes can dominate the ...radical chemistry of the atmosphere in forested areas. In the present study the photochemical degradation mechanism of β-pinene was investigated in the Jülich atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber). One focus of this study is on the OH budget in the degradation process. Therefore, the SAPHIR chamber was equipped with instrumentation to measure radicals (OH, HO2, RO2), the total OH reactivity, important OH precursors (O3, HONO, HCHO), the parent VOC β-pinene, its main oxidation products, acetone and nopinone and photolysis frequencies. All experiments were carried out under low-NO conditions ( ≤ 300 ppt) and at atmospheric β-pinene concentrations ( ≤ 5 ppb) with and without addition of ozone. For the investigation of the OH budget, the OH production and destruction rates were calculated from measured quantities. Within the limits of accuracy of the instruments, the OH budget was balanced in all β-pinene oxidation experiments. However, even though the OH budget was closed, simulation results from the Master Chemical Mechanism (MCM) 3.2 showed that the OH production and destruction rates were underestimated by the model. The measured OH and HO2 concentrations were underestimated by up to a factor of 2, whereas the total OH reactivity was slightly overestimated because the model predicted a nopinone mixing ratio which was 3 times higher than measured. A new, theory-derived, first-generation product distribution by Vereecken and Peeters (2012) was able to reproduce the measured nopinone time series and the total OH reactivity. Nevertheless, the measured OH and HO2 concentrations remained underestimated by the numerical simulations. These observations together with the fact that the measured OH budget was closed suggest the existence of unaccounted sources of HO2. Although the mechanism of additional HO2 formation could not be resolved, our model studies suggest that an activated alkoxy radical intermediate proposed in the model of Vereecken and Peeters (2012) generates HO2 in a new pathway, whose importance has been underestimated so far. The proposed reaction path involves unimolecular rearrangement and decomposition reactions and photolysis of dicarbonyl products, yielding additional HO2 and CO. Further experiments and quantum chemical calculations have to be made to completely unravel the pathway of HO2 formation.
This paper describes a newly developed long‐path differential‐optical‐absorption‐spectroscopy instrument used for the measurement of tropospheric OH radicals. The instrument consists of a high ...resolution echelle spectrometer in conjunction with a multiple‐reflection cell of 38.5 m base length and a UV laser light source that provides a spectral line width of 0.41 nm. Local in situ absorption measurements at total path lengths of either 1.85 or 3.1 km can be performed. The simultaneous observation of six atmospheric OH rotational absorption lines (Q1(2), Q21(2), R2(2), Q1(3), Q21(3), and P1(1)) around 308 nm allows OH measurements with high specificity. A new method to accurately determine the precision and the detection limit of each individual OH measurement data point is presented. Presently, a 2σ‐detection limit of 1.5×106 OH cm−3 is achieved (based on 1.85 km absorption path length and about 6 min integration time), which corresponds to a minimum detectable optical density of 2.5×10−5. The absolute instrumental accuracy was calculated to be better than 6.5%, which emphasizes the qualification of the longpath absorption technique as an absolute method. Examples of field experiments are reported to illustrate the present performance.
PDF
