A survey was conducted in the summer monsoon transition region of China. By combining data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSOs) and Moderate Resolution Imaging Spectroradiometer (MODIS), the effects of East Asian summer monsoon circulation on the spatial distribution of aerosol, as well as the response mechanisms of different aerosols in an abundant and a deficient summer monsoon year, were analyzed. It was found that, in the summer monsoon transition region, the aerosol optical depth (AOD) in abundant monsoon years was lower than that in deficient years. Only in the Gobi Desert region of the Loess Plateau, the AOD in abundant monsoon years was significantly larger than that in deficient years. When the AOD was less than 0.06, the frequency of dust aerosol was higher than that of polluted aerosol in both the abundant and deficient monsoon years. When the AOD was over 0.06, the frequency of polluted aerosol was higher than that of dust aerosol in both the abundant and deficient monsoon years. In summer, the AOD was larger and the frequency of polluted aerosol in abundant monsoon years was higher than that in deficient years.
Aerosol is a general term used to describe solid and liquid particles suspended in the atmosphere. Atmospheric aerosols are the primary atmospheric pollutants affecting human health. Atmospheric aerosols could also affect the radiative equilibrium and cloud microphysical process of the earth-atmosphere system through direct [
In China, the aerosol concentration is higher compared to other regions of the world. In addition, the East Asia is in the monsoon region; thus, the climate characteristics, including temperature, precipitation, and atmospheric circulation, display obvious seasonal changes. The variation over different time scales has a direct impact on regional precipitation and temperature variations. East Asia is a critical area for studies of global climate change. The transportation of atmospheric materials such as water vapor and droplets is also directly affected by the monsoon circulation. Thus, the processes involved in the generation, emission, and transportation of aerosol are also influenced by the atmospheric circulation factor. The temporal variations of aerosol concentrations are significantly impacted by changes in the monsoon circulation.
In recent years, many studies have investigated the interaction between atmospheric aerosols and the Asian monsoon [
Many previous studies considered the effects of aerosols on the intensity and precipitation of the East Asian summer monsoon, but there have been few reports on the influence of the East Asian summer monsoon on atmospheric aerosols. Especially, few people regarded the response mechanisms of different aerosols to abundant and deficient monsoon years and the distribution characteristics of the summer monsoon at different propulsive phases in East Asia. This is also a very prominent scientific problem. It is necessary to conduct thorough systematic research to resolve the issue. In this study, using satellite and reanalysis data, the effects of the East Asian summer monsoon circulation on the spatial distribution of atmospheric aerosol and the response mechanisms of different aerosols to abundant and deficient summer monsoon years in East Asia were studied.
Generally, the effects of the summer monsoon gradually weaken from southeast to northwest in China, and it will transition to the influence area of westerlies. The transition zone is the line representing the edge of summer monsoon activity, and the change of the line determines the drought and flood conditions in China. In the past, various different definitions have been proposed for the edge of the summer monsoon. Based on these previous definitions, Ran and Qian et al. [
Distribution of the summer monsoon border zone in China (the area marked by the oblique line represents the summer monsoon transition zone).
However, it is concerned that the northern edge of the summer monsoon is not fixed, and there is always an obvious interannual and interdecadal swing [
Under the conditions of any landform, a bright surface, thin clouds, and clear sky, the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) was carried by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSOs) satellite, which was launched successfully on April 28th, 2006. It can observe the vertical distribution of aerosol and monitor the vertical features of dust in the transfer process. However, the Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR) cannot provide vertical data. CALIPSO makes up for these shortcomings and provides the vertical distribution of aerosol in different scales. Pan et al. [
The datasets used in this study included the following products: (i) the grid data of the three-level daily data product MOD08, with a 1° × 1° horizontal resolution; (ii) the NCEP daily data of U-wind, with a 2.5° × 2.5° horizontal resolution and from 1000 to 100 hpa; and (iii) a daily dataset of the aerosol layer at level 2 from the CALIPSO. All datasets covered a total period of more than one year (from January 2008 to December 2017).
The backscattering data at different heights, a series of characteristic Lidar signals, the depolarization ratio of volume, backscattering coefficient at 532 nm, type of area, and lift phenomenon were derived from the CALIPSO dataset. The type of aerosol in different heights could be differentiated. Six types of aerosol were determined: clean marine, dust, polluted continental, clean continental, polluted dust, and smoke. The classification rules are shown in Table
Level 2 dataset aerosol classification.
