PROPOSAL DESCRIPTION, THEORETICAL BACKGROUND AND LITERATURE REVIEW: This section must include a general presentation of the problem to investigate. Describe the novel aspects you intend to address and present a critical review of the literature on the state of the art of the research on the proposal topic. The maximum length of this section is 8 pages (Arial or Verdana font size 10 is suggested) . Use additional sheets to list your cited references.
The atmosphere can be seen a large multiphase chemical reactor in which species are chemically and physically transformed via complex processes that couple multi spatial and temporal scales. Clouds in particular, in addition to redistributing and removing trace species, and interacting with short and long wave radiation, they offer a medium in which (photo) chemical reactions occur, and in which water vapor condensates onto atmospheric aerosol particles in air that has become supersaturated (e.g., Fachinni, 2002). Worldwide there is large body of research addressing various aspects of cloud processes, acknowledging more and more the importance of in-cloud chemical and physical transformations, particularly aerosol-cloud interactions (e.g., IGAC,
2006). To a large extent, this interest obeys to the large uncertainties in climate simulations derived from the representation of aerosol-cloud interactions (IPCC, 2001). In this work we approach this complex subject by exploring the size distribution and composition of the aerosols that act as cloud condensation nuclei (CCN) in the persistent stratocumulus deck over the coast of Northern Chile in two aerosol regimes: one around 22°S, dominated by sulfur aerosols; and one around 30°S, with signatures of dust and urban aerosols. Also, we address the origin and regional transport of aerosols.
Clouds are reactors in which multiphase transformations take place, typically at faster rates than in the gas phase. For example, in-cloud oxidation of sulfur dioxide (SO
2
) into sulfuric acid, which in turn condensates forming new particles, is several times more effective than in clean air (e.g., Hermann et al, 2001 and references therein). Particles that act as CCN and dissolution of trace gases provide the chemical species involved in the liquid phase chemistry of clouds. This processing occurring in cloud droplets influences the chemical composition of the tropospheric gas phase as well as the properties of the aerosol after cloud processing. In turn such CCN define the optical properties of clouds and thus the energy transfer affecting actinic fluxes and thereby photochemistry and dynamics. This complexity makes it necessary to study clouds, as a multiphase fully coupled system (e.g., Fachinni 2002).
In addition to dynamical processes that determine where supersaturated environments appear, to address cloud processes and properties one must be able to analyze individual droplets, which have developed from individual aerosol particles. Such studies often use a Counterflow Virtual Impactor (CVI) to sample clouds
(e.g., Durkee et al, 2000). The CVI inertially removes cloud droplets or ice crystals larger than a certain minimum size from the surrounding air and small, inactivated ( interstitial ) aerosol particles. Once sampled, the droplets are evaporated into a carrier gas stream, leaving behind residual aerosol particles that can be characterized chemically and microphysically.
The potential modulation of cloud albedo by aerosols has been a topic of substantial interest for many years now. The nature of the dependence has suggested that the aerosols will have the greatest effect in marine air (Twomey, 1991) and, indeed, the most convincing evidence for aerosol modulation of cloud properties has been derived from the marine atmosphere, specifically, from the marine stratocumulus scenario (Brenguier et al.,
2000; Hegg et al., 2004). This is most prevalent in the eastern regions of the subtropical oceans and is thought to be an important component of the global radiation budget (Stevens et al., 2003). The few characterization experiments that have been performed for marine stratocumulus clouds have provided key-information for improving our understanding of aerosol-cloud-climate processes (Brenguier et al., 2000, 2003; Snider, et al.,
2003). Unfortunately, up to now none of these studies has focused on the extensive stratus deck off the Chilean and Peruvian coast. Only sporadic measurements have been carried out as part of other experiments, and they refer solely to aerosol-cloud interactions over the open ocean (e.g., Bretherton et al, 2004) or merely to dynamical processes (e.g., Garreaud et al, 2001). This situation is now changing as the upcoming regional experiment The VAMOS Ocean-Cloud-Atmosphere-Land Study - Regional Experiment (VOCALS-REx) will take place out over the open ocean off shore of Northern Chile (Wood et al, 2006, http://www.joss.ucar.edu/vocals/ ).
