Isoprene mixing ratio in ambient air across the Southern Ocean in the austral summer of 2016/2017, during the Antarctic Circumnavigation Expedition (ACE).

***** Dataset abstract *****

This dataset contains the mixing ratios of isoprene (C5H8) in ambient air measured during the Antarctic Circumnavigation Expedition around the Southern Ocean in the austral summer of 2016/2017 using the iDirac, an autonomous gas chromatograph (Bolas et al., 2020). Samples were collected at a temporal resolution of approximately 10 minutes, with sizeable gaps in the dataset during Legs 1 and 3 due to instrument malfunction.

Isoprene represents one of the largest biogenic emissions on the planet. While the magnitude and mechanism of its emissions on land are well established, there is still large uncertainty on the drivers of marine isoprene emissions. This dataset, consisting of continuous measurements at a relatively high temporal resolution, represents a unique opportunity to better understand marine isoprene emissions in the Southern Ocean.

***** Original data collection *****

Ambient air was sampled by the iDirac, a custom-built autonomous gas chromatograph coupled with photoionisation detection (GC-PID) (Bolas et al., 2020). The volatiles contained in a 200 mL sample of ambient air were pre-concentrated onto an adsorbent trap (containing ~10 g of Carboxen 1016), and then injected into the chromatography columns. Volatiles contained in the sampled air were separated and detected individually by the photoionisation detector. The temporal resolution for the isoprene measurement was 10 minutes. The timestamps represent the mid-point of each sample run. The instrument detection limit for the ACE campaign was 30 ppt (at the 3σ level).

Each chromatographic run (chromatogram) was stored on a Raspberry Pi computer within the instrument. The software Win-SPC was used to transfer the data from the instrument onto a laptop for analysis.

The instrument was calibrated every 3 hours throughout the ACE campaign. A secondary calibration standard, traceable to a primary reference gas mixture from the National Physical Laboratory (NPL), the UK’s National Metrology Institute, was used to produce a calibration curve.

***** Data processing *****

Gaussian curves were fitted to the chromatographic peaks observed in the calibration and sample runs, and the calibration curve was used to quantify the isoprene in the sample runs (see Bolas et al., 2020 for details).

***** Quality checking *****

Each raw measurement file (chromatogram) is accompanied by a range of “house-keeping” data on the status of the instrument (e.g., flowrate through the instrument, oven temperature, etc.). Whenever a file exhibited anomalous house-keeping values, it was discarded. This primarily involved removing measurement files while the instrument was warming up at the beginning of each leg, and numerous measurement files in legs 1 and 3 due to oven malfunction.

Each value of isoprene mixing ratio carries a 10% uncertainty. Values below 30 ppt (0.03 ppb) are below the detection limit of the instrument.

***** Standards *****
 
Data do not conform to any international or discipline-specific standards.

***** Further information for interpreting the data and using the dataset *****

Filters: Data below 30 ppt (0.03 ppb) are below the instrument detection limit.

Timescales: The data is at a 10 min temporal resolution. While ambient isoprene is known to exhibit a diel cycle driven by temperature and incoming solar radiation, marine isoprene emissions are somewhat more complex and our data shows sudden changes in observed isoprene over the timescales of 1-3 hours.

Interpolation: Given that sudden spikes in isoprene that were observed (especially in Leg 2), interpolation is not encouraged.

Aggregation to lower temporal resolution: The raw data series is at a 10 min resolution. Standard time averages (30 min, 1 h, 3 h etc.) can be applied to lower the temporal resolution.

***** Dataset contents *****

- ace_isoprene_mixing_ratio_ambient_air_v1.1.csv, data file, comma-separated values
- data_file_header.txt, metadata, text
- README.txt, metadata, text
- change_log.txt, metadata, text

***** Version Control *****

The data set was revised in 2021 to account for an improved chromatogram baseline-removal routine in the procedure used to quantify isoprene. Version 1.1 is therefore the preferred and recommended version for all future use. 

***** Dataset contact *****

Valerio Ferracci, Cranfield University. ORCID: 0000-0001-6647-993X. Email: v.ferracci@cranfield.ac.uk

***** Dataset citation *****

Bolas, C., Ferracci, V., Jones, R.L. and Harris, N.R.P. Isoprene mixing ratio in ambient air across the Southern Ocean in the austral summer of 2016/2017, during the Antarctic Circumnavigation Expedition (ACE). (Version 1.1) [Data set]. Zenodo. DOI: 10.5281/zenodo.5674685