Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SARLEP (Simulation and Understanding of the Atmospheric Radical Budget for Regions with Large Emissions from Plants)

Teaser

Approximately one giga-ton carbon per year of volatile organic compounds (VOCs) is released into our atmosphere from man-made and natural sources, of which approximately 90% is from biogenic sources. Most of the organic compounds are processed within the lowest part of the...

Summary

Approximately one giga-ton carbon per year of volatile organic compounds (VOCs) is released into our atmosphere from man-made and natural sources, of which approximately 90% is from biogenic sources. Most of the organic compounds are processed within the lowest part of the atmosphere (troposphere) by oxidation reactions, which lead to oxidized organic compounds, particles and ozone, all of which are harmful to humans and the environment. Actions to improve air quality are guided by the predictions of numerical models, which include chemical reaction schemes. A detailed understanding of the underlying oxidation processes of organic compounds is necessary for accurate predictions. Field studies have shown that there are gaps in the knowledge of the transformation of specifically organic compounds emitted by plants.

The most important oxidant agent in the troposphere is the hydroxyl radical (OH) mainly produced by photochemical reactions driven by sunlight. OH attacks most of the organic compounds. Its outstanding importance for the oxidation capacity of the atmosphere relies on its quasi-catalytic cycling: After VOCs have been attacked, peroxy radicals are produced, which can regenerate OH, so that one OH radical originating from photolysis can oxidize a large number of VOCs before being lost in radical termination reactions. Understanding the recycling mechanisms is essential to predict the removal rate of pollutants out of the atmosphere and the formation of secondary pollutants such as ozone and particles. Recommendations for actions to improve air quality are based on the knowledge of atmospheric oxidation processes driven by radical chemistry.

In order to improve our understanding of atmospheric oxidation processes, simulation experiments in the large outdoor chamber SAPHIR at Forschungszentrum Jülich are performed. In these experiments, trace gas and radical concentrations are quantified by scientific-grade, partly custom-built instruments. In order to ensure high accuracy of data, quality-assurance is essential. Therefore, one objective of the project is to improve instrumentation for the detection of short-lived atmospheric hydroxyl radicals and also to develop new instrumental approaches for the detection of hydroperoxy radicals and oxidized organic compounds. The measurement of radical is challenging due to their very small atmospheric concentrations and their high reactivity towards any surface in the instrument that requires inlet-free sampling. Also atmospheric concentrations of oxygenated organic compound are small and these species are easily lost on surfaces requiring specialized instrumentation and sampling procedures.

Measurements can be used to do budget analysis for radical that allows quantification of the recycling rate of radicals and therefore the oxidation rate of pollutants. In addition, the fate of carbon contained in the organic compounds that are emitted is investigated by calculating the yield of oxidized organic compounds that are formed in their chemical transformation. Therefore, another objective of the project to do budget analysis using only measurements to quantify so far unrecognized radical recycling or unmeasured oxidation products.

Time series of radical and trace gas concentrations are further compared to results of calculations applying state-of-the art chemical models. This gives insights, if current chemical models used to predict air pollution are capable of correctly describing the formation of ozone and particles. Sensitivity model runs can be used to derive potential explanations for observed model-measurements discrepancies leading to the objective of the project, the improvement of chemical models.

Work performed

The analysis of the chemical transformation of organic compounds in the simulation experiments in the SAPHIR chamber requires highly sensitive and accurate detection of radical and trace gas species. The improvement of existing measurement technique and the development of new instrumentation for the detection of trace gas and radical species were achieved.

In the experiments in this project, hydroxyl radical concentrations are measured by established instruments that make use of either long-pass absorption or laser-induced fluorescence. The absorption measurement can be regarded as gold-standard for the detection of atmospheric OH radicals. However, recent findings for similar OH fluorescence instruments as present at the simulation chamber showed significant artefacts in atmospheric OH measurements. These artefacts were discovered by applying a chemical zeroing scheme in order to quantify OH that is potentially internally produced in the instrument. This signal would be indistinguishable from ambient OH, if only the laser is switched between on- and off-resonance of the OH absorption wavelengths as typically done in the past. The chemical zeroing is achieved by mixing a chemical scavenging agent to the sampled air upstream of the fluorescence cell. In this measurement mode, no ambient OH enters the fluorescence cell, but internally produced OH is still detected. In order to ensure that artefacts do not affect the OH detection in the experiments in this project, such a chemical modulation technique was installed and tested for the fluorescence OH instrument at the simulation chamber. So far, no significant unaccounted interference have been observed in the experiments in this project.

