基于船载测量亚微米海洋气溶胶粒径分布特征分析

Size Distributions Analysis of Submicron Marine Aerosol Based on Shipboard Measurement

基于南海北部海域开展的走航观测,测量了颗粒直径为14~680 nm的气溶胶数浓度,分析了气溶胶数浓度时空分布、粒径分布特征。以核模态、Aitken核模态和积聚模态为基础,统计了气溶胶粒径谱谱型并对其分布模式进行了对数正态拟合。同时,讨论了航程中遭遇的一次冷锋过程对气溶胶数浓度、粒径和组分分布的影响。结果表明:沿海海域气溶胶总数浓度超过6800 cm^(-3),远海海域气溶胶总数浓度约为1700 cm^(-3);海洋气溶胶中位数粒径谱符合对数正态分布,近海气溶胶多为双峰型,峰值数浓度在200 cm^(-3)左右,而远海气溶胶多为单峰型,峰值数浓度在60~100 cm^(-3)之间;冷锋过程前的气溶胶样品属于海源气溶胶,呈现海洋本底气溶胶特征,而冷锋过程后的气溶胶样品为受台湾岛污染影响的陆源气溶胶。

Marine aerosol is a significant part of atmospheric aerosol,which has an important impact on the changes of marine meteorology,such as visibility and precipitation.Marine aerosols also play a vital role in Earth's energy budget,atmospheric environment,and climate change due to their directly scattering and absorbing solar and Earth's radiance and indirect effects that modify cloud properties.For marine aerosol size distribution,micron aerosol is the object of analysis in most cases.There are few studies based on aerosol mode,and the research on submicron particles with particle size less than 1000nm is less reported.However,submicron particles in the atmosphere perform a crucial role in aerosol formation processes such as gas-to-particle conversion and cloud condensation nodule formation.It is meaningful to study the temporal and spatial variation characteristics of the number concentration,particle size and composition distribution of submicron aerosols,which is beneficial to grasp the evolution of particle size distribution in the air mass from land to sea and improve the understanding of the formation and evolution process of marine aerosol.A navigation observation lasting 12 days was conducted in the northern South China Sea by a Chinese research vessel“Shenkuo”from June 8 to June 20,2019.Particle number concentrations of submicron aerosols from 14nm to 680nm diameter were measured with SMPS placed at the right frontal side of the ship.The conventional meteorological data(temperature,humidity,atmospheric pressure)on the sea surface mainly comes from the automatic weather station onboard the ship.In addition to the data collected on site,the atmospheric reanalysis dataset(MERRA-2)provides data on sea surface wind speed,wind direction and aerosol composition.In this paper,the HYSPLIT model is used to simulate the trajectory of the continental air mass during a cold front process.After correcting the discrete data and eliminating the pollution data caused by ship discharge,the temporal and spatial change of number concentration and size distribution was analyzed.The size distribution spectra were fitted based on nucleation mode,Aitken mode and accumulation mode with the lognormal function.The influence of a cold front encountered during the voyage on number concentration,size and component distribution was discussed.The cold front weather process was found by combining the data of the shipboard automatic weather station and the meteorological reanalysis dataset.After encountering the cold front,the wind speed,specific humidity,and temperature all decreased significantly,and the wind direction changed from southwest to northeast(Fig.5).Therefore,the aerosols before and after the cold front are divided into marine aerosols and continent aerosols polluted by Taiwan Island.By comparing the differences of aerosol number concentration,size,and component distribution before and after the cold front,it can be seen from the aerosol particle size distribution(Fig.8)that the number concentration of the contaminated land-based aerosol (c, d) is higher than the ocean backgroundlevel, that is, the marine aerosol (a, b) before the cold front process. On June 15 and 16, the peakconcentration of the aerosol appeared in the nuclear mode, which means that there are more newparticles in the aerosol at this time and it is in a polluted state. It can be seen from the changes ofaerosol components (Fig. 7) that except for the decrease in the proportion of sea salt component(SS), the proportion of other components increased on June 15 and 16, especially the sulfatecomponent (SO4) increased significantly. The increase in the total concentration and nuclear modenumber concentration of continent aerosols and the increase in the proportion of sulfate and othercomponents may be due to the air mass impacted by Taiwan Island carrying sulfate, organic carbon,and other components into the observation sea area.The conclusions are as follows: (a)with the increase of offshore distance, the totalconcentration of marine aerosol gradually decreases from the coastal level (6812 cm^(-3)) to thebackground level (1745 cm^(-3)). Compared with the offshore sea, the air in the far sea is cleaner, theproportion of nuclear mode number and concentration gradually decreases (2.35%), the proportionof Egan nuclear mode remains basically stable (52.70%), and the proportion of accumulation modeincreases (44.95%). (b) 62.15% of the median size distributions are single peak and 36.27% aredouble peaks, which agree with the log-normal mode. The median size distributions show doublepeak mode along the offshore with peak value about 200 cm^(-3), but display single peak mode withtotal number concentrations between 60 cm^(-3) and 100 cm^(-3) on the open sea. With the increase ofoffshore distance, the average geometric particle size of the main modes of particle size spectrumincreases and the peak concentration decreases. (c)The aerosol samples obtained before the coldfront process are only affected by the ocean, and the number concentration of marine aerosol islower. The sea salt component is the main component (94.33%), and the particle size distributionpresents a single peak characteristic with the peak appearing in the accumulation mode, whichreflects the characteristics of the background marine aerosol. After the cold front transits, the aerosolis affected by the polluted air mass from Taiwan Island. The sulfate component proportion ofcontinent aerosol is significantly increased (44.73%), and the particle size distribution presents adouble peak characteristic in the nuclear and accumulation mode.