SO2 and aerosol evolution over the very clear atmosphere at the Argentina Andes range sites of San Antonio de los Cobres and El Leoncito

The atmosphere at North-Central Argentina Andes range is exceptionally clear for the placement of astrophysical/astronomical/solar observatories (Piacentini et al, Advances Space Research, 2016). However, this region is part of the Pacific fire belt, due to the large number of active volcanoes. Consequently, the possibility of having strong sporadic emissions of different gases and aerosols needs to be investigated. In the present work, we analyze in particular the SO2 trace gas, since it can affect significantly the solar UVB (280-320 nm) radiation. Also, particulate matter can attenuate this radiation in the UV-visible ranges. One of the most significant contributions to volcanic eruptions that could arrive at the selected San Antonio de los Cobres (SAC) location is the near Lascar volcano. We used satellite images form the OMI/KMNI/Aura/NASA satellite instrument, for deriving the intensity of the eruption at the SAC geographical point. An important eruption was that of the Puyehue/CordónCaulle volcanic complex at Chile Patagonia, in June 2011. No significant influence on the other selected El Leoncito (LEO) location was registered. We present aerosol optical depth (AOD550) satellite data obtained with the Deep Blue Level 2 data provided by the SeaWiFS/SeaStar/NASA satellite instrument for SAC and LEO places, showing that AOD550 for the whole period is extremely low (0.0262 for SAC and 0.0266 for LEO). We also present ground atmospheric aerosol concentration measurements as a function of aerosol diameter with a high quality GRIMM laser instrument for some days of campaing performed in those sites.


Introduction
In the last decades, an important number of large projects for astronomy/astrophysics/solar observatories, have required isolated high altitude sites. These sites ensure skies free of light pollution, and a low aerosol content for observations in very high energy (GeV, TeV and even more) range. Examples of big astrophysics facilities are the Pierre Auger Observatory, for the detection and study of Ultra High Energy Cosmic Rays [1] and the Cherencov Telescope Array (CTA), for the research in high energy Gamma Rays energies [2].Adding the advantage of having a low humidity atmospheric content, observations in microwaves and radiofrequency ranges can be done, such as the Large Latin American Millimeter Array-LLAMA [3] and the Q&U-Bolometric Interferometer for Cosmology (QUBIC) [4] projects. In all these cases, the studies are performed according to the different needs, ensuring well-relieved sites that can be used or offered by the countries as host sites. The mega-astrophysics projects require the joint effort of several countries that contribute to them, involving economic and human resources to reduce risks and ensuring the possibility of installation of other new projects in the characterized area, which can take advantage of existing infrastructure. The emplacement of these facilities contributes to the development of science but also constitutes a benefit for the region by generating jobs and by establishing a synergic relationship with the local community.
The atmosphere at the North and Central Argentinean Andes range is exceptionally clear and very adaptable for the placement of astrophysical/astronomical/solar observatories, as was shown by Piacentini et al [5], since the atmospheric components (gases and aerosols) are present in a very small proportion, with respect to many other places in the world where this type of observatories are placed. However, this region is part of the Pacific fire belt, composed of a large number of volcanoes, some of them very active. Consequently, the possibility of emissions of different gases and of particulate matter (or aerosol) that could contaminate the selected places has to be considered.
In the present work, we analyze in particular the sulfate dioxide (SO2) trace gas, since it can affect significantly the solar UVB (280-320 nm) radiation, due to its large quantum efficiency for the solar photon attenuation in this spectral range. Also, the particulate matter (coming from this and other sources, like biomass burning, traffic, intensive agriculture, etc.) attenuates this radiation in the UV and Visible ranges, as can be seen in photo-images of strong eruptions. Both atmospheric components are good indicators of volcanic activity, since the corresponding aerosol cloud can be detected if it goes in the direction of the analyzed site (see for example references [6][7][8][9][10][11][12]).
Other implications of SO2 and sulfate aerosol propagations for the air quality as well as for possible environmental and economical consequences have been described in the works of Yeh et.al. [13] and Wang et.al. [14].

Measurements
In the past, the source of atmospheric SO2 was mainly the volcanic eruptions, but with the increase in human activities, the contribution of artificial sources of SO2 concentration started to increase significantly, being at present about ¾ of the total (Max Plank Institute for Chemistry, Satellite Group, Mainz, Germany: http://joseba.mpch-mainz.mpg.de/so2t2.htm). The main sources are: fossil fuel consumptions, smelting of metal sulfide ores, coal burning and oxidation of soil organic material. Volcanoes also eject particulate matter which can arrive at the tropopause or even the lower stratosphere if the eruption has enough energy, as was the case for Pinatubo eruption in June 1991 [15].
We analyzed with satellite and ground instruments the air quality at two Argentina Andes range sites, in order to determine the possible arrival of gas (SO2) and aerosol clouds produced by volcanic eruptions. Concerning the selected places, they are the following: • San Antonio de los Cobres (24°02'42.7" S, 66°14'05.8" W, 3607 m asl), Province of Salta, that we will call SAC. It is placed in the highest altitude desert of the world, called Puna of Atacama in the North region of Argentina East Andes range.
• Complejo Astronómico El Leoncito (CASLEO) (31°04'48" S, 69°16'12" W, 2672 m asl), Province of San Juan, that we will call LEO. It is located in a flat region called Pampa El Leoncito, between high mountains of the Central region of Argentina East Andes range.
One of the most significant volcanic eruptions that could arrive at the SAC location is that produced by the near Lascar volcano (23° 22´ S, 67° 43´W, 5592 m asl) and eventually transported to the SAC site by winds flowing from North-East to South-West. One of its latest strong eruptions occurred during the period from 18 to 26 April 1993. Other more recent minor eruptions were

