Abstract. Remote sensing of the gaseous composition of non-eruptive, passively degassing volcanic plumes can be a tool to gain insight into volcano interior processes. Here, we report on a field study in September 2015 that demonstrates the feasibility of remotely measuring the volcanic enhancements of carbon dioxide (CO 2 ), hydrogen fluoride (HF), hydrogen chloride (HCl), sulfur dioxide (SO 2 ), and bromine monoxide (BrO) in the downwind plume of Mt. Etna using portable and rugged spectroscopic instrumentation. To this end, we operated the Fourier transform spectrometer EM27/SUN for the shortwave-infrared (SWIR) spectral range together with a co-mounted UV spectrometer on a mobile platform in direct-sun view at 5 to 10 km distance from the summit craters. The 3 days reported here cover several plume traverses and a sunrise measurement. For all days, intra-plume HF, HCl, SO 2 , and BrO vertical column densities (VCDs) were reliably measured exceeding 5 ×1016 , 2 ×10 17 , 5 ×10 17 , and 1 ×10 14 molec cm −2 , with an estimated precision of 2.2 ×10 15 , 1.3 ×10 16 , 3.6 ×10 16 , and 1.3 ×10 13 molec cm −2 , respectively. Given that CO 2 , unlike the other measured gases, has a large and wellmixed atmospheric background, derivation of volcanic CO 2 VCD enhancements ( CO 2 ) required compensating for changes in altitude of the observing platform and for background concentration variability. The first challenge was met by simultaneously measuring the overhead oxygen (O 2 ) columns and assuming covariation of O 2 and CO 2 with altitude. The atmospheric CO 2 background was found by identifying background soundings via the coemitted volcanic gases. The inferred CO 2 occasionally exceeded 2 × 10 19 molec cm −2 with an estimated precision of 3.7 × 10 18 molec cm −2 given typical atmospheric background VCDs of 7 to 8 × 10 21 molec cm −2 . While the correlations of CO 2 with the other measured volcanic gases confirm the detection of volcanic CO 2 enhancements, correlations were found of variable significance (R 2 ranging between 0.88 and 0.00). The intra-plume VCD ratios CO 2 / SO 2 , SO 2 / HF, SO 2 / HCl, and SO 2 / BrO were in the range 7.1 to 35.4, 5.02 to 21.2, 1.54 to 3.43, and 2.9 × 10 3 to 12.5 × 10 3 , respectively, showing pronounced day-to-day and intra-day variability.

Abstract.A portable Fourier transform spectrometer (FTS), model EM27/SUN, was deployed onboard the research vessel Polarstern to measure the column-average dry air mole fractions of carbon dioxide (XCO 2 ) and methane (XCH 4 ) by means of direct sunlight absorption spectrometry. We report on technical developments as well as data calibration and reduction measures required to achieve the targeted accuracy of fractions of a percent in retrieved XCO 2 and XCH 4 while operating the instrument under field conditions onboard the moving platform during a 6-week cruise on the

Abstract. It is a widely established fact that standard semiLagrangian advection schemes are highly efficient numerical techniques for simulating the transport of atmospheric tracers. However, as they are not formally mass conserving, it is essential to use some method for restoring mass conservation in long time range forecasts. A common approach is to use global mass fixers. This is the case of the semi-Lagrangian advection scheme in the Integrated Forecasting System (IFS) model used by the Copernicus Atmosphere Monitoring Service (CAMS) at the European Centre for Medium-Range Weather Forecasts (ECMWF).Mass fixers are algorithms with substantial differences in complexity and sophistication but in general of low computational cost. This paper shows the positive impact mass fixers have on the inter-hemispheric gradient of total atmospheric column-averaged CO 2 and CH 4 , a crucial feature of their spatial distribution. Two algorithms are compared: the simple "proportional" and the more complex BermejoConde schemes. The former is widely used by several Earth system climate models as well the CAMS global forecasts and analysis of atmospheric composition, while the latter has been recently implemented in IFS. Comparisons against total column observations demonstrate that the proportional mass fixer is shown to be suitable for the low-resolution simulations, but for the high-resolution simulations the BermejoConde scheme clearly gives better results. These results have potential repercussions for climate Earth system models using proportional mass fixers as their resolution increases. It also emphasises the importance of benchmarking the tracer mass fixers with the inter-hemispheric gradient of long-lived greenhouse gases using observations.

