Skip to main content

Reconstructing atmospheric oxidation processes in the past with clumped isotope measurements in O2 trapped in polar firn and ice air

Periodic Reporting for period 1 - Clumped Isotopes (Reconstructing atmospheric oxidation processes in the past with clumped isotope measurements in O2 trapped in polar firn and ice air)

Reporting period: 2017-09-25 to 2019-09-24

In this project we proposed to understand the present and past atmospheric photochemistry using clumped isotopes in O2 in the atmosphere and that trapped in firn and ice core air. Clumped isotopes are the abundances of the isotopologues in which two or more rare isotopes replace the most abundant isotopes i.e. clumping of rare isotopes (e.g. 18O18O in O2). O2 has three clumped isotopes viz., 18O18O, 18O17O and 17O17O. The importance of the clumped isotopes in O2 is that, the abundances of these isotopologues are sensitive to ozone photochemistry because of formation of the highly reactive species O(3p) from ozone (O3) in presence of light, mainly UV. O(3p) reacts with atmospheric O2 and resets its clumped isotope signature depending on the temperature. Therefore, the abundances of these rare isotopologues or clumped isotope values in O2 trapped in firn and ice core air, can be used to understand the past atmospheric O3 variation. There is no direct record related to the past atmospheric ozone due to its fragile nature but its signature is preserved in O2.

The overall objective of the project were to establish the measurements of clumped isotopes in O2, validate the role of ozone on its isotopic ordering by laboratory experiments and measuring the stratospheric and tropospheric air O2 and understand the past tropospheric ozone variation from clumped isotopes in O2 in the firn and ice core air. Also modelling the clumped isotopes in the atmospheric O2 to understand the experimental results was another primary aim of the project.

The method along with clumped isotope values measured in the stratospheric and tropospheric air O2 are published in Rapid Communications in Mass Spectrometry. In the paper we presented the technical details of the measurements and calibrations. We also presented the clumped isotopes data of lower stratospheric and upper tropospheric air O2 collected from GEOPHYSICA aircraft campaign with base station at Kalamata, Greece and lower tropospheric air O2 samples from Netherlands. We found that the tropospheric air is well mixed i.e. the tropospheric mixing time (weeks) is much smaller than the tropospheric isotopic resetting time (months). As expected the lower stratospheric O2 is found to be close to isotopic equilibrium at the ambient temperatures due to abundance of O3 there. But in the troposphere, the O2 is deviated from isotopic equilibrium due to lack of O3 in the troposphere and it carries the partial stratospheric clumping signature.
We also measured the clumped isotopes in the tropospheric O2 from different geographical locations including samples from high latitude and polar regions to understand the role of atmospheric circulation on the clumped isotopes in O2. We observed that the air samples in the regions where stratospheric subsidence takes place are closer to the equilibrium values while those in the upwelling regions are deviated more from the equilibrium. This is due to the fact that the regions with air subsidence have more air mass equilibrated at lower temperatures and higher ozone concentration the upwelling regions are dominated by the lower tropospheric air with lower clumped isotopic values.
We observed a decreasing trend with time in the O2 clumped isotope values in firn air from East Greenland Ice Core Project (EGRIP) and North Greenland Eemian Ice Drilling (NEEM) covering age span of the last ~60 years. This indicates that there was a significant increase in the tropospheric O3 concentration during this period. The Atmospheric Chemistry model (EMAC) also predicts a similar decrease in the clumped isotopic values in response to the increase in the tropospheric ozone burden.
Clumped isotopes in O2 trapped in ice core collected from NEEM was analysed to understand the preindustrial - industrial and glacial-interglacial changes in the tropospheric ozone. We observed that the modern tropospheric O2 are slightly depleted (~-0.05 per mil) compared to the pre-industrial air O2 due to increase in the tropospheric ozone burden. We also observed significant decrease in the tropospheric O2 clumped isotopic values during the mid Holocene, probably due to enhanced stratosphere-troposphere exchange. No significant change in the clumped isotopes in O2 from the glacial period (30-20 kyr) were observed indicating not much variation in the tropospheric ozone. The data are being analyzed and expecting to publish along with model simulations soon.
Following works have been performed during the project
1. Established clumped isotope measurements in O2 using high resolution stable isotope ratio mass spectrometer 253 ULTRA. The measurement technique was highly challenging due to very less abundances (a few ppm) of the rare isotopologues targeted to measure precisely (18O18O and 17O18O) and resolving multiple isobaric interferences associated with the measurement of each isotopologue. Also the calibration of clumped isotopes in international absolute scale was carried out.
2. Measured startospheric and tropospheric air O2 for verifying the hypothesis that ozone photochemistry or more precisely O(3p) produced from ozone photo-dissociation resets the atmospheric O2 clumped isotopic signature which depends on the ambient temperature. Lower startospheric O2 was found to be in close theromdynamic equilibrium at the ambient temperature but tropospheric O2 deviates.
3. Trospospheric air O2 samples from a wide range of latitudes have been measured to verify that the regions with startospheric subsidence and higher ozone concentration have lower clumped isotopic values (more startospheric signature) and upwelling regions (e.g. tropics) with low clumped isotopic values.
4. Firn samples measured for reconstructing recent past atmospheric O2 photochemistry. It is believed that the tropospheric ozone concentration varied in the past. For example, tropospheric ozone increased significantly during the industrial era but there was no direct data available till 2019 CE except model simulations. A very recent publications (appeared in Nature 570, 224−227, 2019) estimated an increase of ozone concentration by 40% near the surface. We found that the model used to estimate the 40% increase in ozone has many shortcoming making the prediction questionable. Our results along with better model simulations will be published soon.
5. O2 trapped in ice core, obtained from Greenland has been used to understand the tropospheric ozone variation during preindustrial-industrial and glacial-interglacial periods. We gathered a comprehensive set of data which along with model simulations will be published soon.
The findings would help to estimate of the past tropospheric O3 variation. This is very important because there is no direct record of the past O3 in the atmosphere. The results would be very helpful for atmospheric and climate modelling community. This will have a wider societal impact as tropospheric ozone has direct influence on human health and the results would help to understand its variation in the past.