Spatial and Temporal Variations of Aerosol Optical Depth and Influence Factors over the East China Sea

Atmospheric aerosols are solid and liquid particles suspended in the air, which has an important effect in macroscopic climate change. The aerosol optical depth (i.e, AOD) is one of the main factors to characterize the composition and the content of the aerosol. This study was conducted over the area of East China Sea (24oN-41oN, 117oE-128oE) based on the monthly average AOD data and monthly average wind data of 10 meter over sea surface during 2003-2013. The AOD distribution has the annual periodic variation, the AOD values increased significantly from January to March, and reached the maximum 0.133 in March, and the AOD values declined from April to August, and they stabilized around 0.1 in September to December. The trend of seasonal changes indicated that the AOD values were the highest in the spring, and slightly higher in the winter. The AOD values of spring and winter were significantly higher than the summer and the autumn. For time-series variation, the maximum of AOD values during certain periods gradually increase from 2003 to 2009, and the maximum of AOD values during certain periods gradually decrease from 2009 to 2013. Analyzing the relationship between spatial distribution of AOD and wind vectors over the sea surface, the spatial and temporal distribution and variation of AOD were closely related to the wind direction and speed over the East China Sea.


Introduction
Atmospheric aerosols are solid and liquid particles suspended in the air. With the rapid development of industry, the combustion of fossil fuel, the emissions of industrial fumes and contaminated gas led to the significant increase of atmospheric aerosol, which not only affects the macroscopic climate, but also reduces the comfort of the living environment and threat to human health. The indirect effect of aerosols on climate occurs by modifying the optical properties ( . The most common parameter derived from satellite observations is the Aerosol Optical Depth (AOD), which is a measure of the extinction of electromagnetic radiation at a given wavelength due to the presence of aerosols in an atmospheric column. AOD is the key factor to calculate the content of the aerosol, evaluating the conditions of the atmosphere and the effect of atmosphere aerosol on climate. It is important for the research of haze and atmospheric correction.
In recent decades, with the development of science and technology, the theoretical knowledge of aerosol science are improved, the research on AOD has been focused on. Since the development of aerosol remote sensing in mid-1970, many AOD retrieval algorithms has been developed (Glantz P., et al. 2009 double-multi-channel reflectance methods (Sundström A.M., et al. 2012), in which high accuracy of AOD inversion results can be obtained from the multi-channel reflectance method. However, the satellite-derived AOD is a measure of light attenuation in the column which is affected by environment conditions. Many scholars have studied the aerosol properties, composition and distribution characteristic of the specific area (Cong Z. Y., et al. 2007, 2009, Chudnovsky A. A., et al. 2014).
The long-term trend of aerosol optical depth over the global oceans has been studied by using a nearly 25-year aerosol product from the AVHRR data. This negative tendency is even more evident for globally and annually averaged AOD with the magnitude of -0.03 /decade. The decreasing tendency in global AOD is consistent with that from the remote sensing data set (Mehta M., 2015, Meij A. de, et al. 2012). The trends in AODs, temporally and spatially was analyzed with MODIS data (2000-2008) over eight typical regions in China (Guo J. P., et al. 2011) The East China Sea is located the western of Pacific Ocean, which is strongly affected by monsoons, massive freshwater outflows, tides and the Kuroshio (Chen C.-T. A., 2009). Therefore the study of the spatial and temporal distribution of AOD and its influence factors are significant to the national economy sustainable development and the environment protection. Aerosol retrieval using MODIS data has been well researched over the past decade (Luo N. N., et al. 2015, Lee J., et al. 2014. This study was conducted over the area of East China Sea (24ºN~41ºN, 117ºE~128ºE) as the research area and bases on the monthly AOD data from MODIS sensor and monthly wind data of 10 meter over sea surface provided by ECMWF during 2003-2013. Then the characteristics of spatial and temporal variation of aerosol optical depth and the effects of the wind field to its distribution were analyzed.

