Abstract
Abstract. During the VOCALS Regional Experiment, the DOE G-1 aircraft was used to sample a varying aerosol environment pertinent to properties of stratocumulus clouds over a longitude band extending 800 km west from the Chilean coast at Arica. Trace gas and aerosol measurements are presented as a function of longitude, altitude, and dew point in this study. Spatial distributions are consistent with an upper atmospheric source for O3 and South American coastal sources for marine boundary layer (MBL) CO and aerosol, most of which is acidic sulfate. Pollutant layers in the free troposphere (FT) can be a result of emissions to the north in Peru or long range transport from the west. At a given altitude in the FT (up to 3 km), dew point varies by 40 °C with dry air descending from the upper atmospheric and moist air having a boundary layer (BL) contribution. Ascent of BL air to a cold high altitude results in the condensation and precipitation removal of all but a few percent of BL water along with aerosol that served as CCN. Thus, aerosol volume decreases with dew point in the FT. Aerosol size spectra have a bimodal structure in the MBL and an intermediate diameter unimodal distribution in the FT. Comparing cloud droplet number concentration (CDNC) and pre-cloud aerosol (Dp>100 nm) gives a linear relation up to a number concentration of ~150 cm−3, followed by a less than proportional increase in CDNC at higher aerosol number concentration. A number balance between below cloud aerosol and cloud droplets indicates that ~25 % of aerosol with Dp>100 nm are interstitial (not activated). A direct comparison of pre-cloud and in-cloud aerosol yields a higher estimate. Artifacts in the measurement of interstitial aerosol due to droplet shatter and evaporation are discussed. Within each of 102 constant altitude cloud transects, CDNC and interstitial aerosol were anti-correlated. An examination of one cloud as a case study shows that the interstitial aerosol appears to have a background, upon which is superimposed a high frequency signal that contains the anti-correlation. The anti-correlation is a possible source of information on particle activation or evaporation.
Highlights
Satellite observations of cloud droplet effective radius indicate a gradient off the shore of Northern Chile
Source regions for aerosol and trace gases have been analyzed by Allen et al (2011) and Bretherton et al (2010) by means of back trajectories calculated for marine boundary layer (MBL) at 950 hPa and free troposphere (FT) at 850 hPa starting from 20◦ S at distances from the shore varying from 70.5◦ W to 90◦ W
A contribution to the FT consisting of return coastal flow from the MBL is hypothesized by Allen et al (2011) based on instances with high humidity and low O3
Summary
Satellite observations of cloud droplet effective radius indicate a gradient off the shore of Northern Chile. According to MODIS retrievals from the Aqua satellite for the month of October, average cloud droplet radius over the Pacific Ocean increased from 8 to 14 μm from the coast to 1000 km offshore (Wood et al, 2007). This gradient is plausibly attributed to anthropogenic SO2 from point sources in Chile and Peru, subsequently oxidizded to aerosol sulfate. Anthropogenic perturbations to marine stratocumulus clouds was one motivation for the VAMOS OceanCloud-Climate-Atmosphere-Land Regional Experiment (VOCALS-REx or VOCALS, for short), an overview of which is provided by Wood et al (2011). The sampling region of the G-1 extended from Arica, Chile, 800 km west to 78◦ W mainly within a restricted latitude band between Arica at 18.35◦ S and 20◦ S
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