Method
The satellite data corresponding to the different atmospheric variables introduced in item 2.1 (AOD500, AOD550 and AI) being physical quantities, can be positive or zero. Consequently, they are distributed around their mean values following a Poisson type statistical distribution (see for example: www.statisticshowto.com/what-is-standard-deviation/), but not a Gaussian (normal) one, since the last one represents a series of values that, in theory could extend even up to minus infinity (in practice, large negative values). Since we are interested in the detection of few but important events (large gas and aerosol clouds produced by volcanic eruptions and arriving at the selected sites), we introduce the following significant events criteria: Vlimit,,L,X =V*L,X + 2.σP (1) where Vlimit,L,X is the lower limiting value of all the registered ones, that are equal or higher than the mean value V*L,X for a given instrument L and location X, plus two standard deviations of the Poisson distribution (σP). The Poisson distribution at a value as given by formula (1), has only several percents of the measured values higher that the imposed limit. So, it is possible to determine in this way the annual frequency of events (mean number of events per year), that we call N*L,X, counting only the variable values equal or larger than Vlimit,L,X, as given by formula (1).

Results
We present results in the following items: satellite measurements of the SO2 cloud evolution (item 3.1) and ground and satellite measurements of the particulate matter cloud propagation over the selected SAC and LEO sites (item 3.2).

Satellite measurements of the SO2 cloud evolution
One important indicator of volcanic eruptions is the emission of SO2 clouds that normally goes up to about several kilometers high and propagates, driven by wind speed and direction at these altitudes. We will analyze this gas emission, from the SO2 total column data obtained by the OMI instrument on board of Aura NASA satellite, during the 2005-2009 period.

Ground and satellite measurements of particulate matter cloud propagation
In what follows, we will present results corresponding to particulate matter (aerosol) propagation up to the selected sites of SAC and LEO.

Ground measurements with aerosol spectrometer at LEO site
The aerosol content analysis was made performing concentration measurements near surface, using a GRIMM aerosol-spectrometer, model 1,109 (manufactured by GRIMM Aerosol Technik GmbH & Co) that belongs to the Institute of Physics Rosario (IFIR, CONICET -National University of Rosario). This device analyzes the fraction PM>0.22 and has a double measuring technique: • A mass of air (charged with aerosols) enters the device and interacts with a laser of 655 nm wavelength. By detecting the scattering signal and internal modeling, it counts particles, determining its size and mass in 31 size intervals (or channels) within the range [0.22 µm -32.0 µm]. There is a final channel for large particles >32.0 µm, but even if the GRIMM instrument cannot determine their size, it can calculate its concentration. The aerosol number (aerosol mass) per unit air volume, corresponding to these 32 size channels are stored in a memory card every one minute. • Aerosols are collected on PTFE (Poly-tetrafluoroethylene) filters for further analysis.
The GRIMM aerosol spectrometer can measure mass concentration (total aerosol mass per unit volume of air) or particle concentration (total particle number per unit volume of air) directly, but it cannot measure in both modes, at the same time. In the present study, we measured aerosol mass concentration. It is possible to transform mass concentration into particle number concentration, making some assumptions: particles are spherical, the aerosol diameter on each size channel is the average value of the corresponding channel and the mass density is assumed to be a mean value of 1.68 g/cm3 [16].
Concentration measurements in LEO site were made from 28 December 2012 (5:36 pm local time = UT -3 hours) to 4 January 2013 (5:37 pm). In SAC site, measurements took place from 6 May 2013 (3:32 pm) to 10 May 2013 (9:16 am).Concentration measurements were made in a period when no volcanic activity or other high contamination events were detected near these locations, so the results presented would show a typical situation for the aerosol content on these sites. As mentioned before SAC and LEO sites. As mentioned before, the sites analyzed are rural sites far from large sources of anthropogenic pollution as well as the resources and instruments needed to perform ground measurements. With these complicated logistics necessary to carry out ground measurements the time extension of each was an important factor to consider, being only possible to perform continouos measurements for several days.  The normalized particle concentration is presented in Figure 3. As expected, there is a large number of PM>10 particles in LEO than in SAC, showing again the PM>10 mode. As it is shown in Figure 3, for small particles, the size distribution for both sites is very similar. A notorious difference appears for LEO, at large particles. In table 2 percentages values of the total particle concentration (PM>0.22) are presented for PM2.5, PM2.5-10, PM10 and PM>10 fractions.  In a previous work [5], we determined with the same GRIMM instrument, a total particle number concentration of (2,800 ± 140) particles/cm 3 in SAC and of (5,860 ± 140) particles/cm 3

