The potential importance of frost flowers, recycling on snow, and open leads for ozone depletion events

[摘要] We present model studies with the one-dimensional modelMISTRA to investigate the potential role of frost flowers, recycling on snow, and open leadsin the depletion oftropospheric ozone in the Arctic spring. In our model, we assumed frost floweraerosols to be the major source of bromine. We show that a major ozonedepletion event can be satisfactorily reproduced only if the recycling on snow ofdeposited bromine into gas phase bromine is assumed. In the model, this cycling is moreefficient than the bromine explosion process andmaintains sufficiently high levels of bromine to deplete ozone down to fewnmol mol⁻¹ within four days. We assessed the influence of differentsurface combinations (open lead/frost flowers) on the chemistry in themodel. Results showed noticeablemodifications affecting the composition of aerosols and the depositionvelocities. A model run with a series of coupled frost flower fields and open leads,separated by large areas of snow, showed results comparable with fieldobservations. In addition, we studied the effects of modified temperature of either thefrost flower field or the ambient airmass. A warmer frost flower field increases therelative humidity and the aerosol deposition rate. The deposition/re-emissionprocess gains in importance, inducing more reactive bromine in the gas phase, and a stronger ozonedepletion. A decrease of 1K in airmass temperature shows in our model that the aerosoluptake capacities of all gas phase species substantially increases, leading to enhanced uptake of acids fromthe gas phase. Consequently, the so-called bromine explosion accelerated andO₃ mixing ratios decreased.In our model representation, variations in wind speed affected the aerosolsource function and influenced the amount of bromine in the atmosphere andthus the ozone depletion strength.Recent studies have suggested the important role of theprecipitation of calcium carbonate (CaCO₃) out of the brine layer for the possible acidification of the liquid phase by acid uptake. Our investigation showed that thisprecipitation is a crucial process for the timing of the bromineexplosion in aerosols. Nevertheless, model runs with either 50% precipitationor complete precipitation displayed a relatively weak difference inozone mixing ratios after four simulated days. By considering conditionstypical for "Arctic Haze" pollution events at the startof the run we obtained a low pH in frost flower aerosols due to a greatermixing ratio of SO₂, and a strong recycling efficiency via large aerosol numberconcentration. The aerosol acidification during a haze event most likelyintensifies the ozone depletion strength and occurrence. The comparisonbetween our modeled deposition on snow and sampled snow at Barrow (Alaska) shows that approximately 75% of deposited bromine may be re-emitted intothe gas phase as Br₂/BrCl. Among several non-halogen fluxes fromthe snow, model simulations showed that only HONOaffects the chemistry.Finally, we investigated the release of Br₂ potentially produced byheterogeneous reactions directly on frost flowers. In this case, we obtainedunrealistic results of aerosol compositions and deposition rates on snowcompared to observations in the Arctic.

[学科分类] 大气科学