Multi-Satellite Observation of an Intense Dust Event over Southwestern China

This study presents the results of a large-scale examination into a dust event that took place over southwestern China, an area that has received relatively little attention in the literature. The sources, transport path, and vertical distribution, as well as formation process of an intense dust storm event that took place over the Sichuan Basin from March 9 to March 14, 2013, were investigated using combined multiple satellite observations and meteorological data. Widespread dust plumes appeared over the Taklimakan Desert, Gobi Desert, and Loess Plateau on March 9. Different from the usual dust storms, the dust plumes did not move downstream directly but settled over the Loess Plateau and Sichuan Basin for several days. Persistent southwestern winds prevailed over eastern China during this period, and could prevent the dust storms from moving directly downstream. Our results suggest that the dust plumes over southwestern China mainly originated from the Gobi Desert. Despite extensive dust plumes over all the Sichuan Basin, PM10 increased sharply to more than 1200 μg/m in the western part of the basin, with only a slight increase in the eastern part. When there was a sharp increase in coarse particles in Chengdu on March 11 and 14, northeasterly winds appeared over northern China. The elevated dust can be blown to the eastern slope of the Tibetan Plateau and deposited in the western Sichuan Basin. However, such intense dust events are not common in southwestern China. In addition, floating dust may not be observed near the surface, but can have an important impact on regional aerosol loading and properties.


INTRODUCTION
Mineral dust from deserts and arid regions is a major component of natural aerosols in the atmosphere.These dust particles play an important role in climate system by changing radiation budget of the Earth-atmosphere system and modifying clouds (Kaufman et al., 2005).The dust radiative effect can alter atmospheric circulation such as the monsoonal moisture transport, which in turn affects precipitation (Zhao et al., 2011).By acting as cloud condensation nuclei or ice nuclei, dust aerosols can influence cloud microphysical properties such as cloud optical depth and water path as well as the their radiative effects (Wang et al., 2010).Moreover, elevated dust plumes originated from deserts, which are usually accompanied by strong winds not only impact atmospheric chemistry in downstream regions, but also largely worsen the air quality (Han et al., 2012).
Deserts and arid regions account for a major part in northwestern China (Fig. 1).Taklimakan Desert in western China and Gobi deserts in Mongolia and northern China are two primary dust source regions in Asia, which are most active in winter and spring (Liu et al., 2008).Dust particles in the Gobi deserts are usually lifted to an elevation of < 3 km and mostly deposit in eastern China and offshore regions (Sun et al., 2001).Satellite observations show that mixing of the dust plumes from Gobi deserts and anthropogenic emissions can lead to regional haze clouds in North China Plain (Tao et al., 2012).By contrast, strong western winds can lift dust materials out of the Tarim Basin to an elevation of > 5 km and result in dust storms associated with longrange transport to southeastern China (Liu and Shiu, 2001) and even North America (Cottle et al., 2013).
Numerous studies have been conducted concerning the sources, formation and transport of the dust events in eastern China due to their marked influences on regional air quality and climate (Huang et al., 2010;Zhang et al., 2010;Bian et al., 2011;Han et al., 2012), but less attention was paid to the situation over southwestern China.Zhao et al. (2010) suggested that dust storms could also arrive in southwestern China based on a 15-month series of weekly sampling of PM 2.5 in Chongqing.As a relative independent region with high loading of particles and acid gases (Zhang et al., 2009), dust particles over Sichuan Basin can play a significant role in regional atmospheric chemistry and climate effects.However, previous studies were mostly concentrated in the more densely populated and developed eastern China, where dust storms usually pass.Dust events over southwestern China were concerned by few observational studies so far.In particular, regional characteristics such as extent, spatial variations, vertical distribution, and optical properties of the dust events are significant in understanding the dust aerosol effects in this region but still less known.
In this study, we provide the first large-scale comprehensive insight into the dust event over southwestern China using integrated multiple satellite observations and meteorological data.During March in 2013, an intense dust event appeared over Sichuan Basin, which provides a unique chance to better understand the dust activities in this region.The sources, transport, vertical distribution as well as formation process of the dust event were investigated.Influence extent of the dust events was discussed by comparing with ground measurements.By examining regional characteristics and transport process of the dust particles, we aim to improve knowledge in the dust aerosols over southwestern China.

