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Development and field-deployment of an original analytical tool: Toward new insights into the VOC budget

Final Report Summary - DEFI-VOC (Development and field-deployment of an original analytical tool: Toward new insights into the VOC budget)

Volatile Organic Compounds (VOCs) play a central role in atmospheric chemistry, as they are involved in chemical processes that affect both regional air quality and global climate change. For instance, the oxidation of VOCs in the lower atmosphere leads to the formation of photochemical pollutants such as ozone and secondary organic aerosols that have detrimental effects on both human health and ecosystems. Furthermore, VOCs and photochemical pollutants mentioned above are important to issues of climate change as they significantly impact the oxidative capacity of the atmosphere and the earth radiation balance.
While it is agreed that source apportionments of VOCs are essential to develop strategies
for pollution control, the likely presence of thousands of compounds in the atmosphere makes exhaustive measurements very challenging and currently unfeasible. A more practical alternative is to characterize the most important VOCs in regard to their importance to current environmental issues. Measurements of total hydroxyl radical (OH) reactivity, the inverse of the OH lifetime, can provide information on the reactivity of these VOCs to assess their relevance for atmospheric chemistry. Indeed, OH reactivity measurements conducted during recent field campaigns are frequently larger by a factor 1.5-3 than that calculated from collocated measurements of trace gases, indicating a poor understanding of the VOC budget.
The research objectives of the DEFIVOC project were to (i) develop an original analytical tool capable of measuring ambient OH reactivity and a full suite of oxygenated VOCs, and to (ii) deploy this instrument during intensive field campaigns of a few weeks to investigate the atmospheric VOC budget.
This project, conducted from August 2011 to July 2015 at Mines Douai, France, led to the following achievements:
- The development and characterization of a Comparative Reactivity Method (CRM) instrument equipped with a Proton Transfer Reaction-Time of Flight Mass Spectrometer (PTR-ToFMS) for concomitant measurements of OH reactivity and VOCs.
- The implementation of an original analytical approach to identify unknown reactive VOCs based on the OH reactivity and VOC measurements.
- Several feasibility studies to extend the PTR-ToFMS measurements to poorly characterized oxygenated VOCs, including dicarbonyls, monocarboxylic acids, and peroxyacylnitrates.
- The deployment of this CRM-PTR-ToFMS instrument during three intensive field campaigns to (i) intercompare it to two other OH reactivity instruments, and to (ii) investigate the chemical composition and the reactivity of air masses at various sites
The CRM-PTR-ToFMS instrument was carefully tested in the laboratory to characterize critical aspects of the CRM technique, i.e. corrections of measurement artifacts that are needed to perform reliable measurements of OH reactivity. The two intercomparison studies showed that OH reactivity measurements performed with this instrument agree well with measurements from a Pump-Probe instrument at an urban site and another CRM instruments at a remote site, highlighting that the measurement artifact mentioned above were well corrected for. These intercomparison exercises were crucial to address the reliability of this new analytical instrument before its extensive use in the field.
Investigations performed to study the feasibility of measuring an extended suite of oxygenated VOCs, including dicarbonyl species, carboxylic acids, and peroxyacylnitrates (PAN) species are discussed below:
- This instrument was involved in an international side-by-side intercomparison of oxygenated VOC measurement techniques as part of the ACTRIS program. Three PTR-ToFMS, five Gas chromatographic instruments, and 2 cartridge samplers from 7 institutions were involved in this study, which was conducted at the Hohenpeissenberg Observatory during October 2013. The results from this study showed good performances for the PTR-ToFMS instruments to selectively measure methanol, acetaldehyde, acetone, methacroleine, and methylvinylketone.
- For PANs, a detection limit of 300 ppt was observed for 10-min measurements of peroxyacetynitrate, which is not good enough for ambient measurements.
- For carboxylic acids, this instrument exhibits figures of merit that are suitable to measure formic, acetic, and butyric acids in the atmosphere, with detection limits of 500, 200, and 90 ppt, respectively, for 10-min measurements. While a good selectivity was observed during a field campaign performed in a remote area, the selectivity has to be checked in other areas due to potential interferences from isobaric species.
- For dicarbonyls, it was found that glyoxal was not quantifiable from the PTR-ToFMS spectra due to peak overlapping with acetone, which is about an order of magnitude more abundant than glyoxal in the atmosphere. In contrast, methyl glyoxal was successfully measured over a 3-weeks period during a field campaign in a remote area. These measurements indicate a detection limit of 22 ppt for an integration time of 10 minutes. The measurement selectivity was confirmed by comparison to offline measurements made using dinitrophenylhydrazine (DNPH) cartridges.
The new methodology proposed to identify unknown VOCs of interest for atmospheric chemistry was successfully implemented and tested during an intensive field campaign conducted in Dunkirk, France. Measured trace gases, including VOCs, were found to only account for 33% of the measured OH reactivity, highlighting a large fraction of unmeasured reactive compounds at this site. The use of the new methodology mentioned above indicated that 6-8% of this missing OH reactivity could be identified by the CRM-PTR-ToFMS instrument, with dialkenes as likely candidates.
The CRM-PTR-ToFMS instrument has been selected to be part of an international side-by-side intercomparison of OH reactivity measurement techniques that will take place during October 2015 at the SAPHIR chamber in Julich, Germany. This study, involving eight institutions and three types of techniques, seeks to determine whether OH reactivity instruments can perform reliable measurements in various environments of different chemical complexity.
These results and the PTR-ToFMS instrument characterized in this project will also likely be of interest for the ACTRIS 2 program, in which our group is involved, to assess the reliability of oxygenated VOC measurements in the atmosphere.
The project outcomes will ultimately benefit the atmospheric science community by adding more constraints to the VOC budget. This research will also advance our knowledge of the VOC chemistry and will lead to a better understanding of the formation of secondary pollutants that are important for both air quality and climate change studies. It is therefore expected that future studies involving the CRM-PTR-ToFMS instrument will help promoting the development of efficient strategies for pollution control, which in turn will benefit to the society.