Birger Bohn

Amplified Trace Gas Removal in the Troposphere

Going Faster The concentrations of most tropospheric pollutants and trace gases are kept in check by their reactions with hydroxyl radicals (OH). OH is a short-lived, highly reactive species that is produced in the atmosphere by photochemical processes, and regenerated in the chain of chemical reactions that follows the oxidative destruction of those molecules. These regeneration mechanisms were thought to be fairly well understood, but now Hofzumahaus et al. (p. 1702, published online 4 June) present evidence of a pathway not previously recognized. In a study of atmospheric composition in the Pearl River Delta, a highly polluted region of China, greatly elevated OH concentrations were observed without the correspondingly high levels of ozone expected from current models. Thus, OH concentrations may be augmented by a process that speeds the regeneration of OH without producing ozone. A yet undescribed pathway for hydroxyl radical production is needed to account for reaction rates of highly polluted air. 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.

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.

Strong daytime production of OH from HNO2 at a rural mountain site

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 HNO 2 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 HNO 2 with a production rate on the order of 2-4 x 10 6 cm -3 s -1 . The data demonstrate an important contribution of HNO 2 to local HO x levels over the entire day, comparable with the photolysis of O 3 and HCHO. On average during the 2004 campaign, 42% of integrated photolytic HO x formation is attributable to HNO 2 photolysis.

Light induced conversion of nitrogen dioxide into nitrous acid on submicron humic acid aerosol

The interactions of aerosols consisting of humic acids with gaseous nitrogen dioxide (NO 2) were investigated under different light conditions in aerosol flow tube experiments at ambient pressure and temperature. The results show that NO2 is converted on the humic acid aerosol into nitrous acid (HONO), which is released from the aerosol and can be detected in the gas phase at the reactor exit. The formation of HONO on the humic acid aerosol is strongly activated by light: In the dark, the HONO-formation was below the detection limit, but it was increasing with the intensity of the irradiation with visible light. Under simulated atmospheric conditions with respect to the actinic flux, relative humidity and NO2-concentration, reactive uptake coefficients γrxn for the NO2→HONO conversion on the aerosol between γrxn <10−7 (in the dark) andγrxn=6×10 were observed. The observed uptake coefficients decreased with increasing NO 2concentration in the range from 2.7 to 280 ppb and were dependent on the relative humidity (RH) with slightly reduced values at low humidity ( <20% RH) and high humidity (>60% RH). The measured uptake coefficients for the NO2→HONO conversion are too low to explain the HONOformation rates observed near the ground in rural and urban environments by the conversion of NO 2→HONO on organic aerosol surfaces, even if one would assume that all aerosols consist of humic acid only. It is concluded that the processes leading to HONO formation on the Earth surface will have a much larger impact on the HONO-formation in the lowermost layer of the troposphere than humic materials potentially occurring in airborne particles. Correspondence to: M. Ammann (markus.ammann@psi.ch)

Missing gas-phase source of hono inferred from zeppelin measurements in the troposphere

On a Zeppelin Nitrous acid (HONO) is an important atmospheric trace gas that acts as a precursor of tropospheric hydroxyl-radicals (OH), which is responsible for the self-cleansing capacity of the atmosphere and which also controls the concentrations of greenhouse gases, such as methane and ozone. How HONO is made is a mystery. Flying onboard a Zeppelin over the Po Valley in Northern Italy, Li et al. (p. 292) discovered HONO in the undisturbed morning troposphere, indicating that HONO must be produced there, rather than mixed from the surface. The high HONO concentrations are likely to have been formed by a light-dependent gas-phase source that probably consumed OH or HO2 radicals, which hints that the impact of HONO on the abundance of OH in the entire troposphere may be substantially overestimated. The tropospheric production of HONO from a light-dependent gas-phase source raises questions about its impact on OH. 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.

