Abstract. The oxidation capacity of the highly polluted urban area of Santiago, Chile has been evaluated during a summer measurement campaign carried out from 8-20 March 2005. The hydroxyl (OH) radical budget was evaluated employing a simple quasi-photostationary-state model (PSS) constrained with simultaneous measurements of HONO, HCHO, O 3 , NO, NO 2 , j (O 1 D), j (NO 2 ), 13 alkenes and meteorological parameters. In addition, a zero dimensional photochemical box model based on the Master Chemical Mechanism (MCMv3.1) has been used to estimate production rates and total free radical budgets, including OH, HO 2 and RO 2 . Besides the above parameters, the MCM model has been constrained by the measured CO and volatile organic compounds (VOCs) including alkanes and aromatics. Both models simulate the same OH concentration during daytime indicating that the primary OH sources and sinks included in the simple PSS model predominate. Mixing ratios of the main OH radical precursors were found to be in the range 0.8-7 ppbv (HONO), 0.9-11 ppbv (HCHO) and 0-125 ppbv (O 3 ). The alkenes average mixing ratio was ∼58 ppbC accounting for ∼12% of the total identified non-methane hydrocarbons (NMHCs). During the daytime (08:00 h-19:00 h), HONO photolysis was shown to be the most important primary OH radical source comprising alone ∼55% of the total initial production rate, followed by alkene ozonolysis (∼24%) andCorrespondence to: J. Kleffmann (kleffman@uni-wuppertal.de) photolysis of HCHO (∼16%) and O 3 (∼5%). The calculated average and maximum daytime OH production rates from HONO photolysis was 1.7 ppbv h −1 and 3.1 ppbv h −1 , respectively. Based on the experimental results a strong photochemical daytime source of HONO is proposed. A detailed analysis of the sources of OH radical precursors has also been carried out.
Daily particle samples were collected in Santiago, Chile, at four urban locations from January 1, 1989, through December 31, 2001. Both fine PM with d a Ͻ2.5 m (PM 2.5 ) and coarse PM with 2.5Ͻd a Ͻ10 m (PM 2.5-10 ) were collected using dichotomous samplers. The inhalable particle fraction, PM 10 , was determined as the sum of fine and coarse concentrations. Wind speed, temperature and relative humidity (RH) were also measured continuously. Average concentrations of PM 2.5 for the 1989 -2001 period ranged from 38.5 g/m 3 to 53 g/m 3 . For PM 2.5-10 levels ranged from 35.8 -48.2 g/m 3 and for PM 10 results were 74.4 -101.2 g/m 3 across the four sites. Both annual and daily PM 2.5 and PM 10 concentration levels exceeded the U.S. National Ambient Air Quality Standards and the European Union concentration limits. Mean PM 2.5 levels during the cold season (April through September) were more than twice as high as those observed in the warm season (October through March); whereas coarse particle levels were similar in both seasons. PM concentration trends were investigated using regression models, controlling for site, weekday, month, wind speed, temperature, and RH. Results showed that PM 2.5 concentrations decreased substantially, 52% over the 12-year period (1989 -2000), whereas PM 2.5-10 concentrations increased by ϳ50% in the first 5 years and then decreased by a similar percentage over the following 7 years. These decreases were evident even after controlling for significant climatic effects. These results suggest that the pollution reduction programs developed and implemented by the Comisió n Nacional del Medio Ambiente (CONAMA) have been effective in reducing particle levels in the Santiago Metropolitan region. However, particle levels remain high and it is thus imperative that efforts to improve air quality continue.
High time resolution chemical characterization of submicron particles was carried out in the South American city of Santiago de Chile using the Aerosol Chemical Speciation Monitor (ACSM). The instrumentation operated for 100 days from August 17 th to November 23 rd 2011 in an urban station located inside the University of Santiago de Chile (USACH) campus. In addition, a semi-continuous OC/EC analyzer was also run in parallel with the ACSM for some of this time. Meteorological conditions varied along the studied period due to the transition from winter to spring time. Atmospheric temperature inversions were responsible for hourly average sub-micron particulate matter levels of up to 80 μg/m 3 , especially during the night time. The average submicron particle mass concentration (± standard deviation) for the whole period was 29.8 ± 25 μg/m 3 . Aerosol particles were composed mainly of organics 59%, followed by nitrate, ammonium, sulfate, black carbon and chloride with contributions of 14, 12, 8, 3 and 3%, respectively. Using positive matrix factorization, the organic fraction was divided into four distinct types of organic aerosol representing fresh automobile exhaust, biomass burning, and two oxygenated organic aerosol factors with different oxidation states. The transition from winter to spring was clearly seen in the composition of OA. The emissions from primary sources, such as vehicle and biomass burning, decreased in the period leading to spring, whereas the amount of oxygenated organic aerosol increased over the same time. This study shows that high time resolution measurements of aerosol chemical composition can lead to better characterizations of the evolution and sources of pollutants in an urban atmosphere.
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