Variation in Global Chemical Composition of PM2.5: Emerging Results from SPARTAN.

The Surface PARTiculate mAtter Network (SPARTAN) is a long-term project designed to maximize the chemical and physical information obtained from filter samples collected worldwide. This manuscript discusses the ongoing efforts of SPARTAN to define and quantify major ions and trace metals found in aerosols. Our methods infer the spatial and temporal variability of PM_2.5 in a cost-effective manner; single filters represent multi-day averaged fine particulate matter (PM_2.5), while an adjacent nephelometer samples air continuously. SPARTAN instruments are collocated with AERONET to better understand the relationship between ground-level PM_2.5 and columnar aerosol optical depth (AOD). We have examined the chemical composition of PM_2.5 at 12 globally dispersed, densely populated urban locations and a site at Mammoth Cave (US) National Park used as a baseline comparison. Each SPARTAN location has so far been active between the years 2013 and 2015 over 2 to 22 month periods. These sites have collectively gathered over 10 years of quality aerosol data. The major PM_2.5 constituents across all sites (relative contribution ± SD) were ammonium sulfate (20% ± 10%), crustal material (12% ± 6.2%), black carbon (11% ± 8.4%), ammonium nitrate (4.0% ± 2.8%), sea salt (2.2% ± 1.5%), trace element oxides (0.9% ± 0.6%), water (7.2% ± 3.1%) and residue materials (43% ± 25%). Analysis of filter samples revealed that several PM_2.5 chemical components varied by more than an order of magnitude between sites. Ammonium sulfate ranged from 1.1 μg m^-3 (Buenos Aires, Argentina) to 17 μg m^-3 (Kanpur, India [dry season]). Ammonium nitrate ranged from 0.2 μg m^-3 (Mammoth Cave, in summer) to 6.7 μg m^-3 (Kanpur, dry season). Equivalent black carbon ranged from 0.7 μg m^-3 (Mammoth Cave) to 8 μg m^-3 (Dhaka, Bangladesh and Kanpur). Comparison with coincident measurements from the IMPROVE network at Mammoth Cave yielded a high degree of consistency for daily PM_2.5 (r² = 0.76, slope = 1.12), daily sulfate (r² = 0.86, slope = 1.03) and mean fractions of all major PM_2.5 components (within 6%). Major ions generally agree well with previous studies at the same urban locations (e.g. sulfate fractions agree within 4% for eight out of 11 collocation comparisons). Enhanced anthropogenic dust fractions in large urban areas (e.g. Singapore, Kanpur, Hanoi and Dhaka) were apparent from high Zn : Al ratios. The expected water contribution to aerosols was calculated via the hygroscopicity parameter κ_v for each filter. Mean aggregate values ranged from 0.15 (Manila and Ilorin) to 0.31 (Rehovot); with the latter included a major sulfate event. The all-site parameter mean is 0.19. Chemical composition and water retention in each filter measurement allowed inference of hourly PM_2.5 at 35% relative humidity by merging with nephelometer measurements. These hourly PM_2.5 estimates compare favorably with a beta attenuation monitor (MetOne) at the nearby US embassy in Beijing, with a coefficient of variation r² = 0.67 (n = 3167), compared to r² = 0.62 when κ_v was not considered. SPARTAN continues to provide an open-access database of PM_2.5 compositional filter information and hourly mass collected from a global federation of instruments. [ABSTRACT FROM AUTHOR]