Gas-phase reaction of the OH–benzene adduct with O2: reversibility and secondary formation of HO2

The reaction of OH radicals with benzene and consecutive reactions of benzene–OH adducts with O2 were studied in the gas phase in N2–O2 mixtures at atmospheric pressure and room temperature. OH was produced by pulsed 248 nm photolysis of H2O2. Time-resolved detection of both OH and benzene–OH adducts was performed by continuous-wave (cw) UV-laser long-path absorption at around 308 nm. The reaction: OH+benzene→products [reaction (1)] was not affected by the presence of O2. Rate constants k1=(1.10±0.07)×10-12 cm3 s-1 and (1.06±0.07)×10-12 cm3 s-1 were obtained in N2 and O2, respectively. In N2 addition of NO2 did not change k1, from which an upper limit of 5% is derived for formation of H atoms in reaction (1). An absorption cross-section of σ(308 nm)=(5.8±1.5)×10-18 cm2 and a self-reaction rate constant of (3.4±1.7)×10-11 cm3 s-1 were determined for the benzene–OH adduct. Upper limits of 5×10-15 cm3 s-1, 1×10-14 cm3 s-1 and 5×10-14 cm3 s-1 were obtained for reactions of the adduct with benzene, H2O2 and NO, respectively. The adduct kinetics in the presence of O2 is consistent with the reversible formation of a peroxy radical: adduct+O2↔adduct–O2 [reaction (2a/-2a)]. An equilibrium constant of K2a=(2.7±0.4)×10-19 cm3 was determined and a rate constant of k2a=(2±1)×10-15 cm3 s-1 was roughly estimated. 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 (2.1±0.2)×10-16 cm3 s-1, or (ii) a unimolecular reaction of the peroxy radical, with a rate constant of (7.6±0.8)×102 s-1. For a reaction of the peroxy radical with O2 an upper limit of 1×10-17 cm3 s-1 is estimated. Addition of NO reveals formation of HO2 in the presence of O2 by recovering OH via HO2+NO. Applying numerical methods, reaction models were tested to describe the observed complex kinetics of OH. The data are consistent with rapid HO2 formation following a peroxy radical+NO reaction with a rate constant of (1.1±0.4)×10-11 cm3 s-1. Extrapolation of HO2 formation rates to [NO]=0 points at a second source of HO2 not preceded by any RO2+NO reaction.

JOYCE: Jülich Observatory for Cloud Evolution

AbstractThe Julich Observatory for Cloud Evolution (JOYCE), located at Forschungszentrum Julich in the most western part of Germany, is a recently established platform for cloud research. The main objective of JOYCE is to provide observations, which improve our understanding of the cloudy boundary layer in a midlatitude environment. Continuous and temporally highly resolved measurements that are specifically suited to characterize the diurnal cycle of water vapor, stability, and turbulence in the lower troposphere are performed with a special focus on atmosphere–surface interaction. In addition, instruments are set up to measure the micro- and macrophysical properties of clouds in detail and how they interact with different boundary layer processes and the large-scale synoptic situation. For this, JOYCE is equipped with an array of state-of-the-art active and passive remote sensing and in situ instruments, which are briefly described in this scientific overview. As an example, a 24-h time series of the ev...