Satellite measurementsof aerosol optical depth at SAC and LEO sites
The solar photon attenuation (absorption + dispersion) by atmospheric aerosols described by the aerosol optical depth (AOD) for a given wavelength has been measured all over the world and particularly at the SAC and LEO sites, by SeaWiFS instrument on board of SeaStar/NASA satellite, through the improved Deep Blue algorithm, level 3 products, with a 1° x 1° pixel resolution. This instrument is described in detail in the corresponding NASA web page: hthttp://oceancolor.gsfc.nasa.gov/SeaWiFS/SEASTAR/SPACECRAFT.html.  Figure 4 (at bottom), the limiting value is AOD550,limit,LEO = 0.353 and the mean number of events in the same period N*SO2,LEO is also null.

Satellite measurements of UV aerosol index at SAC and LEO sites
A large series of measurements of the Aerosol Index (AI), -a variant of the AOD in the UV solar range, have been obtained by the exceptional TOMS (Total Ozone Mapping Spectrometer) and OMI (Ozone Monitoring Instrument) equipments on board of different NASA satellites. In particular TOMS instrument details are given in https://science.nasa.gov/missions/toms. Figure 5 shows AI for the SAC place in the 1998-2016 period, having a mean value AI*TOMS/OMI,SAC = 0.91 (with a Poisson standard deviation of 0.95). Consequently, the limiting value given by formula  It is of interest to note that in the 1990´s decade, the TOMS instrument detected a significant eruptive event that corresponds to Lascar volcano, around 18 April 1993. Figure 6 shows

Frequency of events from different satellite data at SAC and LEO sites
From the different satellite sources of information (SO2 total column, AOD550 and AI), we have the possibility to derive a mean value for the frequency of significant events, as it is shown in Figure 8. The

Discussions and conclusions
SO2 and aerosol clouds generated by volcanic eruptions can propagate due to winds, influencing the air quality of particular geographical places. So, they can be used as a proxy for the determination of significant events. All the analyzed data show that SAC and LEO sites placed at the Argentina Andes range, are very little influenced by volcanic eruptions. These conclusions are supported by ground as well as satellite data. In particular, the following conclusions can be established: • SO2 total column at SAC and LEO sites presents a quite small number of events per year, only a mean of 3.40 and 3.60, respectively.
• Aerosol index also shows a small mean number of events per year (0.50 and 1.53 for SAC and LEO, respectively) and Aerosol optical depth a null result for both sites.
• The general mean frequency of events for each site is: 1.30 events/year for SAC and 1.71 events/year for LEO. These last results imply that less than 0.5 % of the days in a year can be affected by gas or aerosol clouds due to volcanic eruptions (or other significant atmospheric events). In comparison, in observatories placed in desertic regions (or near them) and mainly due to the blow up of the sand by the wind, there is normally much more significant events that those in the Andes range. For example, at the Roque de los Muchachos observatory in La Palma, Canary Islands, in 1984 the Saharan dust period over this site lasted from June 10 to August 30, affecting 45 % of the nights and in 1985 from June 5 to September 5, affecting 35% of the nights [14].
• Concerning the Visibility variable of a given sky, the present results are a reference for the situation of the analyzed places in the time interval considered in the present work, since this variable (measured in Km) is inversely related to the aerosol content of the atmosphere (as given by the Koschmieder equation [17]). In particular, the mean AOD550 measured by SeaWiFS/SeaStar/NASA satellite instrument is very low: AOD*SW,SAC = 0.0262 for SAC and AOD*SW,LEO= 0.0266 for LEO, corresponding to a quite high Visibility (greater than 50 Km), as is confirmed from direct visual observation in the sites (verified during the measurement campaigns).
• When compared with places around the world were astronomical/astrophysical/solar observatories are situated, the SAC and LEO sites are in the range of those with the lowest aerosol content [5], even if these sites are near active volcanoes. Another confirmation of this assessment is given by the volcanic dust database at the Servicio Meteorológico Nacional of Argentina, that in 5 years (2011 -2016 period) informed of only one significant dust deposition event in San Juan Province (where the LEO site is located) and no dust deposition in Salta Province (where SAC is located, www.smn.gov.ar/vaac/buenosaires/cenizaenargentina.php?lang=es).
Future work will be oriented to the analysis of other Argentina Andes range sites for the same purpose, the placement of Observatories and also of Solar power plants.