Optical properties of chain-like atmospheric aerosol particles

Using the generalized multiparticle Mie-solution method, this study examines the optical properties of chain-like particles under different atmospheric conditions and various arrangements. The structural composition of aerosols exhibits a more pronounced impact on their extinction and absorption cross sections when the incident wavelength is below 600 nm, whereas significant changes are observed in backscattering cross sections for incident wavelengths above 600 nm. As the orientation angle between the incident wave and particle chain increases, the extinction cross sections and absorption cross sections exhibit varying degrees of decline. Furthermore, marine atmospheric aerosol chains demonstrate similar extinction cross sections to those of polluted atmospheric aerosols, and their absorption cross sections closely resemble those of clean atmospheric aerosols. In addition, for a particle chain of fixed length, the greater the disparity in particle sizes within the chain, the larger the difference between the backscattering cross section and that of the chains with equal particle sizes. This research provides theoretical support for assessing the climate effects of aerosols and inverting aerosol properties by LiDAR data.

Overview of primary biological aerosol particles from a Chinese boreal forest: Insight into morphology, size, and mixing state at microscopic scale

Biological aerosols play an important role in atmospheric chemistry, clouds, climate, and public health. Here, we studied the morphology and composition of primary biological aerosol particles (PBAPs) collected in the Lesser Khingan Mountain boreal forest of China in summertime using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). C, N, O, P, K, and Si were detected in most of the PBAPs, and P represented a major marker to discriminate the PBAPs. Of all detected particles >100 nm in diameter, 13% by number were identified as PBAPs. We found that one type of PBAPs mostly appeared as similar rod-like shapes with an aspect ratio > 1.5. Size distribution of the rod-like PBAPs displays two typical peaks at 1.4 μm and 3.5 μm, which likely are bacteria and fungal particles. The second most PBAPs were identified as fungal spores with ovoid, sub-globular or elongated shapes with a smooth surface and small protuberances with their dominant size range of 2-5 μm. Moreover, we found some large brochosomal clusters containing hundreds of brochosomes with a size range of 200-700 nm and a shape like a truncated icosahedron. We estimated that mass concentration of PBAPs approximately 1.9 μg m^-3 and contributed 47% of the in situ PM_2.5-10 mass. The detection frequency and concentration of PBAPs were higher at night than in the daytime, suggesting that the relative humidity dramatically enhanced the PBAPs emissions in the boreal forest. Our study also showed that the fresh PBAPs displayed weak hygroscopicity with a growth factor of ~1.09 at RH = 94%. TEM analysis revealed that about 20% of the rod-like PBAPs were internally mixed with metal, mineral dust, and inorganic salts in the boreal forest air. This work for the first time provides the overview of individual PBAPs from nanoscale to microscale in Chinese boreal forest air.

Observation of aerosol number size distribution and new particle formation at a mountainous site in Southeast China

To quantify the physical/chemical properties, and the formation and growth processes of aerosol particles on mountainous regions in Southeast China, an intensive field campaign was conducted from April to July 2008 on the top of Mt. Huang (1840m above mean sea level). The average particle number concentration was 2.35×10³cm^-3, and the ultrafine particles (<0.1μm) represented 70.5% of the total particle number concentration. Excluding the accumulation mode particles, the average daytime particle number concentrations were prominently higher than those measured at nighttime, suggesting there was a diurnal pattern of changes between planetary boundary layer and free troposphere air. The aerosol spectra were classified into two categories: the first category (FCS) exhibited a clear diurnal cycle, with relatively higher number concentration (3.19×10³cm^-3), smaller sizes and air masses from the inland; the second category (SCS) presented less obvious diurnal cycle, with lower number concentration (1.88×10³cm^-3), larger sizes and air masses from coastal regions. Air mass sources, weather conditions, and new particle formation (NPF) events were responsible for the differences of these two particle spectra. Six NPF events were identified, which usually began at 10:00-11:00 LT, with the estimated formation rate J₁₀ in the range of 0.09-0.30cm^-3s^-1 and the growth rate at 1.42-4.53nmh^-1. Wind speed, sulfur dioxide and ozone concentrations were higher on NPF days than those on non-NPF days, whereas temperature, relative humidity, concentrations of nitrogen oxide and carbonic oxide were lower on NPF days. Sulfur dioxide and ozone might be main potentially precursor gases for those NPF events. The NPF events at Mt. Huang corresponded closely to a southwest winds. These results are useful for improving our understanding of the main factors controlling the variation of aerosol size distribution and NPF events in this region.

Internally mixed sea salt, soot, and sulfates at Macao, a coastal city in South China

Direct observation of the mixing state of aerosol particles in a coastal urban city is critical to understand atmospheric processing and hygroscopic growth in humid air. Morphology, composition, and mixing state of individual aerosol particles from Macao, located south of the Pearl River Delta (PRD) and 100 km west of Hong Kong, were investigated using scanning electron microscopy (SEM) and transmission electron microscopy coupled with energy-dispersive X-ray spectrometry (TEM/EDX). SEM images show that soot and roughly spherical particles are prevalent in the samples. Based on the compositions of individual aerosol particles, aerosol particles with roughly spherical shape are classified into coarse Na-rich and fine S-rich particles. TEM/EDX indicates that each Na-rich particle consists of a Na-S core and NaNO3 shell. Even in the absence of heavy pollution, the marine sea salt particles were completely depleted in chloride, and Na-related sulfates and nitrates were enriched in Macao air. The reason could be that SO2 from the polluted PRD and ships in the South China Sea and NO2 from vehicles in the city sped up the chlorine depletion in sea salt through heterogeneous reactions. Fresh soot particles from vehicular emissions mainly occur near curbside. However, there are many aged soot particles in the sampling site surrounded by main roads 200 to 400 m away, suggesting that the fresh soot likely underwent a quick aging. Overall, secondary nitrates and sulfates internally mixed with soot and sea salt particles can totally change their surface hygroscopicity in coastal cities.