Study of Some Aerosol Features in Buenos Aires

We present the analysis of in-situ measurements obtained in an experimental campaign in Buenos Aires. Observations of total condensation nuclei (CN), black carbon (BC), polycyclic aromatic hydrocarbon concentration in particulate phase (PPAH), vertical profiles of aerosol light backscattering and meteorological parameters, were obtained at a coastal site during a one-year period. The study provides detailed and novel information about the contributions to the aerosol load in the atmosphere of Buenos Aires due to the combined act ion of a variety of sources and dispersion mechanisms. Furthermore, they provide a unique dataset to evaluate modeling tools.


Introduction
Buenos Aires (34º38'S, 58º28'W) is a megacity situated on the right coast of the Rio de la Plata. With a surface of nearly 3800 km 2 , it has a population of 11 million inhabitants and ranks as the third in Lat in America. The air pollution in the city is due both to the local sources and regional or remote contributions. In particular, the aerosol load in the urban at mosphere might be an issue of concern not only in the short term as regards public health or visibility, but also by the possible impacts on climate and the hydrological cycle.
A mon g th e stud ies th at repo rted measuremen ts o f part icles, A rkou li et al. (2010) p erfo rmed a on e year campaign and the obtained PM10 and PM2.5 values were higher fro m Ju ly to September. The PM10 daily values were g reater than the European Union limit on 36 occasions. The World Health Organizat ion (WHO) guideline value for daily PM 2.5 was exceeded 21 t imes. As regards the annual averages, the PM10 mean was almost 70% of the Buenos Aires limit and the PM 2.5 annual mean was similar to the limit . Th e annual averag es were above th e W HO air quality gu idelines. Another study repo rted t wenty-four hour averages of PM10 and PM2.5 measured (not simu ltaneously) at a site near downtown Buenos Aires. The values of PM 2.5 concentrations correlated well with the concentrations of carbon monoxide during winter, indicating that direct traffic emissions have an important contribution to PM2.5.
The data were less correlated for PM10, indicat ing that coarse fraction has an important contribution of other sources, for examp le re-suspended material [2].
These works represent the impact of the local sources, mainly the traffic as well as the thermo electrical plants and industries. In addition, the regional pollut ion is related to biomass burning, sea salt and dust. On the side of natural aerosols like sea salt, Dos Santos et al. (2012) found robust evidence that the marine aerosol fro m the South Atlantic Ocean reaches the city of Buenos Aires. Eventually, the western region of South America has also an important number o f active volcanoes that can erupt and release to the atmosphere a variety of pollutants, which can travel far away transported by the free atmosphere winds.
The contribution of biomass burning is more noticeable fro m August to October. These months correspond to the main burn ing activit ies in central South America (northern Argentina, Paraguay, Bo liv ia and central Brazil) [4,5,6]. However, as the practice is common in the agricultural sector, there are also contributions from nearby locations in Argentina [7,8] The latter are most frequent during the austral autumn. Light absorbing carbon, namely black carbon (BC), is mainly due to incomplete co mbustion in a variety of sources. Ep idemio logical studies have suggested that it is associated with increased cardiovascular and respiratory effects. As BC is chemically inert and dry deposition does have little effect on its removal, airborne BC is found not only close to combustion sources but also in areas remote fro m these sources. Similar to BC, particle-bound polycyclic aro mat ic hydrocarbons (PPAHs) are products of inco mplete combustion. Polycyclic aro matic hydrocarbons (PAHs) are semi-volatile organic contaminants that may cause a wide range of effects due to their carcinogenic and mutagenic properties. Atmospheric PAHs are in particu late and gaseous phases. However, the carcinogenic species are predominantl y associated with particles, especially those in the accumulat ion mode. PPAHs have also been associated with induction of oxidative stress and acute respiratory responses. Understanding the short-term temporal behavior between these two co mponents of particulate matter may help to advance in the knowledge of their contribution to local air quality and the mitigation actions.
A research team fro m Un iversidad de Buenos Aires and Universidad Nacional Autónoma de México conducted a collaborative effort during 2011. A suite of sensors was installed at the site of Ciudad Universitaria, Buenos Aires (34º35' S, 58º22'W), and continuous measurements of some properties of aerosols were made. Figure 1 shows the location of the building and the most important nearby sources.
A highway that links the downtown reg ion with the suburban zone passes at about 500 m fro m the building. It is heavily used during the day by cars and buses and the diurnal variation is in accordance to the office hours on workdays. There are also an important number of t rucks related to the transportation of goods to/from the dock. The domestic/regional airport is at 3 km fro m the measurement site. Farther south are situated the thermal power p lants (at about 7 km).
The aims of the experimental campaign were: (1) to provide the first comp rehensive assessment of optical and physical properties of particle aerosols in the city, (2) to study the evolution of these properties in relation with the synoptic situation, atmospheric boundary layer structure and emission sources and (3) to evaluate direct and indirect modelling systems. The results will help to estimate the possible impacts on health, visib ility, h idrological cycle and climate system. Table 1 details the equip ment that provided the data reported in this paper. The suite was fully operative fro m April to December 2011.

