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Department of Atmospheric Science

Seminar: Mon., Feb. 24, 3:10  pm, EN6085 

Persistent Daily New Particle Formation at a Mountain-Top Location


A. Gannet Hallar

Desert Research Institute
Storm Peak Laboratory 
Steamboat Springs, CO

Atmospheric aerosols can impact climate, air quality and visibility, and human health.Specifically, aerosols affect the Earth’s radiation balance directly by scattering sunlight and indirectly through their role as cloud condensation nuclei (CCN) and ice nuclei (IN). In liquid clouds, an increase in the number of CCN leads to more numerous but smaller cloud droplets and increased cloud albedo. This is commonly referred to as the “first indirect effect” or “cloud albedo effect”. The reduction in cloud droplet size affects precipitation efficiency, leading to an increase the liquid water content, the cloud lifetime, and the cloud thickness. This is referred to as a “second indirect effect” or “cloud lifetime effect”. Both of these effects are assumed to have global cooling effect, in other words, a negative net radiative flux change at the top of atmosphere. Currently there are still large uncertainties associated with the radiative impacts of both the first and second indirect effects. Recent work suggest that newly formed particles exhibit enhancedhygroscopicity (i.e. ability to uptake water), may be more effective as CCN, and increasethe indirect radiative effects beyond current estimates. It is thus critical to understand aerosol formation pathways, specifically, nucleation, growth and resulting impact on cloud droplet size and number concentration.

Frequent new particle formation (NPF) has been observed regularly at Storm Peak Laboratory (SPL), a high elevation mountaintop observatory in Colorado. These events occurred during 52% of the 474 measurement days from 2001 to 2009, consistently during the mid-afternoon throughout the spring, summer and winter months.  Average growth rates, condensation sinks, and formation rates for these NPF events will be presented. The strongest correlation factor with NPF is ultraviolet radiation. The events are not related to increased ozone concentration or pre-existing aerosol surface area, implying that nucleation can occur anywhere in the free troposphere.  Yet, these events are associated with wind direction implicating the potential role of coal-fired power plants on NPF.

During the Department of Energy’s Atmospheric Radiation Measurement Storm Peak Lab Cloud Property Validation Experiment (StormVEx), winter 2010-11, an extensive suite of instruments was deployed for the measurement of aerosol and cloud properties at and near SPL.  At SPL, a TSI Nano-Scanning Mobility Particle Sizer (SMPS), a standard SMPS, and three Droplet Measurement Technology Cloud Probes (FSSP, CIP, and PIP) measured aerosol and cloud particle size distributions from 3 nm to 6.2 mm.An SMPS and Condensational Particle Counter were also deployed at the Aerosol Observing System (AOS) below SPL. This presentation will describe several case studies demonstrating the growth evolution of newly formed 3 nm particles observed at SPL.  Concurrent cloud microphysical data collected at SPL will be presented.  Aerosolgrowth episodes observed below cloud base at the AOS site will be discussed.


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