Franz Rohrer

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.

Strong correlation between levels of tropospheric hydroxyl radicals and solar ultraviolet radiation

The most important chemical cleaning agent of the atmosphere is the hydroxyl radical, OH. It determines the oxidizing power of the atmosphere, and thereby controls the removal of nearly all gaseous atmospheric pollutants. The atmospheric supply of OH is limited, however, and could be overcome by consumption due to increasing pollution and climate change, with detrimental feedback effects. To date, the high variability of OH concentrations has prevented the use of local observations to monitor possible trends in the concentration of this species. Here we present and analyse long-term measurements of atmospheric OH concentrations, which were taken between 1999 and 2003 at the Meteorological Observatory Hohenpeissenberg in southern Germany. We find that the concentration of OH can be described by a surprisingly linear dependence on solar ultraviolet radiation throughout the measurement period, despite the fact that OH concentrations are influenced by thousands of reactants. A detailed numerical model of atmospheric reactions and measured trace gas concentrations indicates that the observed correlation results from compensations between individual processes affecting OH, but that a full understanding of these interactions may not be possible on the basis of our current knowledge of atmospheric chemistry. As a consequence of the stable relationship between OH concentrations and ultraviolet radiation that we observe, we infer that there is no long-term trend in the level of OH in the Hohenpeissenberg data set.

Sources and distribution of NO x in the upper troposphere at northern mid‐latitudes

A simple quasi-two-dimensional model is used to study the zonal distribution of NOx. The model includes vertical transport in the form of eddy diffusion and deep convection, zonal transport by a vertically uniform wind, and a simplified chemistry of NO, NO2, and HNO3. The NOx sources considered are surface emissions (mostly from the combustion of fossil fuel), lightning, aircraft emissions, and downward transport from the stratosphere. The model is applied to the latitude band of 40° to 50°N during the month of June; the contributions to the zonal NOx distribution from the individual sources and transport processes are investigated. The model predicted NOx concentration in the upper troposphere is dominated by air lofted from the polluted planetary boundary layer over the large industrial areas of eastern North America and Europe. Aircraft emissions are also important and contribute on average 30%. Stratospheric input is minor about 10%, less even than that by lightning. The model provides a clear indication of intercontinental transport Of NOx and HNO3 in the upper troposphere. Comparison of the modeled NO profiles over the western Atlantic with those measured during STRATOZ III in 1984 shows good agreement at all altitudes.

Climatologies of NOx and NOy: A comparison of data and models

Abstract Climatologies of tropospheric NOx (NO + NO2) and NOy (total reactive nitrogen: NOx + N03 + 2 × N2O5 + HNO2 + HNO3 + HNO4 + ClONO2 + PAN (peroxyacetylnitrate) + other organic ni trates) have been compiled from data previously published and, in most cases, publicly archived. Emphasis has been on non-urban measurements, including rural and remote ground sites, as well as aircraft data. Although the distribution of data is sparse, a compilation in this manner can begin to provide an understanding of the spatial and temporal distributions of these reactive nitrogen species. The cleanest measurements in the boundary layer are in Alaska, northern Canada and the eastern Pacific, with median NO mixing ratios below 10 pptv, NOx below 50 pptv, and NOy below 300 pptv. The highest NO values (greater than 1 ppbv) were found in eastern North America and Europe, with correspondingly high NOy (∼ 5 ppbv). A significantly narrower range of concentrations is seen in the free troposphere, particularly at 3–6 km, with NO typically about 10 pptv in the boreal summer. NO increases with altitude to ∼ 100 pptv at 9–12 km, whereas NOy does not show a trend with altitude, but varies between 100 and 1000 pptv. Decreasing mixing ratios eastward of the Asian and North American continents are seen in all three species at all altitudes. Model-generated climatologies of NOx and NOy from six chemical transport models are also presented and are compared with observations in the boundary layer and the middle troposphere for summer and winter. These comparisons test our understanding of the chemical and transport processes responsible for these species distributions. Although the model results show differences between them, and disagreement with observations, none are systematically different for all seasons and altitudes. Some of the differences between the observations and model results may likely be attributed to the specific meteorological conditions at the time that measurements were made differing from the model meteorology, which is either climatological flow from GCMs or actual meteorology for an arbitrary year. Differences in emission inventories, and convection and washout schemes in the models will also affect the calculated NOα and NOy distributions.