Mark | Aerosol |
|
|
Area | Lift phenomenon |
---|---|---|---|---|---|
1 | Clean marine | <0.075 | <0.0015 | Ocean | No |
2 | Dust | >0.20 | No | ||
3 | Polluted continental | <0.075 | >0.0005 | Land | No |
4 | Clean continental | <0.075 | <0.0005 | Land | No |
5 | Polluted dust | 0.075–0.20 | Land | No | |
6 | Smoke | <0.075 | >0.0005 | Land/ocean | Yes |
The East Asian summer monsoon (EASM) and its variability involve circulation systems in both the tropics and midlatitudes, as well as in both the lower and upper troposphere. Considering this fact, a new EASM index (NEWI) has been proposed based on 200 hPa zonal wind [
New East Asian summer monsoon index (NEWI) of 2008–2016.
In this study, the effects of summer monsoon circulation on the distribution of atmospheric aerosols were investigated in abundant and deficient monsoon years. The summer monsoon transition zone was compared with other regions. The frequency of dust and polluted aerosol with different aerosol optical depths (AODs) and altitude were determined.
Figure
Differences in aerosol optical depth (AOD) between the abundant and deficient EASM years (deficient minus abundant) for (a) June, (b) July, (c) August, and (d) summer.
Throughout the summer, the AOD in abundant monsoon years was lower than that in deficient monsoon years. Only in the Gobi Desert region of the Loess Plateau, the AOD was significantly larger in abundant years than that in deficient years. These characteristics indicated that the spatial distribution of the AOD varied significantly at different stages of monsoon development and under different monsoon intensity backgrounds.
Some previous studies had investigated the frequency of dust aerosol in East Asia [
As shown in Figure
Comparison of the frequency of dust and polluted aerosol in abundant and deficient monsoon years (“blue solid line” is the dust aerosol of abundant monsoon years; “red solid line” is the dust aerosol of deficient monsoon years; “blue dotted line ” is the polluted aerosol of abundant monsoon years; “red dotted line” is the polluted aerosol of deficient monsoon years).
As shown in Figure
Comparison of the frequency of dust and polluted aerosol in summer in the abundant and deficient monsoon years (“blue solid line” is the dust aerosol of abundant monsoon years; “red solid line” is the dust aerosol of deficient monsoon years; “blue dotted line” is the polluted aerosol of abundant monsoon years; “red dotted line” is the polluted aerosol of deficient monsoon years).
Variation in the summer monsoon can change the transmission, sedimentation, concentration, distribution, and chemical reactivity of atmospheric aerosols. In this study, the effects of the East Asian summer monsoon circulation on the distribution of atmospheric aerosols, and the response mechanisms of different aerosols in abundant and deficient summer monsoon years were investigated. The following conclusions were obtained: The distribution of AOD in the transition zone varied in the different stages of the monsoon. At the beginning of the summer monsoon, the distribution of AOD in the southwest-northeast direction was positive-negative-positive. Only in the Loess Plateau region, the AOD was larger in abundant years than that in deficient years. In July, the summer monsoon developed to the mature stage, and there were obvious negative anomalies in the northeast and southwest region of the transition zone, indicating that the AOD in abundant years was larger than that in deficient years. The southwest-northeast direction displayed the “negative-positive-negative” distribution. In summer, the AOD in abundant monsoon years was lower than that in deficient monsoon years. Only in the Gobi Desert region of the Loess Plateau, the AOD was significantly larger in abundant years than that in deficient years. When the AOD was less than 0.06, the frequency of dust aerosol was higher than that of polluted aerosol in both the abundant and deficient years. Moreover, the frequency of dust aerosol and polluted aerosol in the deficient years were higher than that in the abundant years. When the AOD was over 0.06, the frequency of polluted aerosol was higher in both the abundant and deficient monsoon years. In summer, the AOD of polluted aerosol was larger and the frequency was higher in the abundant years than that in deficient years.
The summer monsoon transition zone investigated in this study is very special. In this region, the temperature is low, and is heavily influenced by the winter monsoon. This has an impact on atmospheric pollution in winter, especially the haze that occurs in China. Our next work focuses on the effects of winter monsoon intensity on the distribution of aerosol and the frequency of different aerosols.
The CALIPSO and MODIS data used to support the findings of this study are available from the corresponding author upon request. The MODIS dataset used in this study is available from the Atmosphere Archive and Distribution System (LAADS) of the Distributed Active Archive Center (DAAC) (
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was jointly supported by the Natural Science Foundation of China (41630426) and the National Basic Research Program of China (2013CB430200).