The South East Pacific (SEP) region offers in fact a unique laboratory to study aerosol-cloud-climate interactions. This is the world’s most extended and persistent stratus deck, having a large impact on the regional and global circulations (e.g., Hartmann et al, 1992). The SEP climate is a tightly coupled system defined by intricate interactions between upper ocean dynamics, biology, atmospheric circulation, aerosol and cloud chemistry and dynamics (Wood et al, 2006).
Recent studies reveal that aerosols in the marine boundary layer of the SEP region are predominantly soluble, and are strongly bimodal in nature with an accumulation mode concentration that is highly variable in time and space (e.g., Tomlinson et al. 2006). As far as 1000 km from the coast particle concentrations vary from
20-300 cm-3 with even higher values of up to 500 cm-3 nearer the Chilean coast. This suggests that the MBL over the SEP experiences periods of extremely clean and quite heavily polluted air. This, coupled with the extensive cloud coverage during the Austral spring, indicates that the SEP is an ideal test-bed for investigating the indirect effects of aerosols on clouds (Wood et al, 2006). In fact we have shown (See Figure 1) that increases in cloud droplet number concentration (CDNC) can be observed in connection with easterly wind events downwind from anthropogenic sources, mainly copper smelters in Northern Chile (Huneeus et al, 2006).
An increase in CDNC can be appreciated downwind of the main emission sources in connection with strong easterly wind events (> 5 m/s at 700 hPa). Circulation conditions favorable for the occurrence of easterly wind

events present a synoptic structure typical of the coastal troughing at the onset stage of coastal-low events farther south in Chile (e.g. Garreaud et al., 2002). Assuming a log-log relationship between sulfate concentration and CDNC, the observed changes in CDNC are consistent both in pattern and in magnitude with the changes due to the offshore transport of oxidized sulfur, suggesting a potential anthropogenic impact on the stratus deck.
Although suggestive, these data and simulations do not prove nor quantify an anthropogenic perturbation of the optical properties in the stratocumulus deck off the Chilean coast. It must be pointed out though that other aerosol sources may also be at play. For instance, easterly wind events are concurrent with an enhancement of the low-level jet (LLJ) system (e.g., Garreaud and Muñoz, 2005) that may drive stronger air-sea exchange inducing sea-salt emissions and enhanced biogenic activity. The subsiding easterly winds events may also bring volcanic aerosols over the stratocumulus deck (e.g., Amigo, 2004; Gallardo et al, 2007).
Figure 1.
The upper panels (a and b) show crosssections at latitude 23.3 ºS of mass mixing ratios of SO x
(in ppbm). To the left the monthly averaged distribution and to the right daily averages for July 26 th are shown. The lower panels (c and d) show the burden, i.e., vertically integrated mass of SO x
(in mgS/m 2 ). To the left, average
CDNC for the period July 20 and August 20 2000. In the lower panel, to the right cloud droplet number concentration (CDNC) for July 26 th in connection with the easterly wind event. Displayed on the figure are the location of the power plant Tocopilla (1) and cooper smelters Chuquicamata (2), Noranda (3), Potrerillos (4) and
Paipote (5).
In Figure 1, an area with persistently high CDNC is noticed along the coast and offshore between 30ºS and 26ºS, which is not readily associated with anthropogenic sulfur emissions. This is also apparent in the corresponding images for cloud optical thickness (COT) derived from the Sea-viewing Wide Field-of-view Sensor
(SeaWiFS) imaginery shown in Figure 2 (Glantz, personal communication). Alongshore, this area coincides with the location of the atmospheric low level jet, which in turn is associated with air-sea exchange and upwelling of cold and nutrient rich waters that may be favorable for biogenic emissions of dimethyl sulfide, and therefore of biogenic sulfate aerosols. Also, as air-sea exchange and near-surface strong winds are present, this may lead to emissions of sea-salt aerosols which can act as effective CCN. Further, the semi-desertic areas at the Southern bound of the Atacama desert may also provide particles that may become activated as CCN. Finally, it is

plausible that a signal of somewhat aged air masses exposed to urban emissions from the central region of Chile could also affect the composition of aerosols and CCN at the area of interest.