Study Area
The East China Sea ( i.e., the Bohai, Yellow and East China Sea) is marginal sea surrounded by bordered by Kyushu and the Ryukyu Islands of Japan, Taiwan Island, mainland China, and the Korea Peninsula. The almost enclosed Bohai Sea (BS), with a surface area of 77×10 3 km 2 , is the smallest and shallowest of the three seas with an average depth of only 18 m. The semi-enclosed Yellow Sea (YS) has a surface area of 380×10 3  The study area of the East China Sea (24ºN~41ºN, 117ºE~128ºE) are shown as Fig. 1. It is characterized with high loadings of sediment concentrations in the continental shelf regions. The Yellow River, the liaohe, yalu, and Haihe, also transport significant amounts of sediment into the Bohai, Yellow and East China Sea. It is suffered from the East Asian monsoon, the ecological environment of the Yellow Sea and East China Sea has changed greatly in the past few decades.

Satellite Data Collection of AOD
The standard mapped satellite images from MODIS/Aqua were used to derive the AOD values over the East China Sea. Monthly averaged climatology AOD values from January 2003 to December 2013 were used. These satellite data were obtained from the Ocean Biology Processing Group (OBPG) of the Goddard Space Flight Center (GSFC) (http://oceandata.sci.gsfc. nasa.gov/MODISA/Mapped/ Monthly/9km/aot/) in the compressed Hierarchical Data Format (HDF). The basic algorithm was used for calculating the AOD.

ERA-interim Global Reanalysis Dataset of Wind
The 10m wind field of u-and v-components of horizontal wind dataset produced by the European Centre for Medium-Range Weather Forecasts (ECMWF, http:// apps.ecmwf.int/datasets/data/interim_full_moda/) was processed. The ERA-interim global reanalysis 10m wind field provided global coverage (Stopa J. E. et al, 2014). In this paper, we used the wind dataset retrieved for the same time span from January 2003 to December 2013, and the same domain (24ºN~41ºN, 117ºE~128ºE) used for the AOD retrieval.

Statistical Methods
To analyze the significant variation periods of the longtime series, we introduced the statistical method of discrete power spectral density. The basic principle is to compute the Fourier coefficients. For a sequence x (t = 1,2, … n) with n sizes, the equations represent below (Wei. F. Y.2007).
Where k represents wave number; t is the length of time series; s ) is the power spectral density of k ; T ) is the resonance cycle of k.
In the paper, n equals to 132 representing 132 months from January 2003 to December 2013; k equals from 1to 66 (half of n).
Finally, we introduced F-inspection to inspect the resonance cycle T ) , using 1 − α = 0.95 confidence limit. The F-inspection equation is: Where 2 and n-2-1 represent the freedom of numerator and denominator respectively, S is the variance of the time series. When F ≥ F α( ,EF F ) , it implies that the corresponding period was marked.

Seasonal and Inter-annual Variability of AOD
The monthly average of AOD values over the East China Sea are shown as

Distribution of Spatial Variation of AOD
According to the latitude and longitude the average AOD of pixel point in the same month from 2003 to 2013, the average of AOD values were shown as the distribution of spatial variation of AOD from January to December. April, July, October and January are the months of spring, summer, autumn and winter and the AOD distribution seasonal variation shown in Fig. 5.
In spring, the average values of AOD are high over the whole study area. The AOD values over coastal waters, the new estuaries of the Yellow Sea are higher than that over the central of Bohai Sea. AOD values over the Yellow Sea area are generally high(0.12-0.14), AOD values over the whole East China Sea are high, especially the highest AOD values over Taiwan Strait and the Okinawa appears 0. 16

