Methodology:

Sampling site:

The snow was sampled at several sites representing distinct environments: sea ice, inland close to sea level, and a hilltop ~600 m above sea level (asl). All snow samples collected were transferred to 50 mL polypropylene tubes with screw caps (Corning CentriStar), which prior to field deployment had been rinsed with ultra-high-purity (UHP) water and dried in a class 100 clean laboratory in Cambridge, UK.

Ionic analysis:

All samples were only melted prior to the ion chromatography (IC) analysis, apart from a small portion of the samples that had been melted for salinity measurements. All 2019 samples were filtered using Millex-GP Express PES Membrane, Sterile, 33 mm, 0.22 µm filters (Merck Millipore Ltd., Cork, Ireland). The 2018 snow samples were analysed using Thermo Scientific Dionex ICS-4000 ion chromatography systems, with ions of Na+, Ca2+, Mg2+, K+, NH4+, Cl-, Br-, SO4=, NO3-, F-, acetate, formate, oxalate and MSA measured. The 2019 samples for IC analysis were run on a Dionex ICS-5000+ with ions of Na+, Ca2+, Mg2+, K+, Cl-, Br-, SO4=, NO3-, and MSA measured. Anion analysis was performed using an ionPac AS18-Fast-4 µm column, and cation analysis was performed using an IonPac CS12A column. Multiple samples were analysed to assess precision.

Snow salinity:

Snow salinity was measured using a conductivity meter (SensIon 5, Hach) with a measurement range of 0-200 mScm-1 and a maximum resolution of 0.1 µScm-1 at low conductivities (0-199.9 µScm-1). Conductivity values were converted into psu, approximately equivalent to the weight of dissolved inorganic matter in grams per kilogram of seawater.

MAX-DOAS measurements of BrO:

Multi-axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of BrO partial columns were performed at the PEARL Ridge Lab. Spectra were recorded in the ultra-violet (UV) using a grating spectrometer (spectral resolution 0.45 nm) with a cooled (200 K) charge-coupled device (CCD) detector at 0.4-0.5 nm resolution. Elevation angles of 30°, 15°, 10°, 5°, 2°, 1°, and -1° were used in the elevation scans, and measurements were only taken with solar elevation above 4°. Differential slant column densities (dSCDs) of BrO and the oxygen dimer (O4) were retrieved using the DOAS technique. dSCDs were converted to profiles using a two-step optimal estimation method. The retrievals were performed for 0-4 km altitude on a grid with 0.2-km resolution. Due to the elevation of the measurement site, the instrument often measures BrO in the free troposphere, except during strong wind episodes and storms that generate a deep boundary layer (Bognar et al., 2020).

Surface ozone:

Surface ozone measurements were made by a TEI 49i ozone analyzer deployed at 0PAL.

Meteorology data:

There are two sets of local meteorology data used: one from EWS (the archived data are available at Historical Data - Climate - Environment and Climate Change Canada (ECCC) (weather.gc.ca)) and one from the PEARL Ridge Lab. The PEARL data is not open to the public; to access the data, contact Pierre Fogal at the University of Toronto. The UV index measured during the campaign period in 2018 and 2019 is from the ECCC Brewer spectrophotometer (https://doi.org/10.1029/2004JD004820).

Locations:

The major sampling sites are from a Canadian high Arctic location at Eureka, Nunavut (80°N, 86°W). They include sampling sites at close to sea level: Creek and 0PAL at 79.59.577N, 85.55.634W; Sea ice site at 80.00.130N, 86.07.244W; Onshore site at 80.00.215N, 86.07.145W, and at a hilltop PEARL Ridge lab (600m) at 80.03.208N, 86.25.381W.

Data collection:

Instrumentation:

Ionic analysis: Thermo Scientific Dionex ICS-4000 ion chromatography systems for 2018 samples and Dionex ICS-5000+ for 2019 samples

Conductivity meter: SensIon 5, Hach

BrO profile: Multi-axis Differential Optical Absorption Spectroscopy (MAX-DOAS)

Surface ozone: TEI 49i ozone analyzer

Data quality:

In the ionic analysis for 2019 samples, the limit of detection (LOD) is 3 times standard deviation of filter blank average peak area, and the limit of quantification (LOQ) is 10 times standard deviation of filter blank average peak area for all sequences (~40 samples analysed per sequence). The LOD of Br- is 0.200 um with a relative standard deviation of 0.023 um and the LOD of NO3- is 0.484 um with a relative standard of deviation of 0.037 um.

The accuracy of snow salinity as stated by the manufacturer is +/-0.001 psu at low salinities (<1 psu).