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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - OXFLUX (Oxidised organic vapours in the atmosphere: From fluxes to chemical mechanisms and impacts)

Teaser

Summary

Plants and human activities emit organic vapors, which are oxidized in the air (e.g. by ozone), making them stickier. In particular vapors emitted from trees can oxidize to very sticky vapors with critical effects on atmospheric aerosol particles, and as a consequence on the properties of clouds (e.g. lifetime) that form on those particles, and hence on climate. However, we only understand a fraction of the oxidation mechanisms, which produce 1000\'s of species that interact with aerosol particles. The problem is complicated by other processes, such as the deposition of vapors on surfaces (e.g. leafs in a forest), or natural feedback mechanisms. Vapor-aerosol-cloud interactions have been identified as a major source of uncertainty in our ability to predict the sensitivity of climate to human activities, underlining our need for a more thorough understanding of the underlying processes. The overall goal of the OXFLUX project was to reveal important oxidation products, their life cycles under varying conditions, and their impacts on air chemistry and ultimately on climate.

Work performed

\"The key approach to address this project\'s goals was in the combination of novel measurement techniques. The used instrumentation (chemical ionization mass spectrometers) was able to simultaneously measure the minute atmospheric concentrations of 100\'s of oxidized organics, and in particular rapidly enough to perform eddy covariance analysis methods to derive upward (e.g. due to a strong source) and downward (e.g. due to deposition) fluxes of these compounds.
During the 7-month project, I deployed this instrumentation on a G-1 aircraft (operated by the US Department of Energy) in a multi-institutional field campaign (\"\"HI-SCALE\"\") in Oklahoma, USA. The campaign included 38 flights during two intensive measurement periods (April/May and August/September 2016). The goal of the campaign as a whole was to investigate interactions between aerosol and formation of cumulus clouds that are particularly prevalent in the US Southern Great Plains. By our measurements, we quantified the mixing ratios of numerous oxidation products of biogenic vapors (for the most part of isoprene, one of the most abundant organics emitted by trees globally). Clear correlations associated higher mixing ratios of those products with areas of increased biogenic emissions and of certain products with air masses recently exposed to increased anthropogenic emissions (e.g. from Oklahoma City). Our measurements were made at high frequency, allowing for deriving eddy fluxes when used together with high frequency 3D wind measurements made simultaneously. Data analysis is an on-going collaborative effort, yet processed data of mixing ratios for compounds deemed most important, for the on-going analysis as well as for modelling efforts, is already publicly available (http://www.arm.gov/campaigns/aaf2016hiscale/).
In addition, I analyzed data from pilot eddy flux measurements in the Finnish boreal forest, made previously by the University of Washington in collaboration with the University of Helsinki, for a ca. 1-month period from April to June 2014. Results were published in a peer-reviewed journal (http://onlinelibrary.wiley.com/doi/10.1002/2016GL069599/full). The publication focuses on formic acid, one of the most abundant oxygenated organic compounds in the atmosphere. However, its sources and life cycle remain insufficiently understood, with state-of-the-art models consistently underpredicting observed mixing ratios. Taking into account losses due to deposition, we could derive a time series of formic acid source rates from the boreal forest for most days of the measurement period â€“ the first such data set for a boreal forest to our knowledge â€“ indicating formic acid sources that are missing in emission inventories or oxidation schemes of biogenic vapors, most likely both. We showed that source rates are underestimated by up to a factor of 10, and that a substantial part of these sources must be located in the boreal forest.\"

Final results

The work for this project demonstrated that eddy flux measurements of oxygenated organic trace vapors using state-of-the-art chemical ionization mass spectrometers are feasible - from forest towers (described in detail in our peer-reviewed publication), as well as from aircraft (manuscripts in preparation). These efforts will be helpful for users of same or similar instrumentation when performing future eddy flux measurements.
For the HI-SCALE aircraft campaign, an additional instrumental technique (ionization using benzene ions) was tested, characterized and deployed - the first time for high-frequency airborne measurements. This method has shown promise for future applications for the highly sensitive quantification of mixing ratios of traces gases important to air chemistry, yet not accessible to many other, more common modes of operation (or only at relatively low sensitivity); that is in addition to the wide range of oxygenated organics and acids the instrument has already been very sensitive to. Examples are a wide range of alkenes (e.g. isoprene), aromatic organics, ammonia, or dimethyl sulfide (manuscript in preparation).
The HI-SCALE campaign measurements overall were very successful. However, data analysis is still on-going and the wider impacts of the results remain to be seen. The analysis has been promising, and we expect new discoveries regarding how in detail isoprene oxidation in various air masses affect aerosol and cloud formation, in particular for the US Southern Great Plains, and ensuing improvements in models that aim at predicting cloud formation on regional and larger scales and climatic impacts. By the team\'s outreach efforts and our aim to publish in widely visible high-impact journals, we try for our findings to reach and be of interest to legislators and policy makers dealing with issues in air quality, energy and climate.