Figure 2. Cl oud optical thickness (COT) derived from SeaWiFS imaginery (Glantz, personal communication). To the left the image corresponding to the average condition for the period July 20-August 20 2000. To the right the corresponding image for July 26 2000.
Over the SEP region there are multiple sources of aerosols which may act as CCN. Air-sea exchange of trace gases and particles, e.g., dimethyl sulfide (DMS, CH
3
-S-S-CH
3
) and sea-salt, which provide efficient CCN, is thought to be very important within the Humboldt Current System off Chile and Southern Peru (e.g., Boucher et al, 2003). This is due to the fact that these cold nutrient-rich waters are continuously renewed by wind driven coastal upwelling and exposed to light allowing phytoplankton and zooplankton production, which in turn give rise to the accumulation and degassing of climatically relevant trace species such as carbon dioxide (CO
2
), dimethylsulfide, nitrous oxide (N
2
O), etc (e.g., Torres et al, 1999). Also, the subduction of the Nazca plate under the Andes and the South American continent induces an area of distinct volcanic activity along the Central
Andes where numerous volcanoes show persistent fumarolic activity that probably provides a rather continuous source of sulfate (e.g., Mather et al, 2004), which in connection with down slope winds may supply effective CCN over the stratocumulus deck (e.g., Amigo, 2004; Gallardo et al, 2007). Furthermore, as indicated earlier, there is evidence of a potential perturbation of the subtropical stratocumulus deck due to anthropogenic emissions of oxidized sulfur (SO x
) that occur mainly due to copper smelting along the continental strip of Chile and Peru
(Kuang and Yung, 2000; Huneeus et al, 2006). Anthropogenic sulfate aerosols emitted from smelters located uphill the Andes would reach the stratus deck in connection with strong easterly wind events, whereas coastal emissions would be advected by trade winds. In addition to the copper industry, urban centers, particularly
Santiago (33.5S, 70.5W, 500 m.a.s.l) and Lima (12S, 80W, 50 ma.s.l.), also constitute significant sources of aerosols and trace species that may have an impact on the stratus deck off the coast. Finally, dust, particularly in semi-desertic areas at the Southern bound of the Atacama desert may also provide particles that may become activated as CCN and perhaps more importantly, affect the coastal biochemistry by means of iron deposition
(Jickells et al, 2005).
Except for anthropogenic sources which are monitored on a more or less regular basis within the framework of Chile’s and Peru’s environmental laws (e.g., CONAMA, 1999, 2001, DIGESA, 2005), natural sources of aerosols are poorly constrained in the SEP region.
Using various biogeochemical indicators, estimates of DMS concentration in seawater are available
(Kettle et al, 1999). These estimates show distinct maxima in DMS content in seawater along the coast of
Northern Chile and Southern Peru. These estimates, although based on numerous observations, may not be representative for the coastal area of Chile since this area is clearly under-represented in the empirical database. Moreover, no specific taxonomic or metabolic studies have been carried out to evaluate the production and fate of DMS, which seem to be key parameters to assess the actual production of DMS in seawater (Scholes et al, 2003). Furthermore, the actual transfer rate of DMS in seawater to the atmosphere depends on such factors as wind-stress, wind waves, sea-surface-temperature (SST), turbulence, viscoelastic properties, etc.. These processes are crudely parameterized in nowadays models, mainly in terms of wind speed
(e.g., Gabric et al, 2001; Boucher et al, 2003). This can be particularly sensitive off the coast of Chile given the complex atmospheric and oceanic circulations, strongly modulated by the topography and the strong sea-land gradients (e.g., Rutllant et al, 1998). In addition to this, the formation of sulfate aerosols from DMS, which can act as CCN, depends on complex factors such as the oxidative capacity of the marine boundary layer (MBL), the presence of other aerosols, anthropogenic (e.g., organic compounds) and natural (e.g., sea-salt), etc.. These estimates might be greatly improved during the upcoming VOCALS_Rex as key parameters used in these parameterizations will be actually measured (Wood et al, 2006).
