1. Introduction
The complexity of aerosol sources (primary and aged aerosol) determines the diversity of the composition types and particle sizes, and it has important effects on the weather, climate, air quality, and human health. The direct and indirect effects of aerosols are some of the most poorly–understood variables affecting atmospheric and water circulation. It is necessary to combine aerosol measurements and radiation techniques with model simulations to accurately determine the aerosol effect [
1,
2,
3,
4]. Model simulation uses atmospheric radiation transfer models, such as the Santa Barbara DISORT(Discrete Ordinates Radiative Transfer) Atmospheric Radiative Transfer (SBDART), Global Atmospheric Model (GAME), Moderate Resolution Atmospheric Transmission (MODTRAN), and RSTAR. To estimate the accuracy of the models, the modeled global, diffuse, and Sun’s direct radiation at ground level have been compared with experimental measurements. The Sun–sky radiometric measurements at ground level were also needed to characterize the aerosol properties. The CE–318 sunphotometer is used internationally. Systems such as Prede POM can be used to retrieve the optical properties of aerosols from solar and sky radiance measurements [
5]. Aerosols with different particle sizes have different effects on the environment, transport, and human health [
6,
7]. However, due to the complex composition of aerosols and the uncertainty of their spatiotemporal distribution, the physicochemical and optical properties of aerosols vary [
8], which makes environmental research and assessing the radiation effects of climate change challenging [
9,
10,
11,
12,
13]. Therefore, quantitative remote sensing analysis of aerosol properties in different geographical locations has important theoretical and practical implications.
At present, a number of international and regional aerosol ground–based observation networks have been established. For example, the Aerosol Robotic Network (AERONET), established by the National Aeronautics and Space Administration (NASA), provides the most extensive aerosol database in the world [
14]. The European Skynet Radiometer Network (ESR) is a new type of network in partnership with SKY–Radiometer NETwork–SKYNET (Skynet–Asia) in Japan [
15,
16]. There is also the Global Atmosphere Watch Precision Filter Radiometer Network (GAWPFR NET) [
17]. The Chinese Sun Hazemeter Network (CSHNET), the China Aerosol Remote Sensing Network (CARSNET) [
18], and the Campaign for the Atmospheric Aerosol Research Network of China (CARE–China) were established in China [
19,
20]. ground–based network observation can directly provide basic data for research; it can also provide a reference for the satellite detection data and numerical simulation results and provide an observational basis for the impact of environmental, weather and climate change [
14,
21,
22,
23,
24,
25].
The sunphotometer is one of the most widely used instruments for ground–based passive telemetry to accurately characterize aerosol properties, and its retrieval algorithm is an important component. Dubovik et al. evaluated the physical quantities, errors, and information from different sources. They gave different weights to the data in the retrieval process and applied advanced numerical optimization techniques to obtain the final statistical optimal solution. Through this algorithm, they obtained parameters such as the aerosol particle volume size distribution, complex refractive index, and single scattering albedo by using ground–based observation data [
26]. Olmo et al. applied the Nakajima algorithm to incorporate the randomly distributed ellipsoid approximation to retrieve the aerosol parameters [
27]. The ESR.pack retrieval scheme proposed by Estellés et al. [
28] was based on the Nakajima algorithm and Skyrad.pack to improve and compile the applications of the CE–318, POM, and various sunphotometers. He et al. [
29] compared the retrieval results of the aerosol properties generated by Nakajima et al. [
30] and Dubovik et al. [
26]. Huang et al. used the sun direct radiation data to retrieve the aerosol optical depth (AOD) and Angstrom exponent as well as the single scattering albedo (SSA) and scattering phase functions from the sky scattering data [
31]. At present, the internationally–used, comprehensive aerosol retrieval results come from AERONET (for business retrieval) [
26] and the ESR.pack retrieval scheme that covers ESR and Skynet–Asia [
28]. Estellés et al. performed a comparison of the aerosol properties derived by the ESR.pack with AERONET and obtained good results [
32].
