Measurement report: Significant ozone loss during the Arctic winter 2020 measured from ground based microwave radiometer

Ground-based microwave observations from MIRA2 situated in Kiruna, Sweden, were used to investigate Arctic stratospheric ozone during the Arctic winter 2019/2020. A comparison of O3 retrievals with coincident measurements of Aura MLS between 1 October 2019 and 30 April 2020 show good agreement across the investigated pressure levels (74, 56, 10, and 1 hPa). Remaining differences are well within the retrieval uncertainty of MIRA2. This demonstrates the capability of MIRA2 to provide robust ozone measurements for studies of stratospheric variability. A tracer-based approach was applied to derive cumulative chemical ozone loss on isentropic surfaces. At the 475 K isentropic (at around 50–60 hPa), ozone depletion increased from late winter into early spring, reaching a maximum loss of 2.14±0.90 ppmv in early April 2020. The magnitude and timing of the loss are consistent with the exceptional Arctic ozone depletion stated by model simulations and satellite-based estimates during the Arctic winter 2019/2020. Despite limited temporal sampling, the tracer-based method enables a consistent estimate of seasonal ozone loss from ground-based observations. The results highlight the ability of ground-based microwave radiometers to quantify chemical ozone depletion and its temporal evolution. Our results demonstrate that MIRA2 provides reliable stratospheric O3 measurements, supporting robust monitoring of long-term ozone trends. However, our estimates of chemical O3 depletion currently depend on MLS N2O observations as a passive tracer. With MLS nearing the end of its mission, future chemical ozone loss assessments will require either data sets of other Earth observing satellites, expanded ground-based measurement capabilities for tracer species such as N2O or the use of model-based tracers.