The role of open lead interactions in atmospheric ozone variability between Arctic coastal and inland sites

Abstract

Boundary layer atmospheric ozone depletion events (ODEs) are commonly observed across polar sea ice regions following polar sunrise. During March-April 2005 in Alaska, the coastal site of Barrow and inland site of Atqasuk experienced ODEs (O 3 < 10 nmol mol -1 ) concurrently for 31% of the observations, consistent with large spatial scale ozone depletion. However, 7% of the time ODEs were exclusively observed inland at Atqasuk. This phenomenon also occurred during one of nine flights during the BRomine, Ozone, and Mercury EXperiment (BROMEX), when atmospheric vertical profiles at both sites showed near-surface ozone depletion only at Atqasuk on 28 March 2012. Concurrent in-flight BrO measurements made using nadir scanning differential optical absorption spectroscopy (DOAS) showed the differences in ozone vertical profiles at these two sites could not be attributed to differences in locally occurring halogen chemistry. During both studies, backward air mass trajectories showed that the Barrow air masses observed had interacted with open sea ice leads, causing increased vertical mixing and recovery of ozone at Barrow and not Atqasuk, where the air masses only interacted with tundra and consolidated sea ice. These observations suggest that, while it is typical for coastal and inland sites to have similar ozone conditions, open leads may cause heterogeneity in the chemical composition of the springtime Arctic boundary layer over coastal and inland areas adjacent to sea ice regions.

Springtime Bromine Activation over Coastal and Inland Arctic Snowpacks

“…While ALAR was equipped with an in situ ozone monitor, ALAR primarily flew above 750 m (76.5% of total flight time), which is generally above the ozone-depleted layer . Through examination of ozone vertical profile measurements, we previously showed that ozone conditions at Utqiaġvik during BROMEX reflected regional ozone conditions for eight of the nine flights (excluding March 28) . Therefore, for this work, we classified the eight remaining flights based on near-surface ozone levels as measured at the NOAA Barrow Observatory (Table ), defining flights as occurring during an ODE if the measured ozone at Utqiaġvik was less than 10 nmol mol –1 .…”

“…This lead-induced vertical mixing also causes a recovery in ozone and mercury levels at locations downwind of the open lead as a result of the mixing of these species down from aloft . These lead opening events have also been linked to increased heterogeneity in boundary layer ozone over northern Alaska, with ODEs being observed inland but not at coastal locations . These regional ozone gradients only occurred once during BROMEX (March 28th) and are relatively rare, occurring only 17% of the time from March 19 to April 30, 2005 .…”

“… a Each flight was categorized according to the presence of an ODE (O 3 < 10 nmol mol –1 ) at Utqiaġvik, sign of the ozone rate change at Utqiaġvik (dO 3 /d t ), and whether a sea ice lead was present north of Alaska. O 3 measurements at Utqiaġvik are shown in Figure S4 of the Supporting Information. b As described by Peterson et al, the flight on March 28th occurred when regional spatial gradients in ozone were observed; therefore, this flight was excluded from analyses involving ozone. …”

“…Evidence of this springtime bromine chemistry is frequently observed across the Arctic using satellite-based tropospheric BrO retrievals, ,, which provide basin-scale information about the spatial distribution of tropospheric BrO with temporal resolution of several hours (in the polar regions) and coarse spatial resolution (13 × 24 km for OMI and 40 × 80 km for GOME-2). In contrast, airborne imaging differential optical absorption spectroscopy (DOAS) techniques ,, provide spatial mapping capabilities at a much greater spatial (100 s of m versus 10 s of km) and temporal (seconds versus several hours) resolution, allowing for the investigation of small spatial scale variations in BrO that may provide additional information on reactive bromine source regions. These airborne measurements enable the investigation of the mechanisms underlying the release of reactive bromine to the Arctic boundary layer, including those bromine activation events occurring in shallow layers, which can be difficult to detect with satellite-based observations .…”

Commentary on the outputs and future of Biogeochemical Exchange Processes at Sea-Ice Interfaces (BEPSII)

“…The cycling of the volatile organic compounds, dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP) and dimethyl sulfoxide (DMSO), in Antarctic spring ice was influenced strongly by short-term synoptic events, most likely due to due to the temperature impact on the connectivity and stability of the brine network (Carnat et al, 2016). Differences in ozone vertical profiles at Arctic coastal and inland sites could not be attributed to differences in locally occurring halogen chemistry, but instead were linked to interactions with open water between seaice floes (leads), causing increased vertical mixing and recovery of ozone (Peterson et al, 2016). The applicability of bio-optics for estimating photosynthesis rates of sea-ice algae in the Baltic Sea was assessed by Müller et al (2016).…”