Abstract
Abstract. Accurate measurements of natural aerosol emissions over the ocean are needed to estimate the anthropogenic impact on the environment. In this study, we measured sea spray aerosol (SSA) concentrations with diameters larger than 0.040 μm produced by open-ocean breaking waves over the SEP (southeastern Pacific). Robust statistics were established through repeated airborne flights over 1000 km along 20° S from the coastline of Chile to 85° W during VOCALS-REx (VAMOS Ocean-Cloud-Atmosphere-Land-Study Regional Experiment). Non-volatile SSA number concentrations were inferred using a thermally resolved technique constrained for clean conditions with an Ångström exponent below 0.5, black carbon mass concentration at values lower than 15 ng m−3 and organic aerosol concentration less than 0.02 μg m−3. We found that number concentrations of SSAs active as cloud condensation nuclei (CCN) for a supersaturation of 0.25% varied between 17 and 36 cm−3, but these did not increase with the increasing mean wind speed typically observed further offshore along 20° S. Concurrent increases in mean offshore precipitation rate in excess of about 1 mm d−1 indicate that scavenging of SSAs by precipitation exceeds increases in production at wind speeds above about 8 m s−1. This demonstrates the critical role of precipitation as a major sink of SSA over the remote ocean. Finally, we found that under clean conditions and for estimated stratus supersaturations between 0.20 and 0.43%, SSA represented about 20% of the total potential CCN along 20° S.
Highlights
M technique constrained for clean conditions with an Ångström perturbed atmosphere
A recent model study under pockets of open cells (POCs) conditions present in the same VOCALS-REx region (Kazil et al, 2011) and using the same source function used in this paper found that sea spray aerosol (SSA) production tended to dominate over free troposphere (FT) entrainment as a source of new cloud condensation nuclei (CCN) in POCs
Our preliminary results show that SSAs would represent more than 30 % of the CCN in the clean mature POC observed on 28 October 2008 during RF06
Summary
This study is based on data acquired during VOCALS-REx from 12 October to 15 November 2008 over the SEP. The latter instrument consists of polycarbonate slides exposed directly to aerosols from the air-stream outside the C-130. Due to a very large sample volume (10 L s−1) at typical low-altitude aircraft speed (105 m s−1), the GNI provides accurate measurements (±10 %) of giant nuclei in the dry sizes range of about 1.6 μm up to several tens of micrometers These two very different measurement approaches show good agreement in their common overlap region. These particulate artifacts are removed from the data used in this study
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