Atomic chlorine (Cl) is a strong atmospheric oxidant that shortens the lifetimes of pollutants and methane in the springtime Arctic, where the molecular halogens Cl2 and BrCl are known Cl precursors. Here, we quantify the contributions of reactive chlorine trace gases and present the first observations, to our knowledge, of ClNO2 (another Cl precursor), N2O5, and HO2NO2 in the Arctic. During March – May 2016 near Utqiaġvik, Alaska, up to 21 ppt of ClNO2, 154 ppt of Cl2, 27 ppt of ClO, 71 ppt of N2O5, 21 ppt of BrCl, and 153 ppt of HO2NO2 were measured using chemical ionization mass spectrometry. The main Cl precursor was calculated to be Cl2 (up to 73%) in March, while BrCl was a greater contributor (63%) in May, when total Cl production was lower. Elevated levels of ClNO2, N2O5, Cl2, and HO2NO2 coincided with pollution influence from the nearby town of Utqiaġvik and the North Slope of Alaska (Prudhoe Bay) Oilfields. We propose a coupled mechanism linking NO x with Arctic chlorine chemistry. Enhanced Cl2 was likely the result of the multiphase reaction of Cl– (aq) with ClONO2, formed from the reaction of ClO and NO2. In addition to this NO x -enhanced chlorine chemistry, Cl2 and BrCl were observed under clean Arctic conditions from snowpack photochemical production. These connections between NO x and chlorine chemistry, and the role of snowpack recycling, are important given increasing shipping and fossil fuel extraction predicted to accompany Arctic sea ice loss.

Nitrous acid (HONO) plays an important role in the oxidative capacity of the atmosphere during wintertime via photolysis to produce hydroxyl radicals (OH). While it is known that HONO is emitted from the Arctic snowpack, sparse observations of HONO in the midlatitude urban wintertime environment have hindered our understanding of cold-season atmospheric chemistry. In this study, measurements of ambient HONO, particulate nitrite (pN­(III); N­(III) = NO2(aq) – + HONO(aq) + H2ONO(aq) +), and snow nitrite (sN­(III)) were conducted in Kalamazoo, Michigan during January–February 2018. Elevated levels of HONO and particulate nitrite were observed over snow-covered ground, likely due to emissions of HONO from the snowpack, as well as weak turbulent mixing in the atmospheric boundary layer. The noontime peak in HONO of 87 ± 60 (1σ) parts per trillion (ppt) over snow-covered ground suggests photochemical snowpack HONO production, likely in part through snowpack nitrate photolysis, with only a minor contribution from particulate nitrate photolysis. High concentrations of snow nitrite (0.4 ± 0.3 (1σ) μM) support the hypothesis that the snowpack is a significant source of HONO to the atmosphere. On average, the OH production rate from HONO photolysis, in the near-surface atmosphere (≈ 2 m above ground), was calculated to be about an order of magnitude higher than that from O3 photolysis over snow-covered ground. Future studies are needed to quantify HONO emissions from the midlatitude urban snowpack, given expected HONO production due to high concentrations of snow nitrate and nitrite from anthropogenic particulate nitrate and nitrite deposition.

Bromine and chlorine chemistry in the Arctic atmospheric boundary layer has significant impacts on tropospheric ozone depletion and the fates of atmospheric pollutants, including mercury and hydrocarbons. Bromine chloride (BrCl) produces bromine and chlorine radicals upon photolysis and links these two halogen cycles. However, because of the limited number of BrCl measurements, the relative importance of its production and removal pathways are uncertain. Here we report BrCl observations near Utqiaġvik, AK, during March–May 2016 using chemical ionization mass spectrometry as part of the Photochemical Halogen and Ozone Experiment: Mass Exchange in the Lower Troposphere (PHOXMELT). Two distinct BrCl diel regimes were identified, with daytime BrCl primarily observed in March and nighttime BrCl observed in April and May, demonstrating a dependence on photochemistry. The dominant BrCl production mechanisms for these regimes were explored using a zero-dimensional numerical model constrained to a suite of halogen measurements. Multiphase reactions on the snowpack surface, mainly via Cl2 + Br–(aq) and HOBr + Cl–(aq), are predicted to be the largest contributors to near-surface BrCl production. Average net snowpack fluxes of 1.9 × 108 and 2.2 × 108 BrCl molecules cm–2 s–1 for two case periods in March and May are needed to explain the observations. The findings in this work highlight coupled bromine and chlorine chemistry and important halogen activation pathways in the springtime Arctic boundary layer.

Intermittent transitions between turbulent and non-turbulent states are ubiquitous in the stable atmospheric surface layer (ASL). Data from two field experiments in Utqiagvik, Alaska, and from direct numerical simulations are used to probe these state transitions so as to (i) identify statistical metrics for the detection of intermittency, (ii) probe the physical origin of turbulent bursts, and (iii) quantify intermittency effects on overall fluxes and their representation in closure models. The analyses reveal three turbulence regimes, two of which correspond to weakly turbulent periods accompanied by intermittent behavior (regime 1: intermittent, regime 2: transitional), while the third is associated with a fully turbulent flow. Based on time series of the turbulence kinetic energy (TKE), two non-dimensional parameters are proposed to diagnostically categorize the ASL state into these regimes; the first characterizes the weakest turbulence state, while the second describes the range of turbulence variability. The origins of intermittent turbulence activity are then investigated based on the TKE budget over the identified bursts. While the quantitative results depend on the height, the analyses indicate that these bursts are predominantly advected by the mean flow, produced locally by mechanical shear, or lofted from lower levels by turbulent ejections. Finally, a new flux model is proposed using the vertical velocity variance in combination with different mixing length scales. The model provides improved representation (correlation coefficients with observations of 0.61 for momentum and 0.94 for sensible heat) compared to Monin–Obukhov similarity (correlation coefficients of 0.0047 for momentum and 0.49 for sensible heat), thus opening new pathways for improved parametrizations in coarse atmospheric models.

Bromine radicals (Br•) cause ozone depletion and mercury deposition in the Arctic atmospheric boundary layer, following Polar sunrise. These Br radicals are primarily formed by the photolysis of molecular bromine (Br 2 ), which is photochemically produced in the snowpack. Recently, it was shown that bromine monoxide (BrO•), formed from the reaction of Br• with ozone, is episodically present until the onset of snowmelt in late Arctic spring. To examine the drivers of this late spring shutdown of reactive bromine chemistry, the gases Br 2 , HOBr, BrO, and BrCl were continuously monitored using chemical ionization mass spectrometry during the spring (March−May 2016) near Utqiagvik, Alaska. On May 10th, all four reactive bromine species fell below levels of detection at the same time that air temperature increased above 0 °C, surface albedo decreased, and snowmelt onset was observed. Prior to the cessation of atmospheric bromine chemistry, local surface snow samples in early May became significantly enriched in bromide, likely due to the slowdown of reactive bromine recycling with continued deposition but decreased emissions from the snowpack. Particulate bromide concentrations were not sufficient to explain the quantities of reactive bromine gases observed and decreased upon snowmelt. Low wind speeds during the weeks preceding the cessation of reactive bromine chemistry point to the lack of a contribution to bromine chemistry from blowing snow. Together, these results further highlight the significance of the surface snowpack in multiphase bromine recycling with important implications as the melt season arrives earlier due to climate change.