Satellite Data Set
The Moderate Resolution Imaging Spectroradiometer (MODIS) on Terra and Aqua satellite, which cross equator at 10:30 a.m and 1:30 p.m in local time provides global daily detection of the Earth-atmosphere system by 36 spectral bands in 0.4-14µm with a wide swath ~2300 km.MODIS dark-target algorithm retrieves aerosol optical depth (AOD) over dense vegetation regions with errors in ± (0.05 + 15%) (Levy et al., 2010).Meanwhile, the deep blue algorithm measures AOD over bright surface such as deserts and arid regions with uncertainties within 30% (Hsu et al., 2006).Here Collection 5.1 10 km MYD04 aerosol data was used to analyzed variations of the dust aerosol loading.Moreover, MODIS true color images can provide a direct view of the dust storm.
The Ozone Monitoring Instrument (OMI) onboard the NASA EOS-Aura satellite measures the Earth's reflectance in the visible and ultraviolet spectra (270-500 nm) with spatial resolution of 13 × 24 km at nadir in a swath width ~2600 km .UV Aerosol Index (UVAI) is a widely used parameter to detect the existence of elevated UV-absorbing aerosols such as airborne dust and biomass burning smoke (Torres et al., 2007).UVAI is sensitive to the height of absorbing aerosols but insensitive to the cloudy weather condition and terrestrial surfaces type.We used the OMI Level 2 grid product (0.25 × 0.25°) to detect the extent of dust plumes.
The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) offers height-resolved information of aerosol and clouds with it's unique vertical detection capability at 532 and 1064 nm.In particular, its linear depolarization measurement at 532 nm is sensitive to non-spherical particles such as pure dust (Liu et al., 2008).The volume depolarization ratio (VDR) of clean dust is larger than that of other types of aerosols such as smoke and industrial pollutants.Here we used the V3.30CALIPSO data to obtain information concerning structures of the dust layers.

Ground Measurements and Meteorological Data
The Environmental Protection Agency (EPA) of China began to publish hourly concentration of primary atmospheric pollutants in major cities from January 2013.Hourly concentration of primary pollutants including PM 10 , PM 2.5 , SO 2 , and NO 2 in Chengdu and Chongqing in March 2013 was used to examine the influence of dust event (Fig. 1).Backward trajectories from the HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) were used to track the dust transport path at different altitudes (Draxler et al., 2013).The National Centers for Environmental Prediction (NCEP) reanalysis data was used to analyze regional variations of wind fields (Kalnay et al., 1996).

Intense Dust Event over Sichuan Basin in March 2013
Dust storms usually occurred in eastern China in springtime but were rarely reported in southwestern China.During March in 2013, an intense dust storm invaded Sichuan Basin (103E-108E, 28N-32N) and lasted for several days.As shown in Fig. 2, PM 10 concentration in Chengdu increased sharply to extremely high values exceeding 1200 µg/m 3 during March 11 and 12, and then decreased to < 100 µg/m 3 rapidly on March 13.There was another peak of PM 10 exceeding 900 µg/m 3 on March 14.High concentration of PM 10 > 600 µg/m 3 persisted until March 15.Fig. 3 show  very low visibility during the dust event on March 11.The intense dust event caused severe yellow haze pollution in widespread areas, which was almost the heaviest air pollution in recent years.Compared with dust events in previous studies (Zhao et al., 2010) and records (www.scemc.cn),such intense and persistent dust storm event rarely occurred in Chengdu.
Particle pollution before the dust storm was heavy in Chengdu with both PM 2.5 and PM 10 at high levels (Fig. 2).PM 10 concentration ranged in 200-500 µg/m 3 from March 1 to March 9, and was approximately twice of the PM 2.5 concentration.Moreover, large daily cycles of the particle concentration indicate the existence of high anthropogenic emissions.By contrast, variations of PM 10 in Chongqing show that there were no serious air pollution events in the southeastern edge of the basin, indicating that the dust event had no obvious influence on surface air quality in this region.There was a sudden increase in the fraction of coarse particles in Chongqing on March 15, but PM 10 was just > 300 µg/m 3 , which was much lower than these in Chengdu during March 11-14.To obtain the spatial extent of the dust storm, large-scale satellite observations are needed.
The MODIS true color images present a direct view of the distribution of the dust storm (Fig. 4).There were no notable dust plumes on March 8, but gray haze clouds covered over Sichuan Basin and southwestern China.Long dust belt appeared over Taklimakan Desert and Gobi deserts on March 9.However, it seemed that the dust plumes did not moved downstream.The dust event got weak on March 10, and spread to the central and southwestern China.Though not as intense as the dust storm on March 9, dust plumes were notable in the regions between the Taklimakan Desert and Gobi deserts on March 11.The dust plumes appeared over the Loess Plateau and Sichuan Basin on March 12 and 13.Color of the haze clouds over Sichuan Basin became yellow during this time.
Fig. 5 shows the frequency of UVAI > 1.5 over southwestern China in the spring from 2011 to 2013.Compared the frequent dust plumes over northern China, dust events were much fewer in Sichuan Basin, which is consistent with ground observations (Wang et al., 2013).There were no more than 3-5 days regional dust events in the spring, indicating that dust storms did not usually appear.Moreover, frequency of dust events in 2013 was at the similar levels with the previous two years.Despite the low frequency, it is worth noting that dust events occur almost every year in Sichuan Basin.Intense dust events such as that in March 2013 can have obvious influences on regional air quality.

