## 3 Results and Discussion  
### Identification and Source of Haze  
The experiment was performed at the State Key Laboratory of Information Engineering in Surveying, Mapping, and Remote Sensing site, Wuhan University, where the ground-based equipment mentioned above was installed. The location of the study area is shown in Figure 1. Wuhan is a highly industrialized inland mega city in central China. It is the capital of Hubei Province and is located at the junction of the Yangtze and Han rivers. This area experiences a typical northern subtropical humid monsoon climate with high annual average temperature (15.7-17.5 \({}^{\circ}\)C) and precipitation (1050-2000 mm) [32]. At present, there is no globally consistent definition of haze. The Chinese Meteorological Administration (CMA) defines haze as conditions with visibility \(<\)10 km, whereas some studies have adopted a visibility criterion of \(<\)5 km [46, 47]. For all previous studies, visibility has been adopted as the basic index; however, because both haze and fog result in reduced visibility, on 1 June 2010, the CMA issued a standard for haze observation and forecasting in China. This standard defines haze as visibility \(<\)10 km when relative humidity (RH) is \(<\)95%. However, this study has considered haze as conditions where the horizontal visibility is \(<\)5 km and RH is \(<\)80%, which is consistent with the study of [PERSON] _et al._[23].  
The MODIS active fires distribution during 8-11 June 2012 is shown in Figure 2a. Fires were very dense in northern Anhui Province, northern Jiangsu Province, and Shandong Province. Because it was the crop-harvesting season, large amounts of biomass (such as straw) were combusted and many aerosol pollutants emitted into the atmosphere. It has been discovered that the harvest season in northern Anhui Province is in June (summer) and considerable biomass is burned during this time; thus, the fire density is much greater in June than in other months [48].  
A five-day back-trajectory of air mass at 500 m above sea level was performed to establish the source of the haze and the result is shown in Figure 2b. It is clear that air masses traveled from the East China Sea and across the Yellow Sea and Shandong and Anhui provinces before finally arriving in Wuhan on the morning of 11 June, countering a few pollution sources along their path. It is also apparent from Figure 3a that a northeasterly wind was prevailing in Wuhan on the morning of 11 June.  
Figure 1: Location of study area: (**a**) location of Hubei Province, China and (**b**) location of Wuhan in Hubei Province (solid black line represents the CALIPSO track passing over Wuhan).  
Based on the fires distribution information, it can be speculated that the serious haze event that began in Wuhan on the morning of 11 June was caused by air mass transport bringing large amounts of pollutants, mainly from biomass combustion in northern Anhui Province near Wuhan.  
Figure 2: **(a)** Active fire data during 8–11 June 2012, in Hubei Province and surrounding provinces from MODIS global fire area products (MCD14 ML) (Fire pixels are shown as orange dots); **(b)** HYSPLIT backward trajectories for haze episode in Wuhan during 7–11 June 2012 (Red denotes backward trajectories arriving at heights 500 m above ground level).