Composition and diurnal variability of the natural Amazonian aerosol

As part of the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA)‐Cooperative LBA Airborne Regional Experiment (CLAIRE) 2001 campaign, separate day and nighttime aerosol samples were collected in July 2001 at a ground‐based site in Amazonia, Brazil, in order to examine the composition and temporal variability of the natural “background” aerosol. A combination of analytical techniques was used to characterize the elemental and ionic composition of the aerosol. Major particle types larger than ∼0.5 μm were identified by electron and light microscopy. Both the coarse and fine aerosol were found to consist primarily of organic matter (∼70 and 80% by mass, respectively), with the coarse fraction containing small amounts of soil dust and sea‐salt particles and the fine fraction containing some non‐sea‐salt sulfate. Coarse particulate mass concentrations (CPM ≈ PM10 − PM2) were found to be highest at night (average = 3.9 ± 1.4 μg m−3, mean night‐to‐day ratio = 1.9 ± 0.4), while fine particulate mass concentrations (FPM ≈ PM2) increased during the daytime (average = 2.6 ± 0.8 μg m−3, mean night‐to‐day ratio = 0.7 ± 0.1). The nocturnal increase in CPM coincided with an increase in primary biological particles in this size range (predominantly yeasts and other fungal spores), resulting from the trapping of surface‐derived forest aerosol under a shallow nocturnal boundary layer and a lake‐land breeze effect at the site, although active nocturnal sporulation may have also contributed. Associated with this, we observed elevated nighttime concentrations of biogenic elements and ions (P, S, K, Cu, Zn, NH4+) in the CPM fraction. For the FPM fraction a persistently higher daytime concentration of organic carbon was found, which indicates that photochemical production of secondary organic aerosol from biogenic volatile organic compounds may have made a significant contribution to the fine aerosol. Dust and sea‐salt‐associated elements/ions in the CPM fraction, and non‐sea‐salt sulfate in the FPM fraction, showed higher daytime concentrations, most likely due to enhanced convective downward mixing of long‐range transported aerosol.