Natural sea-salt emissions moderate the climate forcing of anthropogenic nitrate

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.

New Multiphase Chemical Processes Influencing Atmospheric Aerosols, Air Quality, and Climate in the Anthropocene

“… 28 , 29 It is desirable to improve the parametrization of γ for a better understanding of nitrate formation and its environmental impact. 30 , 31 …”

“…For example, reactive uptake coefficients (γ) for N 2 O 5 on atmospheric aerosols varied over a wide range of 10 –5 to 10 –1 and were more strongly correlated with aerosol water content than with other parameters . Zheng et al had to adopt high uptake coefficients close to 0.1 (similar to mineral dust) to reproduce the observed nitrate concentrations during severe haze in Beijing, while observation-based closure studies revealed uptake coefficients from 0.009 to 0.072 in Beijing and southern China. , It is desirable to improve the parametrization of γ for a better understanding of nitrate formation and its environmental impact. , …”

Evaluation of the coupled high-resolution atmospheric chemistry model system MECO(n) using in situ and MAX-DOAS NO<sub>2</sub> measurements

“…ozone production (Vinken et al, 2014;Visser et al, 2019;Mertens et al, 2020a) as well as on heterogeneous processes, e.g. par-V. Kumar et al: Regional model evaluation using MAX-DOAS ticulate nitrate production (Chen et al, 2020). Various studies have shown that significant improvements in satellite retrievals can be achieved through the incorporation of highly resolved a priori trace gas and aerosol fields calculated by high-resolution regional models (Valin et al, 2011;Liu et al, 2020;Ialongo et al, 2020).…”

Sea Salt Aerosol Identification Based on Multispectral Optical Properties and Its Impact on Radiative Forcing over the Ocean

“…This, in turn, will strengthen the nitrate's climate cooling feedback [7][8][9][10], called "mass-enhancement effect" [10]. Coarse particle dominance of SS will shift the total mass of nitrate from sub-to super-micron (coarse) sizes, called the "redistribution effect", counteracting the mass-enhancement effect [10]. In other cases, super-micron SS may also effectively mix with anthropogenic sulphate aerosols originating from continental emissions of sulfur dioxide, causing a reduction in the extinction efficiency of sulphate [11].…”

“…Water-soluble cations originating from SS such as potassium, sodium, magnesium, and calcium react with nitrate, elevating its total column loading through the uptake of nitric acid [6]. This, in turn, will strengthen the nitrate's climate cooling feedback [7][8][9][10], called "mass-enhancement effect" [10]. Coarse particle dominance of SS will shift the total mass of nitrate from sub-to super-micron (coarse) sizes, called the "redistribution effect", counteracting the mass-enhancement effect [10].…”