The tropospheric cycle of H2: a critical review

The literature on the distribution, budget and isotope content of molecular hydrogen (H2) in the troposphere is critically reviewed. The global distribution of H2 is reasonably well established and is relatively uniform. The surface measurements exhibit a weak latitudinal gradient with 3% higher concentrations in the Southern Hemisphere and seasonal variations that maximize in arctic latitudes and the interior of continents with peak-to-peak amplitudes up to 10%. There is no evidence for a continuous long-term trend, but older data suggest a reversal of the interhemispheric gradient in the late 1970s, and an increase in the deuterium content of H2 in the Northern Hemisphere from 80 standard mean ocean water (SMOW) in the 1970s to 130 today. The current budget analyses can be divided in two classes: bottom up, in which the source and sink terms are estimated separately based on emission factors and turnovers of precursors and on global integration of regional loss rates, respectively. That category includes the analyses by 3-D models and furnishes tropospheric turnovers around 75 Tg H2 yr−1. The other approach, referred to as top down, relies on inverse modelling or analysis of the deuterium budget of tropospheric H2. These provide a global turnover of about 105 Tg H2 yr−1. The difference is due to a much larger sink strength by soil uptake and a much larger H2 production from the photochemical oxidation of volatile organic compounds (VOC) in the case of the top down approaches. The balance of evidence seems to favour the lower estimates—mainly due to the constraint placed by the global CO budget on the H2 production from VOC. An update of the major source and sink terms yields: fossil fuel use 11±4 TgH2 yr−1; biomass burning (including bio-fuel) 15 ± 6 Tg H2 yr−1; nitrogen fixation (ocean) 6 ± 3 Tg H2 yr−1; nitrogen fixation (land) 3 ± 2 Tg H2 yr−1; photochemical production from CH4 23 ± 8 Tg H2 yr−1 and photochemical production from other VOC 18 ± 7 Tg H2 yr−1. The loss through reaction of H2 with OH is 19 ± 5 Tg H2 yr−1, and soil uptake 60+30 −20 Tg H2 yr−1. All these rates are well within the ranges of the corresponding bottom up estimates in the literature. The total loss of 79 Tg H2 yr−1 combined with a tropospheric burden of 155 Tg H2 yields a tropospheric H2 lifetime of 2 yr. Besides these major sources of H2, there are a number of minor ones with source strengths > 1 Tg H2 yr−1. Rough estimates for these are also given.

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.

Dependence of the OH concentration on solar UV

OH and the major parameters determining its concentration were measured during a field campaign in August 1994 at Mankmoos, a rural, relatively unpolluted site in northeastern Germany. The measured OH concentrations were previously shown to depend mainly on the intensity of solar UV and on the mixing ratio of NO2. In this paper we develop a simple parameterization of the dependence on solar UV and on NO2. The photolysis of O3 to O1D, of NO2 to NO, and of HCHO to HCO and H, all contribute significantly to the total dependence of OH on solar UV. We demonstrate that the photolysis frequency of O3, JO1D, is a suitable measure for that dependence which is slightly less than linear. The highly nonlinear dependence of OH on NOx is approximated by a Pade function. The parameterization provides a tool for a future quantitative intercomparison of the measured and modeled dependences of OH on UV and NO2. It also allows the removal of the variation in the measured OH induced by the dependences on the variables, UV and NO2, and thus enables a search for dependences on other, less influential parameters.

