Sea salt emission, transport and influence on size-segregated nitrate simulation: a case study in northwestern Europe by WRF-Chem

Abstract

Sea salt aerosol (SSA) is one of the major components of primary aerosols and has significant impact on the formation of secondary inorganic particles mass on a global scale. In this study, the fully online coupled WRF-Chem model was utilized to evaluate the SSA emission scheme and its influence on the nitrate simulation in a case study in Europe during 10-20 September 2013. Meteorological conditions near the surface, wind pattern and thermal stratification structure were well reproduced by the model. Nonetheless, the coarse-mode (PM 1−10 ) particle mass concentration was substantially overestimated due to the overestimation of SSA and nitrate. Compared to filter measurements at four EMEP stations (coastal stations: Bilthoven, Kollumerwaard and Vredepeel; inland station: Melpitz), the model overestimated SSA concentrations by a factor of 8-20. We found that this overestimation was mainly caused by overestimated SSA emissions over the North Sea during 16-20 September. Over the coastal regions, SSA was injected into the continental free troposphere through an "aloft bridge" (about 500 to 1000 m above the ground), a result of the different thermodynamic properties and planetary boundary layer (PBL) structure between continental and marine regions. The injected SSA was further transported inland and mixed downward to the surface through downdraft and PBL turbulence. This process extended the influence of SSA to a larger downwind region, leading, for example, to an overestimation of SSA at Melpitz, Germany, by a factor of ∼ 20. As a result, the nitrate partitioning fraction (ratio between particulate nitrate and the summation of particulate nitrate and gas-phase nitric acid) increased by about 20 % for the coarsemode nitrate due to the overestimation of SSA at Melpitz. However, no significant difference in the partitioning fraction for the fine-mode nitrate was found. About 140 % overestimation of the coarse-mode nitrate resulted from the influence of SSA at Melpitz. In contrast, the overestimation of SSA inhibited the nitrate particle formation in the fine mode by about 20 % because of the increased consumption of precursor by coarse-mode nitrate formation.

A parameterization of the heterogeneous hydrolysis of N<sub>2</sub>O<sub>5</sub> for mass-based aerosol models: improvement of particulate nitrate prediction

“…This is comparable with other models (Im et al, 2015). However, short periods with strong overestimations of [NO − 3 ] were also observed in previous New scheme (this study) 50 % * * Suggested by Sintermann et al (2012), Backes et al (2016), and Chen et al (2016b). studies. This seems to be the case for the HOPE Melpitz campaign simulation, during which COSMO-MUSCAT highly overpredicted [NO − 3 ] over Germany in this study (Fig.…”

“…The emission of NH 3 was reduced by 50 %, since over 90 % of NH 3 emissions in Europe are contributed by agricultural sources (Hertel et al, 2011;Erisman et al, 2008;Reidy et al, 2008), and agriculture emissions of NH 3 are overestimated by ∼ 50 % or even more (Sintermann et al, 2012;Backes et al, 2016). Also, Chen et al (2016b) adopted the same NH 3 emission inventory in the WRF-Chem model and reported that total NH 3 was overestimated by a factory of ∼ 2 at Melpitz during the campaign period. The modelled dust emissions depend on surface wind friction velocities, sur- face roughness, soil particle size distribution, and soil moisture (Heinold et al, 2011).…”

“…We followed the physics-relating configuration according to Chen et al (2016a), which reproduced meteorological conditions γ BC = 0.005 Sander et al (2003) during the HOPE Melpitz campaign well. The sea salt emission (Gong, 2003) was reduced by a factor of 20 in WRFChem, considering that Gong (2003) may highly overestimate sea salt emission (Neumann et al, 2016), and thus leads to an overestimation of sea salt by a factor of 20 during the HOPE campaign at Melpitz (Chen et al, 2016b). The configuration of chemical and aerosol treatments followed Chang et al (2016).…”

“…This seems to be the case for the HOPE Melpitz campaign simulation, during which COSMO-MUSCAT highly overpredicted [NO − 3 ] over Germany in this study (Fig. 3), and WRF-Chem in a previous study (Chen et al, 2016b). In order to evaluate the improvement of the NewN2O5 scheme and quantify the influence of NH 3 emission overestimation on the particulate nitrate prediction, three sensitivity simulations were conducted (Table 2).…”

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

“…However, not only the mass concentration but also the particle mass size distribution (PMSD) are 50 essential for evaluating the direct radiative forcing of nitrate (DRFnitrate) (IPCC, 2013;Murphy et al, 1998;Kok et al, 2017). Besides leading to the increase of total nitrate mass, interactions with sea-salt aerosol also have a 're-distribution effect' on nitrate PMSD which shifts nitrate from sub-to super-micron sizes (e.g., Chen et al, 2016a, see also Fig. 1).…”