Measurements of enhanced H₂SO₄ and 3–4 nm particles near a frontal cloud during the First Aerosol Characterization Experiment (ACE 1)

Abstract

Abstract. Observations of new particle production recorded near a frontal cloud at-6 km above sea level in a remote marine region are reported. Two distinct locations situated near the cloud were found to have enhanced concentrations of sulfuric acid vapor (H2SO4) and freshly formed 3-4 nm particles. Both were in droplet-free air situated above cloudy regions.No evidence for enhanced H2SO4 or nucleation was observed in clear air far from the cloud.In the nucleating region the aerosol size distribution from 3 nm to 600 gm was observed to be trimodal, with a prominent ultrafine mode, and was qualitatively similar to surface-based measurements recorded in regions of postfrontal subsidence. The measurements support the notion that new particle production in the free troposphere occurs preferentially in clear air near clouds via enhanced photochemical production of nucleation precursor gases and that H2SO4 participates. A model simulation suggested that a doubling of ultraviolet intensities above the cloud due to cloud enhanced up-welling radiation or reasonable enhancements in sulfur dioxide concentrations could account for the higher H2SO4 concentration observed near the cloud. In the nucleation regions, H2SO4 and water vapor concentrations were too low for binary nucleation of sulfuric acid and water, according to current nucleation models. The mechanisms of particle formation and growth remain uncertain. The measurements were part of the first Aerosol Characterization Experiment (ACE 1) conducted in the remote South Pacific Ocean (153øE, 47øS) on November 27, 1995. Models, based on observations, suggested that new particles were formed in these regions via binary H2SO4-H20 nucleation [Hegg et al., 1990; Perry and Hobbs, 1994]. The upper perimeters of convective clouds appear to be ideal sites for new particle production due to enhanced actinic fluxes, high relative humidity fields, cold temperatures, and reduced aerosol surface areas associated with higher altitudes. High actinic fluxes from down-welling and cloud reflected up-welling radiation combined with enhanced water vapor concentration could lead to increased photochemical production of OH, which in turn leads to production of low volatility products, such as H2SO4 production from sulfur dioxide (SO2) oxidation. Measurements of high OH concentrations above 24,107

New particle formation and growth from methanesulfonic acid, trimethylamine and water

“…This limits our ability to quantitatively assess the impacts of particles on visibility, human health and climate change as well as to develop effective control strategies. 1,12 Field measurements have shown that sulfuric acid (H 2 SO 4 ) is a key species for NPF, [13][14][15] possibly with the involvement of organic compounds. [16][17][18] The presence of ammonia (NH 3 ) can stabilize nucleating clusters, and enhance nucleation by orders of magnitude.…”

Model of optical response of marine aerosols to Forbush decreases

“…cm −3 s −1 , and κ=[0.5, 1, 1.5, 2, 1000] days. The production values of sulphuric acid were chosen such that the sulphuric acid gas concentration reached peak values of about 1×10 7 cm −3 comparable to the values of and Weber et al (2001). Note that this may be at the high end of the sulphuric acid concentration range.…”

The role of atmospheric ions in aerosol nucleation – a review

“…However the exact mechanism controlling the early stages of aerosol formation is still not understood. Several measurements of atmospheric nucleation have been reported where the observed nucleation rates cannot be explained by the traditional theory of binary homogeneous nucleation by water and sulphuric acid (Hoppel et al, 1994;Weber et al, 1996;Weber et al, 1997;Clarke et al, 1998;O'Dowd et al, 1999;Weber et al, 2001a;Birmili et al, 2003). A ternary nucleating agent such as ammonia has been proposed to enhance the nucleation process by stabilising early cluster formation (Weber et al, 1996;O'Dowd et al, 1999;Weber et al, 1998;Kulmala et al, 2000).…”