Evaluating the impact of blowing-snow sea salt aerosol on springtime BrO and O₃ in the Arctic

We use the GEOS-Chem chemical transport model to examine the influence of bromine release from blowing-snow sea salt aerosol (SSA) on springtime bromine activation and O3 depletion events (ODEs) in the Arctic lower troposphere. We evaluate our simulation against observations of tropospheric BrO vertical column densities (VCDtropo) from the GOME-2 (second Global Ozone Monitoring Experiment) and Ozone Monitoring Instrument (OMI) spaceborne instruments for 3 years (2007–2009), as well as against surface observations of O3. We conduct a simulation with blowing-snow SSA emissions from first-year sea ice (FYI; with a surface snow salinity of 0.1 psu) and multi-year sea ice (MYI; with a surface snow salinity of 0.05 psu), assuming a factor of 5 bromide enrichment of surface snow relative to seawater. This simulation captures the magnitude of observed March–April GOME-2 and OMI VCDtropo to within 17 %, as well as their spatiotemporal variability (r=0.76–0.85). Many of the large-scale bromine explosions are successfully reproduced, with the exception of events in May, which are absent or systematically underpredicted in the model. If we assume a lower salinity on MYI (0.01 psu), some of the bromine explosions events observed over MYI are not captured, suggesting that blowing snow over MYI is an important source of bromine activation. We find that the modeled atmospheric deposition onto snow-covered sea ice becomes highly enriched in bromide, increasing from enrichment factors of ∼5 in September–February to 10–60 in May, consistent with composition observations of freshly fallen snow. We propose that this progressive enrichment in deposition could enable blowing-snow-induced halogen activation to propagate into May and might explain our late-spring underestimate in VCDtropo. We estimate that the atmospheric deposition of SSA could increase snow salinity by up to 0.04 psu between February and April, which could be an important source of salinity for surface snow on MYI as well as FYI covered by deep snowpack. Inclusion of halogen release from blowing-snow SSA in our simulations decreases monthly mean Arctic surface O3 by 4–8 ppbv (15 %–30 %) in March and 8–14 ppbv (30 %–40 %) in April. We reproduce a transport event of depleted O3 Arctic air down to 40∘ N observed at many sub-Arctic surface sites in early April 2007. While our simulation captures 25 %–40 % of the ODEs observed at coastal Arctic surface sites, it underestimates the magnitude of many of these events and entirely misses 60 %–75 % of ODEs. This difficulty in reproducing observed surface ODEs could be related to the coarse horizontal resolution of the model, the known biases in simulating Arctic boundary layer exchange processes, the lack of detailed chlorine chemistry, and/or the fact that we did not include direct halogen activation by snowpack chemistry.

Halogen activation and radical cycling initiated by imidazole-2-carboxaldehyde photochemistry

Atmospheric aerosol particles can contain light-absorbing organic compounds, also referred to as brown carbon (BrC). The ocean surface and sea spray aerosol particles can also contain light-absorbing organic species referred to as chromophoric dissolved organic matter (CDOM). Many BrC and CDOM species can contain carbonyls, dicarbonyls or aromatic carbonyls such as imidazole-2-carboxaldehyde (IC), which may act as photosensitizers because they form triplet excited states upon UV–VIS light absorption. These triplet excited states are strong oxidants and may initiate catalytic radical reaction cycles within and at the surface of atmospheric aerosol particles, thereby increasing the production of condensed-phase reactive oxygen species (ROS). Triplet states or ROS can also react with halides, generating halogen radicals and molecular halogen compounds. In particular, molecular halogens can be released into the gas phase, which is one halogen activation pathway. In this work, we studied the influence of bromide and iodide on the photosensitized production and release of hydroperoxy radicals (HO2) upon UV irradiation of films in a coated wall flow tube (CWFT) containing IC in a matrix of citric acid (CA) irradiated with UV light. In addition, we measured the iodine release upon irradiation of IC ∕ CA films in the CWFT. We developed a kinetic model coupling photosensitized CA oxidation with condensed-phase halogen chemistry to support data analysis and assessment of atmospheric implications in terms of HO2 production and halogen release in sea spray particles. As indicated by the experimental results and confirmed by the model, significant recycling of halogen species occurred via scavenging reactions with HO2. These prevented the full and immediate release of the molecular halogen (bromine and iodine) produced. Recycling was stronger at low relative humidity, attributed to diffusion limitations. Our findings also show that the HO2 production from BrC or CDOM photosensitized reactions can increase due to the presence of halides, leading to high HO2 turnover, in spite of low release due to the scavenging reactions. We estimated the iodine production within sea salt aerosol particles due to iodide oxidation by ozone (O3) at 5.0×10-6 M s−1 assuming O3 was in Henry's law equilibrium with the particle. However, using an O3 diffusion coefficient of 1×10-12 cm2 s−1, iodine activation in an aged, organic-rich sea spray is estimated to be 5.5×10-8 M s−1. The estimated iodine production from BrC photochemistry based on the results reported here amounts to 4.1×10-7 M s−1 and indicates that BrC photochemistry can exceed O3 reactive uptake in controlling the rates of iodine activation from sea spray particles under dry or cold conditions where diffusion is slow within particles.

