CCNAMAZON

Aerosol indirect effects, which represent the impact of aerosols on climate through influencing the properties of clouds, remain one of the main uncertainties in climate predictions (IPCC, 2007). Reducing this large uncertainty requires both improved understanding and representation of aerosol properties and processes in climate models, including the cloud activation properties of aerosols. The Atmospheric System Research (ASR) science program plan of Jan 2010 states that “A key requirement for simulating aerosol-cloud interactions is the ability to calculate cloud condensation nuclei and ice nuclei (CCN and IN, respectively) concentrations as a function of supersaturation from the chemical and microphysical properties of the aerosol.” Observations and Modeling of the Green Ocean Amazon (GoAmazon 2014), a field campaign approved by ARM Climate Research Facility (ACRF), seeks to understand how aerosol and cloud life cycles are influenced by pollutant outflow from a tropical megacity (Manaus), in particular the differences in cloud-aerosol-precipitation interactions between polluted and pristine conditions. One key question of GoAmazon 2014 is “What is the influence of the Manaus pollution plume on the cloud condensation nuclei (CCN) activities of the aerosol particles and the secondary organic material in the particles?” To address this question, we propose the measurement of size-resolved CCN spectra, a critical measurement identified during the recent GoAmazon 2014 workshop, at the main research site (T3 site).

The pollution plume from Manaus meanders north and south on two to three day cycles so that the main research site will be in and out of the Manaus plume and pristine conditions every few days. The contrasts between pristine air and the pollution plume will provide excellent opportunities to look into how and to what extent different aerosol size and compositions impact the CCN activity of aerosol. The proposed measurement provides CCN spectra and activation fraction of size selected particles, therefore allowing a clear separation of the impact of aerosol composition from size. The diurnal and seasonal variations of CCN activity, and the influences from both aerosol size and composition will be examined under a variety of conditions ranging from pristine to heavily polluted. The influence of aerosol composition on CCN activity is manifested in particle hygroscopicity, which will be derived from measured size-resolved CCN spectra. When combined with aerosol composition measurements, the derived particle hygroscopicity also allows us to characterize the average value and range of hygroscopicity for major aerosol components, including secondary organics aerosols (SOA). Earlier modeling study shows that the uncertainty in the hygroscopicity of SOA could lead to substantial uncertainty in simulated surface CCN concentration and global aerosol indirect forcing (Liu and Wang, 2010). The long-term deployment (one-year) and the opportunities to sample both pristine air and Manaus plumes will provide a sufficient data set for examining the hygroscopicities of both primary organic aerosol (POA) and SOA from the urban region, as well as SOA formed by natural Biogenic precursors (including possible modification by the anthropogenic influence). These datasets and results can be incorporated into advanced process models for improved representation of CCN concentrations and cloud droplet formation to address the influence of tropical megacities on climate.

Timeline

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