Constraining New Particle Formation in Global Models with Global-Scale Measurements of Aerosol Size Distributions

Aerosols nucleated in the atmosphere may account for over half of the global cloud condensation nuclei (CCN; Merikanto et al., 2009), yet the relative importance of different particle formation mechanisms remains poorly understood (Westervelt et al., 2014). The spatial distribution of newly formed particles is controlled by nucleation mechanisms, sinks for condensable vapors and small particles, and concentrations of precursor vapors. Constraining factors controlling new particle formation in models is particularly important for pre-industrial aerosol-climate interactions, where the effect of newly formed particles on climate is amplified compared with the present day (Gordon et al., 2016). The specific mechanisms of new particle formation alter the concentrations and spatial distributions of these particles both of which affect their influence on climate.

We compare contiguous global datasets of size distributions from the NASA Atmospheric Tomography mission (ATom) with output from two global models with online aerosol microphysics: GEOS-Chem-TOMAS and CESM-CARMA. Aerosol size distributions from 0.003 to 4.8mm were measured with a suite of fast-time-response instruments on two sets of flights of the ATom mission in August 2016 and February 2017. Flights covered the Pacific and Atlantic basins from ~80°N to ~65°S latitude, constantly profiling between 0.2 and ~13km altitude. The DC-8 aircraft was equipped with instrumentation for measuring various aerosol properties as well as greenhouse, reactive and trace gases.

Measurements of aerosol size distributions from ATom reveal a pattern of high nucleation mode aerosol concentrations at high altitude over the tropics, coincident with low surface area concentrations. The major features of this were reproduced by both models run with binary neutral nucleation schemes, and we are testing the influence of ions, ammonia and organics in nucleation schemes in the models on the level of agreement between model outputs and ATom data. Average size distributions over this region show a continuous increase in modal diameter with decreasing altitude between the upper troposphere and top of the boundary layer, demonstrating that these particles grow to CCN sizes and can influence planetary albedo and radiation balance in the remote atmosphere. Both observations and models suggest that this process spans from ~30°S to ~30°N, representing half of the Earth's global oceanic surface area.

The full aerosol size distributions measured during ATom constrain pre-existing concentrations of larger particles that act as sinks for new particle formation and growth. Precursor gas concentrations and nucleation mechanisms are evaluated through comparison of the resulting spatial distribution of small particles from different realizations of the models (Snow-Kropla et al., 2011) with our ATom data. Since the ATom mission focuses heavily on remote regions, the observed spatial patterns help distinguish between various nucleation mechanisms (Kerminen et al., 2010).

Our observations support a mechanism whereby marine or transported tropical continental air is lofted through deep convective clouds, which scavenge existing aerosol, leaving behind air masses with precursors of low-volatility species and low condensation sinks. Nucleation and growth occur as these air-masses exit the top of the cloud. Particles grow to CCN-active size by condensation during slow, multi-week descent outside of convective cores. This mechanism was initially proposed by Clarke et al. (2002), based on previous observations over the Pacific. We evaluate this mechanism using back-trajectories from ATom flights to the point of convective outflow, and combine this with the TOMAS box model to evaluate nucleation and growth mechanisms and rates.

ACKNOWLEDGEMENTS

This work was supported by NASA’s Earth System Science Pathfinder Program under award NNH15AB12I and by NOAA’s Health of the Atmosphere and Atmospheric Chemistry, Carbon Cycle, and Climate Programs. A. Kupc is supported by the Austrian Science Fund FWF's Erwin Schrodinger Fellowship J-3613. We are grateful to James C Wilson and the University of Denver Aerosol Group for the loan of a Nucleation Mode Aerosol Size Spectrometer.

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