Andreas Wahner

A large source of low-volatility secondary organic aerosol

Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol, which is known to affect the Earth’s radiation balance by scattering solar radiation and by acting as cloud condensation nuclei. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere–aerosol–climate feedback mechanisms, and the air quality and climate effects of biogenic emissions generally.

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.

Rapid aerosol particle growth and increase of cloud condensation nucleus activity by secondary aerosol formation and condensation: A case study for regional air pollution in northeastern China

[1] This study was part of the international field measurement Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing-2006). We investigated a new particle formation event in a highly polluted air mass at a regional site south of the megacity Beijing and its impact on the abundance and properties of cloud condensation nuclei (CCN). During the 1-month observation, particle nucleation followed by significant particle growth on a regional scale was observed frequently (~30%), and we chose 23 August 2006 as a representative case study. Secondary aerosol mass was produced continuously, with sulfate, ammonium, and organics as major components. The aerosol mass growth rate was on average 19 μg m -3 h -1 during the late hours of the day. This growth rate was observed several times during the 1-month intensive measurements. The nucleation mode grew very quickly into the size range of CCN, and the CCN size distribution was dominated by the growing nucleation mode (up to 80% of the total CCN number concentration) and not as usual by the accumulation mode. At water vapor supersaturations of 0.07-0.86%, the CCN number concentrations reached maximum values of 4000-19,000 cm -3 only 6-14 h after the nucleation event. During particle formation and growth, the effective hygroscopicity parameter κ increased from about 0.1-0.3 to 0.35-0.5 for particles with diameters of 40-90 nm, but it remained nearly constant at ~0.45 for particles with diameters of ~190 nm. This result is consistent with aerosol chemical composition data, showing a pronounced increase of sulfate.

European scientific assessment of the atmospheric effects of aircraft emissions

The purpose of this report prepared on behalf of the European Commission is to review the current understanding of chemical and dynamical processes in the upper troposphere and lower stratosphere, and to assess how these processes could be perturbed as a result of current and future aircraft emissions. Specifically, perturbations in the atmospheric abundance of ozone and in climate forcing, as predicted by atmospheric models, will be presented. The goal is to compile and evaluate scientific information related to the atmospheric impact of subsonic and supersonic aircraft emissions and to review the state of knowledge concerning the various aspects of this problem. In Section 2, the issues and scientific questions relevant to the problem of aircraft perturbations will be presented. The key physical and chemical processes occurring in the troposphere and stratosphere will be discussed in Section 3. Estimates of air traffic and aircraft emissions will be given in Section 4. Section 5 and 6 will review the understanding of the atmospheric impact of aircraft emissions at small and large scale, respectively. The effect of aircraft emissions on climate forcing will be discussed in Section 7. Finally, conclusions will be provided in Section 8.

New particle formation in forests inhibited by isoprene emissions

It has been suggested that volatile organic compounds (VOCs) are involved in organic aerosol formation, which in turn affects radiative forcing and climate. The most abundant VOCs emitted by terrestrial vegetation are isoprene and its derivatives, such as monoterpenes and sesquiterpenes. New particle formation in boreal regions is related to monoterpene emissions and causes an estimated negative radiative forcing of about -0.2 to -0.9 W m-2. The annual variation in aerosol growth rates during particle nucleation events correlates with the seasonality of monoterpene emissions of the local vegetation, with a maximum during summer. The frequency of nucleation events peaks, however, in spring and autumn. Here we present evidence from simulation experiments conducted in a plant chamber that isoprene can significantly inhibit new particle formation. The process leading to the observed decrease in particle number concentration is linked to the high reactivity of isoprene with the hydroxyl radical (OH). The suppression is stronger with higher concentrations of isoprene, but with little dependence on the specific VOC mixture emitted by trees. A parameterization of the observed suppression factor as a function of isoprene concentration suggests that the number of new particles produced depends on the OH concentration and VOCs involved in the production of new particles undergo three to four steps of oxidation by OH. Our measurements simulate conditions that are typical for forested regions and may explain the observed seasonality in the frequency of aerosol nucleation events, with a lower number of nucleation events during summer compared to autumn and spring. Biogenic emissions of isoprene are controlled by temperature and light, and if the relative isoprene abundance of biogenic VOC emissions increases in response to climate change or land use change, the new particle formation potential may decrease, thus damping the aerosol negative radiative forcing effect.

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.

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.

Nitrate effect in the heterogeneous hydrolysis of dinitrogen pentoxide on aqueous aerosols

The heterogeneous hydrolysis of N2O5 was investigated on NaHSO4, Na2SO4, and NaNO3 aerosols. The experiments were performed in the large Aerosol Chamber at FZ Julich at room temperature and ambient pressure for several relative humidities. These salts are components of aerosols in the marine and coastal boundary layer. For the sodium sulfate aerosols at relative humidities of 50–70% the reaction probabilities γN2O5 were in the range of 0.02–0.04. For NaNO3 aerosol at similar relative humidities we observed γN2O5 of 0.0018–0.0032. With increasing relative humidity, i.e. with increasing dilution of the nitrate concentration in the aerosol droplets, γN2O5 increases to 0.023 at 90% relative humidity. Our observation of decreasing γN2O5 with increasing nitrate concentration can be explained within the framework of an ionic mechanism for the hydrolysis of N2O5, if the recombination reaction of NO2+ with NO3- to N2O5 is considered. By a steady state analysis we derived analytical expressions of γN2O5 as a function of the nitrate concentration for a reaction either throughout the aerosol volume or in a thin surface shell. Accordingly, increasing nitrate concentration should enhance the lifetime of physically dissolved N2O5(aq) and as a consequence the heterogeneous hydrolysis of N2O5 should change from a near-surface to a volume reaction. The observation of such a specific nitrate effect can be regarded as further experimental evidence for the ionic reaction mechanism in the uptake of N2O5 on aqueous aerosols. A nitrate effect may gain (local) importance in the atmosphere if increasing NOX emissions translate in an increasing nitrate fraction in the secondary aerosol of anthropogenic origin.

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.

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.