Characterizing spatial and temporal variations in an aerosol is critical for a thorough understanding of its formation, transport, and accumulation in the atmosphere. The Pearl River Delta (PRD) region is one of the most densely urbanized regions in the world and one of the main hubs of China's economic growth. In this study, long-term (2000-2013) Moderate Resolution Imaging Spectroradiometer (MODIS) level 2 aerosol products were used to study the spatial and temporal distributions of both aerosol optical depth (AOD) and fine aerosol optical depth (FAOD) over the PRD region.
To investigate the variation characteristics of particulate pollution in the PRD region for a long time, we used long-term (2000-2013) MODIS AOD and FAOD data with 10 km resolution to calculate the monthly, seasonal, annual, and 14-year means. In this process, we screened the cloud pixels.
Results indicate that the spatial distribution of AOD of higher values are located in the central region of the PRD region (e.g., Foshan, north of Zhongshan, and southwest of Dongguan), whereas that of lower values are located in the western and eastern parts (e.g., Guangzhou, Huizhou, Zhaoqing, and Jiangmen). The spatial variability of FAOD is less significant than that of AOD, and the highest value of FAOD is located in Zhaoqing. Both annual mean AOD and number of areas with high AOD increase from 2000 to 2006. Between 2006 and 2013, although the annual AOD level over the PRD region fluctuated, the condition of particulate matter pollution has generally improved since 2006 because of the strict air pollution control implemented in the PRD region. The FAOD data set shows an increasing trend from 2000 to 2012, and the spatial variation in FAOD over the PRD region becomes increasingly significant. Therefore, a further improvement in the air quality in the PRD region requires a stricter regulation of fine particulate matter concentrations. A comparison of the annual MODIS FAOD values and the annual AERONET FAOD values at the PolyU site (114.18°E, 22.30°N) shows that, from 2006 to 2013 both variables have a good agreement and the absolute differences between them are all lower than 0.08. The lowest AOD (0.39±0.10) and FAOD (0.25±0.05)occur in winter. The maximum AOD (0.72±0.13) in the PRD region occurs in spring, whereas the FAOD (0.45±0.10) reaches the maximum in autumn. From 2000 to 2013, the maximum fluctuation in AOD appears in summer, with the AOD standard deviation (std) reaching 0.28 in July. The season with the minimum fluctuation in AOD occurs is winter, with the AOD std being only 0.086 in January. Similar to AOD, the maximum (std=0.21) and minimum (std=0.038) fluctuations in FAOD occur in June and January, respectively.
A long-term MODIS AOD and FAOD series can reflect the spatio-temporal variation trend of particulate matter pollution over the PRD region. Although aerosol extinction has decreased significantly since 2006, the fine aerosol extinction has still increased to a certain extent, indicating that further control on fine particulate matter pollution is necessary in the PRD region.