Halogen activation and radical cycling initiated by imidazole-2-carboxaldehyde photochemistry

Atmospheric aerosol particles can contain light absorbing organic compounds, also referred to as brown carbon (BrC). In the context of the ocean surface and of sea spray aerosol deriving from the latter, light absorbing organic species are also referred to as chromophoric dissolved organic matter (CDOM). Many BrC or CDOM species (especially carbonyls, dicarbonyls or aromatic carbonyls such as imidazole-2-carboxaldehyde (IC)), referred to as photosensitizers, form triplet excited states upon UV-VIS light absorption. These triplet excited states are strong oxidants and may initiate catalytic radical reaction cycles within atmospheric aerosol particles and at their surface, therefore increasing the reactive oxygen species (ROS) production within atmospheric aerosol particles. Triplet states (or ROS resulting from them) can also react with halides generating halogen radicals and additionally molecular halogens compounds, which can be released into the gas phase and may thus contribute to halogen activation. In this work we study the influence of bromide and iodide on the photosensitized HO2 production and release upon UV irradiation of films in a coated wall flow tube (CWFT) containing IC in a matrix of citric acid (CA). Additionally we measured the iodine release upon irradiation of IC/CA films in the CWFT. We use a kinetic model to interpret our results and to assess radical production and iodine release in sea-spray particles. As indicated by the experimental results and confirmed by the model, significant recycling of halogen species occurs via scavenging reactions with HO2, to prevent the full and immediate release of the molecular halogen (bromine and iodine) produced, while partially shutting down the HOx chemistry. The recycling efficiency is higher and affected by diffusion limitations at high viscosity. Our findings also show that halides can increase substantially the BrC or CDOM photosensitized HO2 production (which in turn promotes radical and ROS production) by reacting with triplet statesin sea-spray particles. The iodine production within sea salt aerosol particles due to iodide oxidation by ozone is estimated at 5.9 × 10−5 M s−1 assuming ozone equilibration in the particle. Under diffusion limitation this activation can drop several orders of magnitude in an aged, organic-rich sea-spray derived aerosol (1.1 × 10−7 M s−1 for an ozone diffusion coefficient of 10−12 cm2 s−1). The estimated iodine production from BrC photochemistry amounts to 2.5 × 10−7 M s−1. This indicates that BrC photochemistry can exceed O3 reactive uptake in controlling the rates of iodine activation from sea spray particles under dry or cold conditions where diffusion is slow within particles.

Case study of ozone anomalies over northern Russia in the 2015/2016 winter: Measurements and numerical modeling

Episodes of extremely low ozone columns were observed over the territory of Russia in the Arctic winter of 2015/2016 and the beginning of spring 2016. We compare total ozone columns (TOC) obtained using different remote sensing techniques (satellite and ground-based observations) and results of numerical modelling over the territory of the Urals and Siberia for the above period. We demonstrate that the provided monitoring systems (including new Russian Fourier- spectrometer IKFS-2) and modern 3-dimensional models are able to capture the observed TOC anomalies. However, the results of observations and modelling show discrepancies of up to 20–30 % in TOC measurements. Analysis of the role of chemical and dynamical processes demonstrates that it is unlikely that observed short-term TOC variability may be a result of local photochemical destruction initiated by heterogeneous halogen activation on particles of polar stratospheric clouds that formed under low temperatures in the mid-winter.