Abstract
Abstract. Natural sea-salt aerosols, when interacting with anthropogenic emissions, can enhance the formation of particulate nitrate. This enhancement has been suggested to increase the direct radiative forcing of nitrate, called the “mass-enhancement effect”. Through a size-resolved dynamic mass transfer modeling approach, we show that interactions with sea salt shift the nitrate from sub- to super-micron-sized particles (“redistribution effect”), and hence this lowers its efficiency for light extinction and reduces its lifetime. The redistribution effect overwhelms the mass-enhancement effect and significantly moderates nitrate cooling; e.g., the nitrate-associated aerosol optical depth can be reduced by 10 %–20 % over European polluted regions during a typical sea-salt event, in contrast to an increase by ∼10 % when only accounting for the mass-enhancement effect. Global model simulations indicate significant redistribution over coastal and offshore regions worldwide. Our study suggests a strong buffering by natural sea-salt aerosols that reduces the climate forcing of anthropogenic nitrate, which had been expected to dominate the aerosol cooling by the end of the century. Comprehensive considerations of this redistribution effect foster better understandings of climate change and nitrogen deposition.
Highlights
Particulate nitrate (NO−3 ) is one of the most important anthropogenic aerosol components that exerts a climate cooling effect (IPCC, 2013; Haywood and Schulz, 2007)
Sea salt is emitted into the marine planetary boundary layer (PBL) with a mass concentration dominated by coarse particles, usually with a short lifetime and a limited transport range
The interaction between natural sea-salt aerosols and anthropogenic nitrate leads to the redistribution effect, which can shift the particulate nitrate from sub- to super-micron sizes
Summary
Particulate nitrate (NO−3 ) is one of the most important anthropogenic aerosol components that exerts a climate cooling effect (IPCC, 2013; Haywood and Schulz, 2007). This is because sea-salt aerosol can be transported over industrialized regions, interacts with anthropogenic precursors of nitrate and enhances the total nitrate column loading in the atmosphere through heterogeneous uptake of HNO3 and its precursors (Liao et al, 2004; Liao and Seinfeld, 2005; Seinfeld and Pandis, 2006; Xu and Penner, 2012; Ravishankara, 1997; Lowe et al, 2015) Such sea-salt-induced nitrate mass increase is believed to strengthen the DRF and climate cooling of nitrate (Liao and Seinfeld, 2005), called the “mass-enhancement effect”. Compared to the fine particles, coarse particles have a significantly lower extinction efficiency in the visible part of the spectrum (IPCC, 2013; Murphy et al, 1998); the sea-salt-induced redistribution effect tends to weaken DRFnitrate, which counteracts the mass-enhancement effect The competition between these two effects will determine the net impact of sea-salt aerosol on anthropogenic DRFnitrate. A 1-year simulation with the EMAC (ECHAM5/MESSy Atmospheric Chemistry) model is used to demonstrate the importance of the redistribution effect on a global scale (Jöckel et al, 2010)
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