The Yunnan–Kweichow Plateau (100–111°E, 22–30°N) is located in the southeastern part of the Qinghai–Tibet Plateau in southwestern China and is a typical low–latitude plateau monsoon climate zone. Its geographical location (it is adjacent to Southeast Asia and South Asia, and it is an important source of aerosols in southwest China due to the strong influence of the monsoon circulation) [
33,
34] and climate (the annual temperature difference is small and the daily range is large, the dry season and wet season are distinct, and the sunshine and ultraviolet radiation are strong) are unique. It is also an important path for water vapor transport of convection and advection in China. Research shows that aerosols have important effects on global and regional water vapor variation [
35,
36] and influence the regional climate and environment. However, there are few ground–based stations in the Yunnan–Kweichow Plateau, and there is a lack of systematic research on aerosols. Therefore, this paper explores the relationship between the seasonal and interannual variation characteristics of the East Asian monsoon and South Asian monsoon and the variation of the aerosol optical radiation properties, which were based on the combination of the aerosol properties retrieved from the CE–318 observation data from the Kunming Atmospheric Ozone Monitoring Station, No. 209 of the Global Ozone Observing System (GO3OS), and the monsoon circulation index. It is important to know and understand the aerosol variation and monsoon activities in the Yunnan–Kweichow Plateau and their impact on the environment and climate in specific areas.
6. Analysis of the Causes of the Seasonal Variation
Kunming is located in the central part of the Yunnan–Kweichow Plateau and is affected by the East Asian monsoon and the South Asian monsoon. The annual sunshine is about 2200 h, and the ultraviolet radiation is strong. The monthly mean temperature is between 9.1 °C and 20.7 °C, and the monthly precipitation is between 11.3 mm and 204.0 mm. The precipitation in the rainy season from May to October accounts for 85% of the whole year and for more than 60% in summer. The southeast’s warm and humid airflow from the western Pacific and the southwest’s warm and humid airflow from the Indian Ocean meet over the Yunnan–Kweichow Plateau (which contains Yunnan), and a variety of aerosol species combine with water vapor to generate aerosol particles, which are transported over long distances (Southeast Asia–South Asia and the Iranian plateau to North Africa) [
33,
61], or the aerosol particles are hygroscopically grown. Under the conditions of a high temperature, high humidity, and strong ultraviolet radiation, chemical and photochemical reactions are favored to generate new aged aerosol particles. Therefore, due to the increase in the type and quantity of aerosol particles in summer (which makes the AOD reach its maximum), there is an increase in the volume concentration of the aerosol particles and the concentration of the coarse and fine particles. Diversification of the particle size (the Angstrom Exponent (
α) is higher than the annual mean value) affects the degree of the atmospheric turbidity (the
β coefficient reaches the largest value). Because the physical, chemical, and optical properties (absorption and scattering cross section) of the aerosol changed greatly, the extinction effect was enhanced. Moreover, the increase of the water vapor (the CWV reaches the maximum value) enhances the extinction of the solar radiation absorption and scattering, which leads to the enhancement of the secondary or multiple scattering of the aerosol particles and promotes the variation of the aerosol properties.
Precipitation in the dry season from November to April of the following year accounts for only 15% of the whole year. There is little rain and plenty of sunshine in winter. It is mainly influenced by the west wind circulation and cold air from the higher latitude continent. The source and the physicochemical properties of the aerosol are different from those in the summer, which leads to the characteristic parameters of the AOD, CWV, and β coefficient change with the seasons and the AOD, CWV, and β coefficient reach the minimum values in winter.
In the spring, there are few clouds and little rain, the air is dry, the evapotranspiration is strong, the solar ultraviolet radiation is enhanced, and the diurnal range of the temperature is large. The temperature drops rapidly in the autumn, which is about 2 °C lower than that in the spring, the precipitation is reduced and less than 30% of summer (but more than that in spring and winter), and the air is dry. The values of the aerosol properties in spring and autumn are between those of summer and winter, but they are slightly different in the two seasons. Comparing the spring and autumn, the aerosol properties are higher (larger) to different degrees. The main reason is that the wind speed in the near–surface layer in spring is much larger than that in the autumn, so that the flying dust and floating dust generated by the exposed surface can enter the atmosphere, which makes the amount of aerosol particles increase slightly. The increase of the desert dust aerosol in spring makes the average AOD higher than that in autumn, and the particle radius changes obviously make the turbidity stronger. Due to the influence of the summer monsoon circulation, the CWV values in spring and autumn are quite different (there is no obvious difference and regularity). The wet season starts at the end of spring and ends in autumn, but the variations of the CWV in the two seasons are slightly different. A more specific difference analysis is needed to combine the characteristics of the actual monsoon circulation evolution with different years, characteristics of water vapor transport, and variation in meteorological factors.
