Marine aerosol is a significant part of atmospheric aerosols, which has an important impact on the changes in marine meteorology, such as visibility and precipitation. Marine aerosols also play a vital role in Earth's energy budget, atmospheric environment, and climate change as they can directly scatter and absorb solar and Earth's radiance and indirectly modify cloud properties. In studies of the size distribution of marine aerosols, micron aerosols are the object of analysis in most cases. There are few studies based on the aerosol mode, and the research on submicron particles with a particle size of less than 1000 nm is scarcely reported. However, submicron particles in the atmosphere perform a crucial role in aerosol formation processes such as gas-to-particle conversion and the formation of cloud condensation nuclei. It is of great significance to study the temporal and spatial variation characteristics of the number concentration, particle size, and component distribution of submicron aerosols. This can help 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 aerosols.
A navigation observation lasting 12 days is conducted in the northern South China Sea by a Chinese research vessel "Shenkuo" from June 8 to June 20, 2019. The particle number concentrations of submicron aerosols with a diameter of 14-680 nm are measured by scanning mobility particle sizer (SMPS) placed on the right frontal side of the ship. The conventional meteorological data (temperature, humidity, and atmospheric pressure) on the sea surface mainly comes from the automatic meteorologic station onboard the ship. In addition to the data collected on site, the atmospheric reanalysis dataset (MERRA-2) provides data on the sea surface, such as 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. After correcting the discrete data and eliminating the polluted data caused by ship discharge, we analyze the temporal and spatial change in the number concentration and size distribution. The size distribution spectra are fitted on the basis of the nucleation mode, Aitken mode, and accumulation mode with the log-normal function. The influence of a cold front encountered during the voyage on the number concentration, particle size, and component distribution is discussed.
The meteorologic process of a cold front is found through the combination of the data from the shipboard automatic meteorologic station and the meteorological reanalysis dataset. When a cold front is encountered, the wind speed, specific humidity, and temperature all decrease significantly, and the wind direction changes from southwest to northeast (Fig. 5). Therefore, the aerosols before and after the cold front are divided into marine aerosols and continental aerosols polluted by the Taiwan Island. The differences in aerosol number concentration, particle size, and component distribution before and after the cold front are compared. It can be seen from the aerosol particle size distribution (Fig. 8) that the number concentrations of the contaminated continental aerosols (? and ? in Fig. 8) are higher than the marine background level, that is, the level of marine aerosols (? and ? in Fig. 8) before the cold front. On June 15 and June 16, the peak number concentration of aerosols appears in the nucleation mode, which means that there are more new particles in the aerosols at this time, and they are in a polluted state. It can be seen from the changes in aerosol components (Fig. 7) that except for the decrease in the proportion of sea salt (SS), the proportions of other components increase on June 15 and June 16, especially the sulfate component (SO4). The increase in the total number concentration and nucleation-mode number concentration of continent aerosols and the increase in the proportions of SO4 and other components may be due to the air mass, impacted by the Taiwan Island, carrying sulfate, organic carbon, and other components into the observation sea area.
Firstly, with the increase in offshore distance, the total number concentration of marine aerosols gradually decreases from the coastal level (6812 cm-3) to the background level (1745 cm-3). Compared with the situation of the offshore sea, the air in the far sea is cleaner, and the proportion of the nucleation-mode number concentration gradually decreases (2.35%), the proportion of the Aitken mode remains stable (52.70%), and the proportion of the accumulation mode increases (44.95%). Secondly, the fitted spectra show that 62.15% of the median size distributions are single-peak, and 36.27% are double-peak, which agrees with the log-normal distribution. The median size distributions show the double-peak mode along the coast with a peak value of about 200 cm-3, but display the single-peak mode with a total number concentration between 60 cm-3 and 100 cm-3 on the open sea. As the offshore distance grows, the average geometric particle size of the main modes of particle size spectra increases, and the peak number concentration decreases. Finally, the aerosol samples obtained before the cold front are only affected by the ocean, and the number concentration of marine aerosols is lower. The SS component is the main component (94.33%), and the particle size distribution presents a single-peak characteristic, with the peak appearing in the accumulation mode, which reflects the characteristics of the background marine aerosol. After the cold front transits, the aerosol is affected by the polluted air mass from the Taiwan Island. The SO4 proportion in continental aerosols is significantly increased (44.73%), and the particle size distribution presents a double-peak characteristic in the nucleation and accumulation modes.
