Spatial and temporal distribution of sea salt aerosol mass concentrations in the marine boundary layer from the Arctic to the Antarctic

1. Sampling information The samples were collected from the Chinese Research Vessel (R/V) Xuelong, which undertook eight Antarctic cruises and five Arctic cruises, in summer, between 2008 and 2018. Briefly, all cruises used a large-flow air sampler to collect TSP (total suspended particulates) samples as well as field blanks. To avoid contamination, the large flow samplers were placed on the top deck during all cruises. All samples were collected using the relevant filter at a flow rate of 1.05 m3/min up to 57 h (most of them are 24h). After sampling, the samples were stored in a refrigerator at - 4 °C, prior to laboratory analysis. 2.Ion chromatography 467 samples were measured for ion concentrations. In summary, a punch sample of each filter was extracted by sonication and filtered through pre-washed 0.22-µm filters. Na+ in the samples was measured by ICS-2100 ion chromatography (Thermo Fisher Scientific Inc., USA). Each test had at least two reagent blanks, two field blanks, and one recovery. The recovery rate of all recovered samples was between 90% and 110%. The typical analytical precision of the instrument was better than 10 % relative standard deviation (RSD) for all ions. For comparison between the GEOS-Chem model and the observations, we converted the observed Na+ mass concentrations to SSA mass concentrations using a factor of 3.256, based on the mass ratio of Na+ in seawater. 3.GEOS-Chem model The GEOS-Chem global tropospheric chemistry model v12.2.6 (http://acmg.seas.harvard.edu/geos/) was used to simulate SSA concentrations with a spatial resolution of 2° × 2.5° at an hourly resolution (Text S3). Briefly, the model is driven by the Modern-Era Retrospective Analysis for Research and Applications-2 (MERRA-2). As our samples were TSP samples, the default SSA radius setting in the model (dp: particle diameter ~16 µm) was selected. SSA source functions in GEOS-Chem with an empirical dependence on SST derived by Jaegle et al. (2011). We simulated SSA concentrations every hour along the specific ship tracks and using the meteorology for the same dates as the measurements, and hourly-mean output for grid boxes extracted to match ship position. After simulating the hourly SSA concentration values along the ship tracks, all the simulated values during the sampling period (such as 24 hours) were arithmetic averaged to obtain the final SSA simulated value for each sample, which was compared with the observed value. 4.Auxiliary data The U10 data analyzed in this study were obtained with the meteorological detection system atop R/V Xuelong (33.5 m) during the Arctic and Antarctic cruises. SST, T2M (temperature at 2 m in the atmosphere), relative humidity (RH), sea ice fraction (FRESEAICE) were also acquired from the MERRA-2 meteorological field in the GEOS-Chem model, more details about the MERRA-2 data were given in Molod et al. (2015).1. 采样信息 样品采集自执行过8次南极科考航次与5次北极科考航次的中国“雪龙”号极地科考船（R/V Xuelong），采样时间为2008年至2018年的夏季。简言之，所有航次均采用大流量空气采样器采集总悬浮颗粒物（TSP, total suspended particulates）样品以及现场空白样。为避免污染，所有航次中该大流量采样器均部署于船舶露天甲板。所有样品均通过配套滤膜采集，采样流量为1.05 m³/min，单次采样时长最长可达57小时，多数采样时长为24小时。采样完成后，样品被存放于-4℃冰箱中，直至实验室分析。 2. 离子色谱分析 本研究共对467份样品进行了离子浓度测定。简言之，每份滤膜取打孔试样，经超声萃取后，通过预先洗涤过的0.22 μm滤膜进行过滤。样品中的钠离子（Na+）采用美国赛默飞世尔科技公司的ICS-2100离子色谱仪进行测定。每批次测试至少设置2份试剂空白、2份现场空白以及1组平行回收样。所有回收样品的回收率介于90%至110%之间。该仪器针对所有离子的典型分析精密度优于10%相对标准偏差（RSD, relative standard deviation）。为将GEOS-Chem模型模拟结果与观测数据进行对比，本研究基于海水中钠离子的质量占比，采用3.256的转换系数将实测的钠离子质量浓度换算为海源气溶胶（SSA, sea spray aerosol）质量浓度。 3. GEOS-Chem 模式 本研究采用GEOS-Chem全球对流层化学模式v12.2.6（网址：http://acmg.seas.harvard.edu/geos/）模拟海源气溶胶浓度，模式空间分辨率为2°×2.5°，时间分辨率为逐小时（详见补充材料Text S3）。简言之，该模式由现代回顾性研究与应用再分析资料第2版（MERRA-2, Modern-Era Retrospective Analysis for Research and Applications-2）驱动。由于本研究采集的为总悬浮颗粒物样品，因此采用模式默认的海源气溶胶粒径设置（dp：颗粒物直径约16 μm）。GEOS-Chem模式中的海源气溶胶排放函数采用Jaegle等人2011年提出的、依赖于海表温度（SST, sea surface temperature）的经验方案。本研究沿船舶特定航迹逐小时模拟海源气溶胶浓度，采用与观测同期的气象场数据，并提取对应船舶位置网格点的逐小时平均输出结果。在得到航迹沿线逐小时的海源气溶胶浓度模拟值后，对单份样品采样时段（如24小时）内的所有模拟值进行算术平均，得到该样品最终的模拟浓度，进而与实测浓度进行对比。 4. 辅助数据 本研究用到的10米高度风速（U10, 10-meter wind speed）数据，由“雪龙”号科考船33.5米高处的气象探测系统在南北极科考航次中采集得到。海表温度（SST, sea surface temperature）、2米高度气温（T2M, temperature at 2 m in the atmosphere）、相对湿度（RH, relative humidity）以及海冰覆盖率（FRESEAICE, sea ice fraction）数据则从GEOS-Chem模式所用的MERRA-2气象场中获取，MERRA-2数据的详细说明参见Molod等人2015年的研究。