Distribution of Sea Surface 10m Wind over the East China Sea
The each pixel point of the wind vector data over sea surface 10m are averaged according to the latitude and longitude in the same month from 2003 to 2013. The results of wind vector data show the spatial distribution of wind vector from January to December. April, July, October and January are the months of spring, summer, autumn and winter and the seasonal variation were shown in Fig. 6, in which color indicates wind speed, arrows indicate wind direction.
In the spring, the wind speed over the East China Sea was low, the wind direction over Bohai Sea changed from south to southwest with the increasing of longitude. Wind direction over the North Yellow Sea area was WSW. In the South Yellow Sea, wind direction changed from south-eastern in coastal clockwise to west near the shore in the Korean Peninsula, which consistent with the time when Bohai and Yellow Seas appeared cold wave. This anticyclone flow was the average performance of cold anticyclone. In the East China Sea, wind direction over the Taiwan Strait was the northeast, while wind direction was east over the east of the Taiwan island, in the other regions the wind direction changed from northwest to northeast near the Jeju island, which consistent with the time when the East China Sea appeared cold wave. The change of wind direction reflected the characteristics of cold anticyclone.
In summer, the wind speed increased compared with the spring, the wind direction was the south over the central of the Bohai Sea. The wind direction was the south over the North Yellow Sea beside the coastal waters of Korea Bay whose direction was WSW. In the South Yellow Sea, the wind direction changed from the south-southeast to the southwest with the increasing of longitude. In the East China Sea, the wind direction was the south over the regions of 122ºE~125ºE, and it was southwest in the top of the Taiwan Strait, it is south-southeast over the regions of the east of 125ºE and the south of 26ºE.
In autumn, the wind speed over the Bohai Sea and Yellow Sea was low, and high over the other regions. The wind direction was WNW over Bohai Sea and northwest in North Yellow Sea respectively. The wind direction was south over the South Yellow Sea. In the East China Sea, the wind direction changed from north to northeast with the decrease of latitude.
In winter, cold air from north to south blows off the mainland toward the sea forming the winter monsoon. The wind speed over the East China Sea was higher than the other seasons. The wind direction was NNW over Bohai Sea and North Yellow Sea area. In the South Yellow Sea, the wind direction was north-northwest over the west of the Yellow Sea trough and it was northwest over Yellow Sea trough. In the East China Sea, the wind direction changed from NNW to northeast with the decrease of latitude.

Relationship Between the Wind Vector and AOD
Over the Bohai Sea and the North Yellow Sea, in spring, the wind direction was southwest, i.e., from the land to the ocean. There was a range of high sandstorm in northern China, which is the main reason for AOD values became the largest and most widely scattered in four seasons. In summer, the wind direction was from the ocean to the land, which forced the sea aerosol to transport outward, therefore the AOD decreased significantly in summer. In autumn, well atmosphere condition leaded to the reduction of the landbasic sources of aerosol and the AOD values over Bohai Sea continued to decrease. The regions with higher AOD values were disappeared. In winter, the spatial distribution of wind direction was similar to the autumn, so the AOD values over the whole Bohai Sea continued to decline significantly.
In the South Yellow Sea, the AOD remained at a high value (0.12-0.14) in spring. In summer, since the wind blew from the East China Sea to the land, the AOD values reduced over the south of the Yellow Sea, while the AOD values grew significantly over the north of the Yellow Sea. The AOD values has increased over the east of Yellow Sea Trough, which may be the reason of the accumulation of aerosol substance, the wind speed over the central of the South Yellow Sea slightly greater than the speed at the junction of the South Yellow Sea and North Yellow Sea. In autumn, although the wind direction over the South Yellow Sea was from the land to the East China Sea, the wind speed over the South Yellow Sea was significantly higher than the North Yellow Sea, the AOD values decreased significantly compared with the summer. In winter, the AOD values increased over the entire west of the Yellow Sea compares with the autumn, which may be relationship between the dust in the land and the wind direction.
The AOD values over the East China Sea were high in the spring, especially in the Taiwan Strait and Okinawa Trough waters, the AOD values appeared a high value about 0.16. In summer, due to the wind direction was from the top of the East China Sea to the land and the wind speed is large, the AOD values over the entire East China Sea show a clear downward trend compared to the spring, especially in the Taiwan Straits, Okinawa Trough reduced significantly. The wind direction in the autumn was from land to the East China Sea, the AOD values over most of the East China Sea showed significant growth trend. In winter, the AOD values over the East China Sea increased clearly and showed an upward trend, especially in the Okinawa Trough waters and Jeju Island, which mainly related to the presence of the dust from the inland in the winter, the pollution of human activity and the wind blow from the land to the East China Sea.

Conclusions
The AOD presented obvious annual periodic variation over the East China Sea, the maximum of AOD values appeared in February or March during springtime, while the minimum appeared in autumn. The AOD values increased firstly and decreased in the course of a year, and significant increase from January to March, the maximum of AOD values reached 0.146 and a progressive declined from April to August then it stabilized around 0.1 from September to December. The seasonal changes trend indicated the AOD in spring was slightly higher than in the winter and significantly higher in summer and autumn, in other words the AOD reached the highest value in spring. Spatial and temporal variation of AOD is affected by many factors, especially the monsoon. By analyzed the AOD and wind vector data, there were closely relationship between the AOD and wind field in different sea area near Chinese coast. Overall, the spatial and temporal and distribution and variation of AOD are closely related to the wind direction and speed above the corresponding sea area.