Atmospheric aerosols in the South Pacific: A regional baseline and characterization of aerosol fractions

Atmospheric aerosols are a major component of the earth's climate system and modulate air quality at local and regional scales. One region that has seen little attention is the South Pacific, despite air quality being of increasing concern for human health in Pacific Island Countries and Territories (PICTs). A review of literature on sources of pollution emissions in the Pacific region reveals that while most of the studies have focused on specific emission events, the annual cycle and long-term variability of the different aerosol types is rarely considered. Hence, this study provides a regional overview of aerosols over the Pacific using two global reanalyses (NASA/MERRA-2 and ECMWF/CAMS) and one satellite-based product (VIIRS). The uncertainties of the different data sources are discussed. The spatio-temporal variability of total Aerosol Optical Depth (AOD) and different aerosol species is shown and discussed in relation to the pollution sources previously investigated. In general, CAMS exhibits a higher total AOD (0.11) compared to MERRA-2 (0.09), both lower than VIIRS (0.12). Both reanalyses show that sea salt is the predominant aerosol source in the region, whereas sulphates and organic matter aerosols are also significant pollution sources, particularly over the islands in Melanesia. Both reanalyses capture the annual variation of AOD similarly, with higher total AOD levels occurring from September to February (SON and DJF), and lower levels from March to May (MAM). Besides capturing the aerosols seasonal variability, both reanalyses also capture extreme aerosol peaks, which evidence the long-range transport of dust from the Australian outback, black carbon and organic matter from major Australian bushfires, and sulphates from volcanic eruptions. This study serves as a baseline for further air quality assessment in the PICTs, considering that the seasonal variation presented here has not been thoroughly analyzed before but can be expected to play an important role in regional atmospheric and ocean biochemistry as well as potential impacts on human health in the Pacific.