Kinetics of the OH + C2H2 reaction in the presence of O2

Room-temperature rate constants for the removal of OH radicals in the gas-phase reaction with acetylene have been determined using pulsed H2O2 photolysis production of OH followed by cw UV-laser long-path absorption detection. The pressure dependences of the rate constants in N2, O2 and synthetic air as buffer gases have been investigated in the fall-off range between 1.5 and 100 k Pa. In N2 the pressure dependence can be described by the empirical expression of Gardiner: k={(k∞)a+(k0[M])a}1/a, using the parameter a=–⅓. This results in the rate constants k∞=(1.07 ± 0.07)× 10–12 cm3 s–1, and k0=(3.9 ± 1.6)× 10–29 cm6 s–1 for the given pressure range. Smaller effective rate constants were obtained in the presence of O2, owing to a fast regeneration channel for OH. In terms of the above expression the rate constants k∞=(4.6 ± 0.2) and (3.1 ± 0.1)× 10–13 cm3 s–1 and k0=(1.8 ± 0.2) and (1.9 ± 0.5)× 10–29 cm6 s–1 were obtained for O2 and synthetic air as buffer gases, respectively. Compared with N2 there is virtually no difference in the fall-off behaviour in O2 and synthetic air, indicating that the OH regeneration yield is independent of pressure. On the other hand, this yield was found to decrease with the O2 mixing ratio. A kinetic model is proposed that qualitatively explains this effect with different chemical properties of non-thermalized OH–acetylene adducts with regard to O2. This model is confirmed by measurements showing a significantly stronger dependence of the regeneration yield on the O2 mixing ratio in O2–He mixtures.

Model-aided radiometric determination of photolysis frequencies in a sunlit atmosphere simulation chamber

In this work diurnal and seasonal variations of mean photolysis frequencies for the atmosphere simulation chamber SAPHIR at Forschungszentrum J ülich are calculated. SAPHIR has a complex construction with UV permeable teflon walls allowing natural sunlight to enter the reactor volume. The calculations are based on external measurements of solar spectral actinic flux and a model considering the time-dependent impact of shadows from construction elements as well as the influence of the teflon walls. Overcast and clear-sky conditions are treated in a consistent way and different assumptions concerning diffuse sky radiance distributions are tested. Radiometric measurements inside the chamber are used for an inspection of model predictions. Under overcast conditions we obtain fractions of 0.74 and 0.67 of external values for photolysis frequencies j (NO2) (NO2+hν→NO+O(P)) andj (O1D) (O3+hν→O2+O(D)), respectively. On a clear sky summer day these values are time-dependent within ranges 0.65–0.86 and 0.60–0.73, for j (NO2) andj (O1D), respectively. A succeeding paper ( Bohn et al., 2004) is dealing with an on-road test of the model approach by comparison with photolysis frequencies from chemical actinometry experiments within SAPHIR.

Formation of HO2 from OH and C2H2 in the presence of O2

Pulsed production of OH in a gas-phase system containing acetylene, O2 and NO resulted in biexponential OH-decay curves, indicating formation of HO2 in secondary reactions. Production and detection of OH were performed by 248 nm photolysis of H2O2 and cw-laser long-path absorption at 308 nm, respectively. Measurements were made at room temperature in O2 or N2–O2 mixtures containing 5% O2 at total pressures between 10 and 100 kPa. Analysis of the decay curves resulted in effective rate constants for the removal of OH and the formation of HO2 by acetylene in the presence of O2 in the range (1.4–3.5)×10-13 cm3 s-1, dependent on total pressure and O2 concentration. HO2 is thought to be formed from HCO and O2, with HCO originating in a reaction of an intermediate acetylene–OH adduct with O2. HO2 yields were found to vary between 1.13 and 1.01 and tending to higher values at lower total pressures. These yields are higher than the expected value of 1, which can be explained by a dissociation of a small fraction of vibrationally excited glyoxal formed, together with OH in a second channel of the acetylene–OH adduct+O2 reaction. In order to check whether the increased HO2 yields are real, CO was used instead of acetylene. In this case, an HO2 yield of 0.99 was found, in good agreement with expectations, and a rate constant of (1.66±0.25)×10-13 cm3 s-1 for the OH+CO reaction in 20 kPa O2 was determined. In addition, a rate constant for the HO2+NO reaction of (9.5±1.5)×10-12 cm3 s-1, rate constants for the OH+NO reaction in the range (1.3–7.4)×10-12 cm3 s-1, depending on total pressure, and upper limits for the rate constants of possible reactions HO2+C2H2 (k⩽5×10-15 cm3 s-1) and HO2+CO (k⩽3×10-15 cm3 s-1) were derived. Error limits include statistical (2σ) and possible systematic errors.