As in the case of DMS, the abundance and distribution of sea salt aerosols is not known in the SEP region. These aerosols are generated by various physical processes, especially the bursting of entrained air bubbles during whitecap formation, resulting in a strong dependence on wind speed (Penner et al, 2001 and references therein). Sea salt aerosols may be the dominant contributor to both light scattering and cloud nuclei in those regions of the marine atmosphere where wind speeds are high and/or other aerosol sources are weak

(e.g., Quinn et al., 2000). Sea salt particles are very efficient CCN. For example, Feingold et al. (1999) showed that in concentrations of 1 particle per liter, giant salt particles are able to modify stratocumulus drizzle production and cloud albedo significantly. In addition to their role as CCN, their reactions with chemical species can affect ozone concentrations, hydrocarbons and cloud condensation nuclei (Nenes et al., 2002). Again, in this case one has to use model based estimates of the emissions (e.g., Gong et al, 1998).
The subduction of the Nazca plate under the Andes and the South American continent induces an area of distinct volcanic activity (e.g., de Silva and Francis, 1991). This geological condition is known to result in a substantial contribution to the sulfur budget over the region through volcanic emissions of sulfur compounds during explosive eruptions (e.g., Andres and Kasgnoc, 1998). However, a less explored source of sulfur compounds from volcanoes is the emissions that occur through fumarolic activity, i.e. quiescent degassing, a phenomenon which appears to be very common among Andean volcanoes (e.g., Mather et al, 2004). Various volcanoes along the Central Andes have been reported to present a semi-permanent fumarolic activity, typically rich in sulfur (e.g., http://www.volcano.si.edu/ ). These volcanoes are located at high altitude (>5000 m.a.s.l.), in areas usually under the influence of strong winds. These conditions facilitate the injection of sulfur emissions in the upper troposphere and most likely spread their potential effects far downwind. However, only a few of the more than 100 volcanoes located along the Andes have been to some extent monitored regarding sulfur emissions. In Chile only three volcanoes have been partially assessed in terms of their emissions, namely the
Láscar (23.4ºS, 67.7ºW, 5592 m.a.s.l.), Lonquimay (38.4ºS, 71.6ºW, 2865 m.a.s.l.), and Villarrica (39.4ºS,
71.9ºW, 2400 m.a.s.l.) volcanoes. These volcanoes emit under fumarolic conditions about 2300, 2400 and 470
MgSO
2
/day respectively (Anders et al, 1991; Witter et al, 2001; Mather et al, 2004). These amounts are higher or of the same order of magnitude as those emitted from large copper smelters in Northern Chile and Southern
Peru, even prior to when control measures were enforced for these anthropogenic mega sources. These estimates can now be improved by the combination of such sporadic measurements and the use of state-of-thescience satellite retrievals. Carn et al (2007) have recently shown such estimates of emissions for large industrial point sources, and volcanoes for Peru, Ecuador and Colombia. South of 20S these estimates are somewhat hampered by the South American Anomaly (SAA), which is a field of trapped protons in the magnetic field of the
Earth that perturb the satellite retrievals. Nevertheless, this perturbation can be smoothed out by means of longterm averaging. This technique was applied by Dr. Carn who kindly provided us a preliminary SO
2
burden estimate for Northern Chile (Cf. Figure 3), where the presence of copper smelters and Láscar (23.4ºS, 67.7ºW,
5592 m.a.s.l.) and Lastarria (25.17S, 68.50W, 5697 m.a.s.l.) volcanoes are apparent. The pattern along the coast might be an artifact due to clouds (Carn, personal communication).
