Transport, vertical structure and radiative properties of dust events in southeast China determined from ground and space sensors

Abstract

Two dust events were detected over the Yangtze Delta region of China during March 14–17 and April 25–26 in 2009 where such dust events are uncommon. The transport behavior, spatio-temporal evolution, vertical structure, direct radiative effects, as well as induced heating rates, are investigated using a combination of ground-based and satellite-based measurements, a back-trajectory analysis, an aerosol model and a radiative transfer model. Back-trajectories, wind fields and aerosol model analyses show that the first dust originated in northern/northwestern China and the second generated in the Taklimakan desert in northwest China, and traveled across the Hexi corridor and Loess Plateau to the Yangtze Delta region (the so-called “dust corridor”). The mean lidar extinction-to-backscatter ratio (LR) during the two dust events was 38.7 ± 10.4 sr and 42.7 ± 15.2 sr, respectively. The mean aerosol depolarization ratio (δ_a) for the first dust event was 0.16 ± 0.07, with a maximum value of 0.32. For the second, the mean δ_a was around 0.19 ± 0.06, with a maximum value of 0.29. Aerosol extinction coefficient and δ_a profiles for the two events were similar: two aerosol layers consisting of dust aerosols and a mixture of dust and anthropogenic pollution aerosols. The topmost aerosol layer is above 3.5 km. The maximum mean aerosol extinction coefficients were 0.5 km⁻¹ and 0.54 km⁻¹ at about 0.7 km and 1.1 km, respectively. Significant effects of cooling at the surface and heating in the atmosphere were found during these dust events. Diurnal mean shortwave radiative forcings (efficiencies) at the surface, the top-of-the-atmosphere and within the atmosphere were −36.8 (−80.0), −13.6 (−29.6) and 23.2 (50.4) W m⁻², respectively, during the first dust event, and −48.2 (−70.9), −21.4 (−31.5) and 26.8 (39.4) W m⁻², respectively, during the second dust event. Maximum heating rates occurred at 0.7 km during the first dust event and at 1.1 km during the second dust event, with a maximum value of 2.74 K day⁻¹ for each case. This significant atmospheric heating induced by elevated dust aerosol layers can affect convection and stability in the lower troposphere.

Introduction

Mineral dust aerosols are a particularly important aerosol type because they can induce significant dynamical perturbations of the synoptic flow (Alpert et al., 1998), absorb and scatter radiation to produce intense direct radiative effects, and play an important role as cloud condensation nuclei. The latter can modify the microphysical properties of cloud and indirectly change climate through cloud radiation processes (Forster et al., 2007). Determining the vertical distribution of dust aerosols is crucial because as a strongly absorbing aerosol, it can influence radiative effects at the top-of-the-atmosphere (Gadhavi and Jayaraman, 2006), modify vertical profiles of radiative heating (Ramanathan et al., 2007), change the stability of the atmosphere and affect convective and turbulent motions and clouds (McFarquhar and Wang, 2006). Dust can have a warming or cooling effect depending on where dust layers are above or below more absorbing atmospheric layers (Meloni et al., 2005, Raut and Chazette, 2008).

However, due to limited observations of dust aerosols on a regional and global scale, especially regarding their vertical distribution, its radiative forcing has not been well quantified. Ground-based and space-based instruments, such as the sun photometer from the Aerosol Robotic Network (AERONET), ground-based lidar, the Ozone Monitoring Instrument (OMI), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), can be used to study dust aerosol properties. Combining measurements from these instruments, we can identify dust aerosol sources and transport route, and characterize columnar and vertical optical properties. In particular, lidar technology can help us understand processes occurring in dust aerosols aloft and understand their vertical variations.

Asian dust is a dominant aerosol component in northern Asia. Northeasterly winds associated with the Asian monsoon and continental anticyclones may transport dust to southern and southeastern Asia. During long-range transport, dust aerosol properties, such as hygroscopicity, may change through chemical reactions with other kinds of aerosols. Elevated dust layers can lead to instantaneous atmospheric heating rates of approximately 1–4 K day⁻¹ (Kim et al., 2010). In recent years, several observational campaigns focusing on aerosols in Asia were carried out, such as the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) field campaign (Shimizu et al., 2004) and the 2008 China–U.S. joint field campaign (Li et al., 2011). Studies regarding spatial, temporal and vertical variations of dust aerosols have been carried out (Shimizu et al., 2004, Zheng et al., 2008, Huang et al., 2008, Huang et al., 2010), as well as studies focused on the long-range transport of Asian dust (Han et al., 2008, Liu et al., 2009). Most of the research focused on dust source areas in northwestern China but few have studied those affecting southeastern China, an area with high aerosol loading, complex combinations of aerosols, and high humidity (Li et al., 2007). There are also fewer lidar systems deployed in this region.

A comprehensive observation campaign, which involved the collection of radiation, aerosol, and cloud data, was conducted at Taihu from March 2008 to December 2009. The site was established at the edge of Lake Taihu (31.702N, 120.358E, 10 m above sea level) surrounded by large cities in the Yangtze Delta region, such as Shanghai (east of the site), Nanjing (west of the site) and Hangzhou (south of the site). The region has intensive industrial and agricultural activities, with high aerosol loading (Li et al., 2007). In this study, we investigate two dust events that occurred in the spring of 2009 over the Yangtze Delta region regarding their: (1) sources and transport over time, using ground-/satellite-based measurements, back-trajectory analysis and an aerosol model, (2) optical properties and vertical distribution, and (3) direct radiative effects and atmospheric heating rates.

Brief descriptions of the ground and space-borne instruments and their products, as well as the radiative transfer model, are provided in Section 2. In Section 3, dust source identification, transport behavior, vertical properties, as well as direct radiative effects, are described. Finally, a summary is given in Section 4.