Figure 13 shows the characteristics of the seasonal variation and inter–annual activities of the EAMI and SAMI and aerosol properties normalized time series from March 2012 to February 2014. EAMI and SAMI are consistent with the variation in the aerosol properties, and the CWV and CWV (AIRS) are exactly the same as the seasonal variation of the monsoon index. The positive (negative) phase of the summer (winter) oscillation variation is significant, while those of spring and autumn are relatively weak, reflecting the difference in the aerosol type and source and the optical–radiation properties in different seasons. Except for the influence of other factors, it is mainly affected by the variation in the monsoon circulation activities (
Figure A1 and
Figure A3). The interannual variation difference of the monsoon circulation can also be reflected in the variation of the aerosol properties. The positive phase of the EAMI and SAMI variations in the summer of 2012 is much longer than that in 2013, resulting in the difference of the variation of the aerosol properties in the summer during those two years.
The linear fitting regression analysis of the aerosol properties and the monsoon index normalized sequence shows that y (the response of the aerosol properties to the monsoon circulation) = kx (the variation of the monsoon circulation). Only the influence of the monsoon circulation variation is considered here. The physical meaning of the slope k is the sensitivity of the aerosol properties to the monsoon circulation variation, while kx×% characterizes the relative influence rate of the monsoon circulation on the variation of the aerosol properties.
Figure 14 and
Figure 15 show the correlation analysis of the normalized sequence between EAMI and SAMI and aerosol properties containing CWV (AIRS CWV). The CWV and CWV (AIRS) have the best linear relationship with the monsoon index. The relationship not only reflects the transport of the water vapor from the atmospheric circulation but also verifies the reliability of the aerosol properties retrieval. The linear relationship between the AOD at 8 bands, SSA at 440 nm, CWV (AIRS CWV), EAMI, and SAMI all pass the significance test of more than 99%, but the linear relationship between the Angstrom Exponent (
α) and the monsoon index is relatively insignificant.
Table 2 shows the sensitivity
kE of the aerosol properties to the EAMI variation and the influence rate (%) of the variation of the East Asian monsoon circulation on the relative variation of the aerosol properties.
During the dry season, from November to March (or April) of the following year, the aerosol characteristic parameter value decreases with the weakening of the East Asian monsoon circulation (negative phase) and reaches the minimum value in January. During the spring/summer transition from March (or April) to May, the positive phase of the East Asian monsoon circulation variation gradually increases. At this time, the variation of the aerosol characteristic parameter value under the influence of the monsoon circulation starts to change from decreasing to increasing. During the rainy season from May to October, the values of the aerosol properties increase rapidly in the period of the East Asian monsoon circulation enhancement (positive phase). It should be noted that the aerosol properties reached a maximum value in August 2012. However, the maximum value not only was delayed by two months but was relatively small. It shows that the interannual activity of the East Asian monsoon circulation is significantly different. The negative phase of the East Asian monsoon circulation variation gradually increases during the autumn–winter transition from October to November. At this time, the variation of the aerosol characteristic parameter value begins to change from increasing to decreasing due to the variation of the monsoon circulation. The transformation of the positive and negative (negative and positive) phases of the monsoon circulation affects the seasonal variation characteristics of the aerosol properties. However, the activity intensity of the monsoon circulation in different years and the difference in the transition period have different effects on different aerosol properties. Except for the CWV (AIRS CWV), the effect of the SSA on the 440 nm band is the most significant, and the effect on the AOD at different wavelengths is more consistent.
Table 3 shows the sensitivity
kS of the aerosol properties to the SAMI variation and influence rate (%) of the South Asian monsoon circulation on the relative variation of the aerosol properties. Compared with the East Asian monsoon circulation, the South Asian monsoon circulation weakens 1 month earlier. From October to April of the following year, the South Asian monsoon circulation weakens, and the aerosol characteristic parameter values decrease accordingly, also reaching a minimum value in January. During the transition period from April to May, the monsoon circulation has little effect on the variation of the aerosol properties due to the aerosol characteristic parameter values’ change from decreasing to increasing.
From May to September, the South Asian monsoon circulation increases, and the aerosol characteristic parameter values continued to increase, reaching a maximum value in August. During the transition period from September to October, the South Asian monsoon circulation begins to weaken, which changes the aerosol characteristic parameter value from increasing to decreasing and the impact of the variation of the monsoon circulation on the aerosol characteristic parameter values is also small.