Within this project, the measurement of hydroperoxy radicals (HO2) using chemical ionization (CIMS) combined with a high resolution time of flight mass spectrometer from Aerodyne Research Inc. employing bromide as primary ion was developed and characterized. An custom-built inlet system and ion flow-tube was developed that is optimized for the challenging detection of radicals by minimizing wall contact of the sampled air. The instrument was characterized in laboratory experiments and successfully installed at the chamber for experiments in this work. The sensitivity was proven to be high enough to detect atmospheric HO2 concentrations below 1 parts per trillion.

HO2 radicals are part of the radical recycling chain reaction that leads to the reformation of OH radicals in the reaction with nitric oxide (NO). The reaction of HO2 with NO makes often a large fraction of the total OH production in the radical budget, so that quantification of unrecognized OH production from other reaction channels that is one objective of the project can only be achieved, if accurate HO2 measurements are done. So far, HO2 in chamber experiments in SAPHIR have been measured by chemical conversion of HO2 to OH that is then detected by fluorescence. Both instruments, CIMS and chemical conversion + LIF, are now available at the chamber. Agreement of measurements by both instruments gives confidence that measurements are not affected by interferences.

The use of mass spectrometry applying Br- as ionization reagent is not only of great value for the detection of HO2 radicals, but also for the quantification of oxygenated organic compounds that are not accessible with standard instrumentation like gas chromatography. During experiments, when this new instrument was measuring in the chamber, mass spectra were also evaluated to determine concentrations of oxidation product of the isoprene chemistry. The number organic molecules detected in the experiments was complemented by a second new instrument applying mass spectrometry. This scientific grade commercial instrument from Aerodyne (VOCUS-Proton-Transfer-Reaction Mass Spectrometer, VOCUS-PTR) was purchased for this project. Characterization and calibration for organic species that are produced in the experiments needed

Final results

Instrumentation for the detection of atmospheric constituents specifically of radical species in the atmospheric chamber SAPHIR are of highest quality. This is, for example, ensured by the unique feature of the simultaneous detection of radical species by independent techniques. OH radicals are detected by fluorescence and absorption instruments and HO2 radicals by chemical conversion to OH that is detected by fluorescence and by the newly developed instrument applying chemical ionization mass spectrometry (CIMS). The CIMS instrument is the first one worldwide for the detection of HO2 that was tested against established HO2 detection methods. Concerning the detection of organic compounds, the SAPHIR chamber is one of the first facilities worldwide that is now equipped with a VOCUS-PTR instrument. It allows measuring a much wider range of different oxygenated organic compounds than previous PTR instruments.

The photo-oxidation and night-time oxidation were performed under atmospherically relevant conditions unlike laboratory experiments which are typically done at high concentrations of reactants. The experiments with isoprene show that the recent implementation of the oxidation mechanism in the widely used “Master Chemical Mechanism” does not describe the observations in the chamber regarding the recycling of the OH radical. The analysis of experiments further reveals that the importance of the subsequent chemistry of oxidation products is much higher than previously thought. Based on experimental observations in chamber experiments and quantum mechanical calculations, the mechanistic understanding of the isoprene photooxdiation has been extended over a wide range of chemical conditions. Implementation of the modifications of the chemical process in global predictions for OH radical concentrations demonstrates the importance of the new chemistry for regions with high isoprene emissions. For example, in isoprene dominated environments like the Amazonian rainforest, OH concentrations are three times higher if the new chemistry is taken into account compared to model calculations without the additional chemistry. It can be expected that the experiments investigating the night-time oxidation of isoprene by the nitrate radical will further improve our understanding of the impact of isoprene oxidation on air quality.

In the case of the MVK photo-oxidation experiments, the yield of one of the major oxidized organic product (glycolaldehyde) was more than a factor of two higher than expected and radical concentrations were 50% lower. This could be explained by additional RO2 chemistry that has not been taken into account in the oxidation scheme of MVK so far.

The analysis of the experiments with monoterpenes demonstrates the reaction of -pinene and -pinene with the OH radical favours different pathways than currently assumed in models. This is demonstrated by the smaller yield of organic oxidation products than predicted in these models and the radical budget, which shows that there is a significant gap of HO2 production in the radical budget, if only so far known reaction channels are taken into account. These results help to explain previous field observations where similar results were found in environments that were dominated by monoterpene emitting plants.

In the further project, results from the investigation of single compounds that were mainly conducted and evaluated in the first part of the project will be extended by the investigation of complex mixtures of emitted species and oxidation products. This will be achieved by either using emissions from real plants that are housed in a Teflon bag or by investigating ambient air in the simulation chamber. The forest nearby the SAPHIR chamber will provide a mixture of natural forest emissions. The experiments with complex mixtures will further bridge the gap of the understanding of atmospheric chemistry for biogenic environments and laboratory experiments investigating sing