Urban pollution substantially increases particle number concentration through organic new particle formation in the Amazon rainforest

Abstract:

A major challenge in quantifying aerosol radiative forcing—one of the largest uncertainty in anthropogenic forcing of the climate—is to understand how human activities change aerosol properties relative to the preindustrial baseline. The Amazon rainforest is one of the few places on Earth where the atmosphere transitions between preindustrial and human-influenced conditions. Here we combine chemical transport simulation and field measurements to quantify the effect of urban pollution on aerosol fields through various new-particle formation mechanisms and primary particle emissions near Manaus, an isolated megacity in central Amazon with various types of emission sources. We incorporate in the Weather Research and Forecasting model with Chemistry (WRF-Chem) seven NPF pathways and a novel experimentally-constrained radical Two-Dimensional Volatility Basis Set (2D-VBS). The radical 2D-VBS systematically simulates the temperature-dependent formation chemistry and thermodynamics of low volatility organic compounds that drive organic NPF. The model developments bring simulated particle number and size distribution into good agreement with aircraft- and ground-based measurements. Observations and simulations show that the urban pollution from Manaus increases particle number concentration and cloud condensation nuclei (CCN) by a factor of 10–50 and 3–8 over a large downwind region compared to background conditions. Simulations indicate that NPF contributes over 80% of the total particle number over this downwind region. Among the seven NPF pathways considered in our model, the ternary NPF of sulfuric acid and organics dominates (> 90%), indicating a close interaction between anthropogenic and biogenic emissions. This study provides unprecedented insight into the mechanisms by which anthropogenic pollution affects aerosol number budget in a preindustrial-like environment, which will help better estimate anthropogenic aerosol forcing from preindustrial era to present day.