Data and Methodology
The measurements fro m the condensation nuclei counter, nephelometer and particle soot absorption photometer were recorded every second. The mass concentration of particle bound polycyclic aro matic hydrocarbons was recorded every 5 s. The weather meteorological station measured the state parameters and wind and the recording rate was every minute.
Black carbon (BC) mass concentration was derived from the absorption coefficient, B abs , using the factory recommended specific absorption coefficient of 10 m μg −1 .
This conversion factor was applied after the absorption coefficients had been corrected to account for d ifferences fro m factory specifications in deposit area and flow rate and for the effects of light scattering. The reported concentrations from the PPAH analyzer are discussed with respect to relative changes because they were not co mpared with an analytical method, as reco mmended by the manufacturer.  values are within the order of magnitude than similar locations worldwide [9].

Results and Discussion
In general, the measurements show the higher values during winter suggesting that the health risk due to exposure in winter is higher than in summer. During the cold period, the concentrations exh ibited a high variab ility. The seasonal behaviour is similar to the variations in aerosol properties observed elsewhere [9]. These variat ions are likely due to a combination of changes in emissions rates and meteorology. In winter the increase in anthropogenic emissions associated with domestic heating combine with unfavourable dilution ability mainly due to shallow at mospheric boundary layer [10]. The variab ility is mainly related to the mid-latitude transient systems that reach the city more frequently during the cold season. Another source of variability was the arrival of the ash plu me fro m the Puyehue-Cordon Caulle in Chile that erupted on 4 June and reached the city several times till October. Figure 3 shows the relationships between the concentrations of BC, PPAH and CN for d ifferent wind sectors: NE (fro m the de La Plata River), SE (fro m the power plants and industries), SW (fro m the city and suburbs) and NW (fro m the northern reg ion of the city). In the NW sector the variables are well correlated and have the largest concentrations, in the SE sector relat ively lo wer BC and PPAH concentrations are registered; the SW sector has a bifurcation for high CN values and the NE sector corresponds to clean air masses. High concentrations of CN, PPAH, BC are associated with winds fro m W and NW, which points to urban common sources. Intermediate BC concentrations, low CN concentrations and very low PPAH concentrations are present with variable wind direct ions. Strong easterly winds that bring clean air fro m the La Plata River are related to intermed iate BC, very low CN and PPAH concentrations.  The ceilo meter measurements provided the vertical distribution of aerosols and the boundary layer diurnal variation ( Figure 5). An interesting feature is that the intrusion of the volcanic ash plu me fro m the Puyehue-Cordon Caulle was detected (days 159 and 165). There is also excellent agreement with the aerosol optical thickness derived fro m the AERONET site (A Erosol RObotic NETwork) fro m NASA (National At mospheric and Science Ad min istration) at CEILAP-BA (34.5º S, 58º W) (http://aeronet.gsfc.nasa.gov).

Conclusions
The condensation nuclei (CN) reached 60000 cm -3 , black carbon (BC) peaked at 800 ng m -3 and polycyclic aromat ic hydrocarbon (PPAH) concentration in part iculate phase exceeded 500 ng m -3 . These values are as large as those often found in other urban areas with populations much larger than in Buenos Aires. The different relationships between pollutants according to the wind sector clearly show the influence of sources. To the knowledge of the authors this is the first observational campaign that reports the concentrations of CN, BC and PPAH in Buenos Aires, for nearly a year period. The adverse effects of BC and PPAH demand this type of characterization to contribute to manage local, regional as well as global air pollution control strategies.