Vertically-Resolved Measurements of Nucleation Precursors

Abstract

Background and Motivation: Atmospheric nucleation, a process where gases react to form stable particles, contributes the majority of the seed particles for cloud formation at altitudes above the surface layer. Consequently, higher altitude nucleation plays an important role in cloud properties and Earth’s radiative balance. Freshly formed particles have been observed in the upper boundary layer yet the nucleation processes that formed these particles remains poorly understood. This is primarily due to instrument limitations that prevent measurement of nucleation precursor gases above the ground level. As a result, little is known about how precursors are transported to higher altitudes and how their resulting nucleation rates impact aerosol number concentrations aloft.  

Project Objectives: The purpose of this project is to determine to what extent sulfuric acid-driven nucleation occurs aloft by measuring how nucleation precursor concentrations vary vertically and temporally at different altitudes above Lamont, Oklahoma. We propose three research objectives: (1) Develop and calibrate compact nucleation precursor counters capable of being launched on a tethered balloon system, (2) measure nucleation precursors and particles at various altitudes at the field site, and (3) understand how the measured vertical distribution of nucleation precursor concentrations contribute to sulfuric acid nucleation and how these distributions can be explained by meteorological conditions and vertical transport of gases.

Technical Approach: (1) We will develop and deploy two, 1 nm condensation particles counters to measure gaseous concentrations of sulfuric acid and grouped contribution of compounds that enhance sulfuric acid-driven nucleation. In collaboration with Dr. Chongai Kuang (Brookhaven National Laboratory), these instruments will be developed and calibrated for operation on the tethered balloon system. (2) The precursor instruments, along with ground-based instrumentation including a high-resolution chemical ionization mass spectrometer and scanning mobility particle sizer will be deployed to the field site in coordination with Kuang and his campaign to measure vertically-resolved nucleation mode particles. We will measure sulfuric acid concentration and effective concentration of stabilizing compounds as a function of altitude. Ground-based measurements of sulfuric acid, other speciated nucleation precursors, and particle size distributions will also be conducted. (3) Measured nucleation precursors will be used to model sulfuric acid nucleation rates and concentration of nucleation mode particles as a function of time of day and vertical height. Predicted particle concentrations will then be compared to measured concentrations to determine if the sulfuric acid is the key driver of nucleation in Lamont, Oklahoma. Vertical distributions of nucleation precursors will be related to atmospheric turbulence and other meteorological conditions to determine how precursors are transported from the surface sources.

Impact of Project: This project will provide process-level understanding on how nucleation in the boundary layer impacts cloud properties and thus Earth’s radiative budget. The project will produce a novel technique to measure gaseous nucleation precursors using a condensation particle counter that can be deployed in diverse locations. In addition, the project will provide insights into other aerosol processes beyond nucleation as these precursor gases also serve important roles in aerosol growth, aqueous chemistry, and cloud activation. Thus, vertically-resolved measurements of nucleation precursors will broadly improve understanding on how aerosol particles impact Earth’s radiative budget.