The Sources and Transport of the Dust Storm in Sichuan Basin
MODIS AOD indicates high loading of dust aerosols over northwestern China during the polluted period (Fig. 6).From March 9 to March 11, AOD was >1.0 in most of the areas in Taklimakan Desert and Gobi deserts, where anthropogenic emissions was limited (Zhang et al., 2009).Consistent with the true color images, high AOD values > 1.0 over the Loess Plateau imply that the dust plumes had moved downstream in March 12-13.Floating dust still prevailed over northwestern China with moderate AOD values ranging in 0.4-0.6.
Wind fields show that northwesterly air masses were predominant at 700 hPa (≈3 km) over the deserts and arid regions (Fig. 7).From March 9, southwesterly airflows prevailed in eastern China, and met with the northwestern winds in 35 N-40 N, which may prevent the downstream transport of the dust plumes.Meanwhile, northeasterly winds dominated central China at 850 hPa (≈1.5 km) on March 10 and 13, when there was a sharp increase in amount of coarse particles.The northeastern winds can blow the dust plumes to the Sichuan Basin.
Backward trajectories in Chengdu show that air masses at 500 and 1500 m mainly originated from the Gobi deserts (Fig. 8).Transport of these airflows was below 3 km except in the source regions.By contrast, the airflows at 3000 m were directly from the Taklimakan Desert, which can transport over the Tibetan Plateau to the northwestern Sichuan Basin.However, satellite observations show that there were no obvious UV-absorbing aerosols over the northeastern Tibetan Plateau (Fig. 7).Consistent with the backward trajectories within boundary layer, satellite observations and wind fields demonstrate that the dust plumes in Sichuan Basin were transported from the Gobi deserts.Dust particles in the Taklimakan Desert may first be carried to the Gobi deserts, and then moved to downwind regions.The northeastern airflows played a predominant role in blowing the dust plumes over Loess Plateau and central China to the Sichuan Basin.