By combining
Table 2 and
Table 3, it is clear that the effects of the East Asian monsoon and the South Asian monsoon on the variation in aerosol properties are similar. Although there is a seasonal difference, the synergy between the East Asian monsoon and the South Asian monsoon is obvious, and the variation in the aerosol properties is consistent with their seasonal variations. The sensitivity of the CWV and CWV (AIRS) to the EAMI and SAMI variations is 0.83–0.90 and 0.93–0.95, respectively. The CWV (AIRS CWV) is more significantly affected by the South Asian monsoon than the East Asian monsoon, which indicates the contribution of the water vapor from the atmospheric circulation. The sensitivity of the SSA in the 440 nm band to the EAMI and SAMI is 0.72 and 0.77, while the sensitivity of the AOD for different
λ to the EAMI and SAMI ranges from 0.58 to 0.66 (mean value is 0.62) and 0.48 to 0.69 (mean value is 0.56), which indicates that the extinction of the aerosol particles is deeply affected by the variation of the monsoon circulation activity. It is also noted that the sensitivity of the AOD to the EAMI is greater than that of the SAMI, and the influence of the inter–monthly variation of the EAMI on the aerosol properties is more drastic than that of the SAMI. This may be due to the complexity of the East Asian monsoon and the South Asian monsoon in the process of converging over the Yunnan–Kweichow Plateau, which needs to be studied in detail.
In summary, aerosol properties that vary with the seasons are affected and controlled by the variation in the monsoon circulation activity. For example, the AOD is closely related to the seasonal variation of the CWV. The increase of the water vapor content can affect the variation of the aerosol particle size and quantity, and the physical and chemical reactions of aerosol particles, which promotes the increase of the AOD caused by the extinction of the aerosol. In 2012 and 2013, the variation difference of the monsoon circulation can affect and control the difference in the aerosol sources and types, and the variation in the intensity of the seasonal and interannual activity may also change the number of aerosol particles and the formation difference of the aged aerosol. However, since only two years of data were used, it is necessary to verify whether it is credible through the evaluation of longer–term data. In addition, it should be noted that except for the influence of the monsoon circulation, there is an impact from the increased local emissions of fugitive dust on the aerosol properties. For example, the differences between the interannual and monthly variation in the aerosol properties in Kunming in 2012 and 2013 are not only related to the interannual differences in monsoon circulation but also related to the increase in the local emissions.
7. Conclusions
The relationship between the influence of the East Asian and South Asian monsoon index on the seasonal variation of the aerosol properties is discussed below based on the retrieval of the aerosol optical and radiation properties from the observational data of the CE–318 sunphotometer at the Kunming atmospheric ozone monitoring station. The conclusions are as follows.
The AOD decreases with the increase of λ and is consistent with the typical Mie scattering properties. The seasonal variation of the AOD, Angstrom turbidity coefficient β, and CWV is unimodal, the summer (winter) is the largest (smallest), and the spring is greater than the autumn. The CWV values in spring and autumn showed no significant difference, and the seasonal variation of the Angstrom Exponent (α) was not obvious. The turbidity in the atmosphere was relatively low. This indicates that the aerosol properties of the low–latitude plateaus in southwestern China are different from those in other parts of China. Although they are affected by anthropogenic emissions from the Southeast Asia–South Asia region and biomass burning aerosols, the correlation between the intensity of the seasonal activities of the monsoon circulation and the local anthropogenic emissions and meteorological factors will affect the aerosol properties, so further in–depth simulation studies are needed.
The seasonal and annual variations of the aerosol particle size distributions are bimodal. The complex refractive index and single scattering albedo showed that the summer aerosol particles had a stronger scattering power and a relatively weak absorption power; in winter, they exhibited the opposite. In summer, the water vapor content was high, and after the hygroscopic aerosol particles absorbed water, the particle size and volume increased, resulting in enhanced scattering. In order to explain the enhancement of the aerosol absorption capacity in winter, the retrieval result for the FI needs to be further discussed.
The variation of the aerosol properties with the season was significantly affected by the monsoon circulation activity, and the synergistic effect of the East Asian monsoon and South Asian monsoon made the variation of the aerosol properties have the same seasonal oscillation characteristics as that of the monsoon. In the transition period of the season (especially during the transition periods of the dry and wet seasons in Kunming), the monsoon circulation had little effect on the variation of the aerosol properties. The AOD was closely related to the seasonal variation of the CWV. The increase or decrease of the water vapor content did affect the variation of the aerosol particle size and quantity, thus affecting the physical and chemical reactions of the aerosol particles. Furthermore, the extinction of the aerosol was promoted to cause an increase in the AOD. Only in 2012 and 2013 did the variation difference of the monsoon circulation affect the source and type of aerosol. The variation in the intensity of the seasonal and interannual activity may have also changed the number of aerosol particles and the formation difference of the aged aerosol. It should also be noted that, in addition to the influence of the monsoon circulation, there was an impact from the increased local emissions of fugitive dust on the aerosol properties.