An extremely low visibility event over the Guangzhou region: A case study

Over the Guangzhou region, also called the Pearl River Delta (PRD) of Southern China, there are often high aerosol concentrations. The satellite (MODIS) data show that the aerosol optical depth is often higher than 0.6 in this area, which is known as Asian Brown Cloud region. In some extreme events, these high aerosol levels have caused very low visibilities. In this paper, we will show that on 2 November 2003, the daily average visibility is less than 2 km (the instantaneous value is less than 200 m), and the aerosol optical depth is greater than 1.2. We analyze the cause of this particular event. The analysis indicates that this event is related to the hurricane of "Melor 0319". The hurricane is located at the northwest Philippines on 2 November 2003, and the Guangzhou region is outside the hurricane. The hurricane produces a strong descent motion in the lower troposphere, a weak surface wind speed, and a relatively low planetary boundary layer (PBL) in the Guangzhou region. As a result, the strong subsidence causes descending air motions to force the aerosol particles into a very shallow layer, and the weak horizontal wind acts to keep the aerosol particles inside the source region, producing very high aerosol concentrations and extremely low visibility in the Guangzhou region on 2 November 2003. This case analysis will be useful to better understand the causes of the poor air quality over Guangzhou, which is a first step for ultimately improving it in the future. For example, one can reduce emissions when synoptic conditions are expected to cause recurrences of the phenomenon discussed in the paper.