Influence Extent of the Dust Event and Its Regional Implications
High values of UVAI (> 2.0) indicate the existence of widespread UV-absorbing aerosols (Fig. 7).Although there were no extensive biomass burning fires during this period in southern China, prevailing fire smoke over southeastern Asia in this season can also be transported to southwestern China.CALIPSO vertical detections (Fig. 9) show that top of the pollution plumes can reach 4-5 km.UVAI shows smoke plumes over southern China from the southwestern Asia on March 10 (Fig. 7), but there was no marked smoke transport in other days.By contrast, high VDR values (> 0.2) imply that dust was dominant in Sichuan Basin on March 13.
Although the combined satellite observations show  widespread dust plumes over the Sichuan Basin, there was a large increase in coarse particles in the western region but not in the eastern part of the basin.As shown in Fig. 9, the dust layers were 1-2 km above the surface and homogeneous in both vertical and horizontal scales.Deposition of the elevated dust plumes can be affected by meteorological conditions, height of the dust layers, and topography.The northeasterly airflows can transport the dust particles to the eastern slope of the Tibetan Plateau.The dust transport can be blocked by the plateau and deposited in the western regions of the Sichuan Basin.
The pure dust over deserts can be mixed with anthropogenic pollutants during the transport path (Hung et al., 2010).VDR values in the basin were lower than these in Loess Plateau and northern deserts on March 10 and 11, indicating that the fire smoke and anthropogenic pollutants had mixed with the dust.As shown in Fig. 4, gray fog-haze pollution was prevalent in the basin before the dust storm.There was a decrease in the gaseous pollutants in Chengdu during the dust period (Fig. 2).It was worth noting that the fraction of coarse particles accounted for almost one half of PM 10 in Chengdu before the dust storm.The notable daily cycle of PM 10 and PM 2.5 with consistent trends indicates that the coarse particles may mainly come from local blowing dust.
Although ground sites observed no obvious dust events in the eastern areas of the Sichuan Basin, satellite observations show that elevated dust plumes can cover the whole basin.It is worth noting that high UVAI values also existed over regions without notable dust plumes such as these on March 8 (Fig. 7), indicating that floating dust was prevalent during this periods.Dust particles at high altitudes can be transported to this region easily in certain meteorological conditions, which may not be observed near surface but have an important impact on regional aerosol loading and properties.However, as shown in Fig. 5, intense dust plumes were not common in southwestern China.There were only several regional dusty days in Sichuan Basin in the spring, which were much less than these in northern China.Considering the inhomogeneous distribution and complex background, further study is still in need to character interactions between dust particles and local pollutants as well as estimate their regional climate effects.

CONCLUSIONS
Here we conducted the first large-scale study on the dust event over southwestern China.From March 9 to March 14, 2013, strong dust storm occurred in Sichuan Basin.The intense dust event led to two extremely severe air pollution events in the western part of the basin, with PM 10 exceeding 1200 µg/m 3 .We investigated the sources, transport, vertical distribution and formation process of the intense dust event using combined multiple satellite observations and meteorological data.
Satellite observations show that intense dust storm appeared over Taklimakan Desert and Gobi deserts, Loess Plateau, Sichuan Basin and central China successively.Different from the usual dust storms, the dust plumes on March 9 did not move downstream to long-range regions but settled over Loess Plateau and Sichuan Basin for several days.Strong northwesterly airflows met persistent southwestern winds over the North China Plain at 700 hPa, which could prevent the dust storms moving downstream directly.When there was a sharp increase in coarse particles in Chengdu on March 11 and 14, northeasterly winds appeared over northern China at 850 hPa.
Despite widespread dust plumes over Sichuan Basin, there was a large increase in coarse particles near surface in the western part but not in the eastern part.CALIPSO vertical detections show that the top of the dust layers reached 4-5 km and were distributed 1-2 km above the surface.The elevated dust particles were blown to the western part of basin and blocked by the eastern slope of Tibetan Plateau, leading to massive dust deposition in the western part of the basin.Such intense dust event was not common in southwestern China.Floating dust may not be observed near surface but can have an important influence on regional aerosol loading and properties.The results revealed in this study can increase our understanding of the dust aerosols over Sichuan Basin as well as improve regional chemistry and climate modeling.

Fig. 1 .
Fig. 1.The terrain (a) and true color image of mainland China (b).

Fig. 3 .
Fig. 3. Dust storm in Chengdu on March 11, 2013.The photo was taken in the urban area of Chengdu around noon at the altitude of 25 m near surface.

Fig. 6 .
Fig. 6.Aqua MODIS 10 km AOD at 550 nm with winds fields at 850 hPa at 2 pm in local time.

Fig. 7 .
Fig. 7. OMI UV Aerosol Index with wind fields at 700 hPa at 2 pm in local time.

Fig. 9 .
Fig. 9. CALIPSO total attenuated backscattered at 532 nm and volume depolarization.The while dashed lines denote location of the Sichuan Basin.