On the use of hydrocarbons for the determination of tropospheric OH concentrations

This paper explores a new approach to estimating atmospheric hydroxyl radical concentrations from regional measurements of a suite of hydrocarbons. The approach is guided by the study of a suite of synthetic tracers, i, with uniform continental sources and constant but different lifetimes of 1, 2, 5, 20, 50, and 100 days, whose global distributions are calculated from a three-dimensional chemical tracer model. With the help of the model we show that in a grid box the standard deviation σi divided by the average concentration Mi¯ is a unique function of the chemical lifetime τi. In favorable cases, for instance, in surface air within a specific region sampled by the Pacific Exploratory Mission (PEM) West B campaign, that function takes a simple form: σi/Mi¯ = A × τi-α, with α = 0.48, very close to 1/2. An analogous relation is found for the alkanes, ethane through n-hexane, measured during the PEM West B campaign in the same domain, with their reaction rate constant with OH, kOH. That relation has the form σi/Mi¯ = B × (kOH,i)α′, with α′ = 0.49. Using the alkenes, for example propene, which also react with O3, the dependence on kOH,i can be related to a dependence on τi. This allows us to estimate the OH concentration, 6×105cm−3, with an error of roughly a factor of 2 for this region (boundary layer, 30°N-40°N latitude, and 135°E-155°E longitude in March). This estimate is essentially based on empirical relations only and the assumption that the considered hydrocarbons have the same source distribution. As a by-product, we show that the α defined above is related to the slope in the (logarithmic) correlation plot between the mixing ratios of two trace gases with different lifetimes. We also show that the global distribution of a appears to be a useful tool to diagnose fast regional transport.

Free radicals and fast photochemistry during BERLIOZ

The free radicals OH, HO2, RO2, and NO3 are known to be the driving force for most chemical processes in the atmosphere. Since the low concentration of the above radicals makes measurements particularly difficult, only relatively few direct measurements of free radical concentrations have been reported to date. We present a comprehensive set of simultaneous radical measurements performed by Laser Induced Fluorescence (LIF), Matrix Isolation — Electron spin Resonance (MI-ESR), Peroxy Radical Chemical Amplification (PERCA), and Differential Optical Absorption Spectroscopy (DOAS) during the BERLIner OZonexperiment (BERLIOZ) during July and August of 1998 near Berlin, Germany. Most of the above radical species were measured by more than one technique and an intercomparison gave good agreement. This data set offered the possibility to study and quantify the role of each radical at a rural, semi-polluted site in the continental boundary layer and to investigate interconnections and dependencies among these free radicals. In general (box) modelled diurnal profiles of the different radicals reproduced the measurements quite well, however measured absolute levels are frequently lower than model predictions. These discrepancies point to disturbing deficiencies in our understanding of the chemical system in urban air masses. In addition considerable night-time peroxy radical production related to VOC reactions with NO3 and O3 could be quantified.

Comparison of measured OH concentrations with model calculations

The influence of chemical precursors and sunlight on the atmospheric OH abundance is investigated by a comparison of locally measured tropospheric OH with model calculations. The latter are based on the gas phase reaction mechanism of the regional acid deposition model (RADM2) which incorporates an explicit inorganic and a comprehensive organic chemistry. The experimental data were obtained in the planetary boundary layer during two sets of campaigns. In Deuselbach (1983) and Schauinsland (1984), rural conditions were encountered with NOx concentrations on the average of 2.2 and 0.9 ppb, respectively. This data set was already compared with model calculations based upon an older and less detailed chemical reaction scheme (Perner et al., 1987). Since then the experimental data were reanalyzed leading to modified measured OH concentrations and also to modified precursor concentrations. For a consistent comparison with the more recent campaigns in Julich (1987 and 1988) we have redone the calculations. The modeled and measured OH concentrations of the campaigns in 1983 and 1984 correlate well with a coefficient of correlation of r = 0.73. The model overpredicts OH by about 20%. Under more polluted conditions in Julich with average NOx concentrations of 4 ppb the correlation coefficient between experimental and modeled data are significantly smaller (r = 0.61). Possible reasons are the influence of not measured precursors, for example isoprene, and the inapplicability of a quasi-steady state model under the spatially inhomogeneous conditions in Julich. Again the model overpredicts the OH concentration by about 15%, which is somewhat smaller than for the rural case. The precision of the comparison is limited by the uncertainties of the chemical reaction rate constants.