PNNL

Abstract

Agricultural activities change natural aerosol population by lofting organic matter, soil dust, and plant debris into the atmosphere, which modify mass loadings, mixing state and composition of aerosols existing in the region (e.g. secondary organics, sulfates, and biomass burning aerosols). Vegetation and crops in agricultural areas are a significant source of both biogenic secondary organic aerosol and of directly emitted primary biological atmospheric particles (PBAP) composed of plant fragments/debris, fungal spores, pollen, and bacteria among others. Variability of PBAP and internal complexity of the corresponding individual particles depends on many environmental variables such as biogenic species, ecosystem health, and seasonal/meteorological cycles. In addition, seasonal crop harvesting significantly intensifies regional PBAP emissions and aerosolizes soil organic matter and dust particulates. The atmospheric contributions and impact of PBAP has been insufficiently investigated due to limited real-time observations and ambiguity of their detection by existing measurement techniques. We collected ambient particles onboard an instrumented Beechcraft BE76 Duchess research aircraft flying above actively harvested agriculture fields. Particle samples were analyzed using complementary chemical imaging techniques, including computer-controlled scanning electron microscopy coupled with elemental microanalysis to describe particle-type population and scanning transmission X-ray microscopy to provide quantitative metrics on particle internal heterogeneity. We show the particle-class contribution is consistent between corn and soybean harvesting across altitudes of 100-300 m AGL accessed by the aircraft. The contribution of PBAP is estimated to be 10-12% of ~39000 analyzed particles grouped by k-means clustering algorithm. X-ray microanalysis has shown that the fine-mode organic particles collected from this study occur within the viscous region (105-108 Pa s), while viscosity of the PBAP fragments can be in solid region (>108 Pa s). The observation of small, solid carbonaceous fragments of biogenic origin has implications to the regional CCN and IN budget. Therefore, seasonal harvesting of crops may play an important, yet unrecognized, role in regional cloud formation and climate.