Figure 3.
Vertically integrated mass burden (expressed in Dobson Units) of sulfur dioxide as observed from the
Ozone Monitoring Instrument. Author: Simon Carn.
Considered globally, soil dust is a major contributor to aerosol loading and optical thickness, especially in sub-tropical and tropical regions (Penner et al, 2001 and references therein). Surface features play an important role in both the location of the source regions and the intensity of dust emissions (e.g., Marticorena et al, 1997). The uplifting process is parameterized using wind speed, soil water content and vegetation cover (e.g.,
Marticorena and Bergametti, 1995). The value of surface wind speed that dust starts to be raised is called the threshold wind speed and its corresponding friction velocity is called the threshold friction velocity. Most of studies on the threshold friction velocity and/or the threshold wind speed have been conducted on dust uplifts at

the Sahara desert, which differs greatly with the Atacama desert. The influence of mineral dust particles from
Atacama desert located in Northern Chile, north of 25°S is uncertain and most probably small. As stated earlier, the southern bound of the Atacama desert may show larger soil dust emissions.
In order to ascertain the nature of the CCN in the coastal stratocumulus of Northern Chile, and the origin of the gases and aerosols which serve as CCN, one has to combine sophisticated measurements and model simulations. We propose to measure, in addition wind, humidity and temperature profiles, the composition and size distribution of CCN at two coastal sites where stratocumulus are persistently present, and where two aerosol regimes can be identified , one around 22°S, and one at around 30°S, exposed to distinct signal of natural and anthropogenic aerosols. These measurements require complementary characterizations of aerosol precursors (e.g., sulfur dioxide) and particulate matter on a regional scale. In addition to this, we will measure sulfur dioxide content in volcanic emissions as derived from OMI (Ozone Monitoring Instrument) satellite timeseries data sets (Carn et al., 2007), and from in situ measurements using ground ultraviolet (UV) spectrometers
(e.g., McGonigle, 2007). These latter observations will be carried during a fieldtrip to both active volcanoes that principally contribute to SO
2 volcanic emissions in northern Chile: Lascar (23.4ºS, 67.7ºW, 5592 m.a.s.l.) and
Lastarria (25.17S, 68.50W, 5697 m.a.s.l.) volcanoes. These measurements will be carried out during a few weeks in spring 2008 and 2009 when the extent of the stratocumulus deck is at maximum.
The data collected will be further analyzed and put into a regional framework by means of high resolution meso-scale simulations using a weather forecast model as well as dispersion simulations including natural and anthropogenic oxidized sulfur sources. Satellite data (e.g., cloud droplet radii, cloud cover, liquid water path, SO
2 loading, etc.) will also complement these analyses. The model exercise will consist of regional scale weather simulations obtained from the dynamical downscaling of reanalysis fields using the techniques described by
Rummukainen al (2001). These simulations will be evaluated against remotely sensed data and in situ meteorological measurements, with emphasis in the years 2000 and 2001, when smaller campaigns have been performed (e.g., Garreaud et al, 2001, Bretherton et al, 2004). Further, these meteorological outputs will be used to drive a regional scale emission-transport-deposition model (e.g., Robertson et al, 1999; Huneeus et al, 2006) to simulate the fate of oxidized sulfur emitted from copper smelters, volcanoes, and the fraction derived from
DMS oxidation. Emission data will be compiled from official records, previous studies, and by implementing a parameterization of DMS emissions (e.g., Boucher et al, 2003). Satellite data, particularly that provided by the
Ozone Monitoring Instrument (OMI), will be used to constrain large industrial emissions and volcanoes (Carn et al, 2007). Once evaluated, the same modeling system, now using forecasted fields, will allow an interpretation and contextualization of the data collected in situ. Estimates of the contribution from other aerosol sources will be addressed by other groups participating in VOCALS.

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