Abstract. A portable Fourier Transform Spectrometer (FTS), model EM27/SUN, is deployed onboard the research vessel Polarstern to measure the column-average dry air mole fractions of carbon dioxide (XCO2) and methane (XCH4) by means of direct sunlight absorption spectrometry. We report on technical developments as well as data calibration and reduction measures required to achieve the targeted accuracy of fractions of a percent in retrieved XCO2 and XCH4 while operating the instrument under field conditions onboard the moving platform during a six week cruise through the Atlantic from Cape Town (South Africa, 34° S, 18° E) to Bremerhaven (Germany, 54° N, 19° E). We demonstrate that our solar tracker typically achieves a tracking precision of better than 0.05° toward the center of the sun throughout the ship cruise which facilitates accurate XCO2 and XCH4 retrievals even under harsh ambient wind conditions. We define several quality filters that screen spectra e.g. when the field-of-view is partially obstructed by ship structures or when the lines-of-sight cross the ship exhaust plume. The measurements in clean oceanic air, can be used to characterize a spurious airmass dependency. After the campaign, deployment of the spectrometer side-by-side the TCCON (Total Carbon Column Observing Network) instrument at Karlsruhe, Germany, allows for determining a calibration factor that makes the entire campaign record traceable to World Meteorological Organization (WMO) standards. Comparisons to observations of the GOSAT satellite and concentration fields modeled by the European Centre for Medium-Range Weather Forecasts (ECMWF) within the project Monitoring of Atmospheric Composition and Climate – Interim Implementation (MACC-II) demonstrate that the observational setup is well suited to provide validation opportunities above the ocean and along interhemispheric transects.

Abstract. Remote sensing of the gaseous composition of non-eruptive, passively degassing volcanic plumes can be a tool to gain insight into volcano interior processes. Here, we report on a field study in September 2015 that demonstrates the feasibility of remotely measuring the volcanic enhancements of carbon dioxide (CO2), hydrogen fluoride (HF), hydrogen chloride (HCl), sulfur dioxide (SO2), and bromine monoxide (BrO) in the downwind plume of Mt. Etna using portable and rugged spectroscopic instrumentation. To this end, we operated the Fourier Transform Spectrometer EM27/SUN for the shortwave-infrared (SWIR) spectral range together with a co-mounted UV spectrometer on a mobile platform in direct-sun view at 5 to 10 km distance from the summit craters. The three days reported here cover several plume traverses and a sunrise measurement. For all days, intra-plume HF, HCl, SO2, and BrO vertical column densities (VCDs) were reliably measured exceeding 5 × 1016 molec/cm2, 2 × 1017 molec/cm2, 5 × 1017 molec/cm2, and 1 × 1014 molec/cm2, with an estimated precision of 2.2 × 1015 molec/cm2, 1.3 × 1016 molec/cm2, 3.6 × 1016 molec/cm2, and 1.3 × 1013 molec/cm2, respectively. Given that CO2, unlike the other measured gases, has a large and well-mixed atmospheric background, derivation of volcanic CO2 VCD enhancements (ΔCO2) required compensating for changes in altitude of the observing platform and for background concentration variability. The first challenge was met by simultaneously measuring the overhead oxygen (O2) columns and assuming covariation of O2 and CO2 with altitude. The atmospheric CO2 background was found by identifying background soundings via the co-emitted volcanic gases. The inferred ΔCO2 occasionally exceeded 2 × 1019 molec/cm2 with an estimated precision of 3.7 × 1018 molec/cm2 given typical atmospheric background VCDs of 7 to 8 × 1021 molec/cm2. While the correlations of ΔCO2 with the other measured volcanic gases confirm the detection of volcanic CO2 enhancements, correlations were found of variable significance (R2 ranging between 0.88 and 0.00). The intra-plume VCD ratios ΔCO2 / SO2, SO2 / HF, SO2 / HCl, and SO2 / BrO were in the range 7.1 to 35.2, 5.02 to 10.5, 1.54 to 3.43, and 2.9 × 103 to 12.5 × 103, respectively, showing pronounced day-to-day and intra-day variability.