Volatile Organic Compounds measurement in tropical and temperate climates: Concentration levels in urban and rural areas

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Rapid urbanization and economic development in past decades have led to air pollution problems such as excess of nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs) and Semivolatile Organic Compounds (SVOCs). Motor vehicle exhaust, industrial emissions, chemical solvents as well as biogenic emissions are the major anthropogenic sources of VOCs. Depending on the area where the measurements are carried out. the external conditions and pollutants can be very different. For instance in Thailand, the air is hot and very humid and pollutant concentrations can vary from very high near cities to very low by the sea. The goal of tins study is to identify and quantify automatically and continuously VOCs and SVOCs in tropical and temperate climates in different environments (industrial area, sea side, city center, forest) using the same monitoring system. The types of VOCs and SMVOCs emitted in each places is studied and compared. Two automated Thermal-Desorber Gas Chromatographs equipped with Flame Ionization Detectors (auto TDGC-FID) were used to monitor continuously the concentrations of VOCs and SMVOCs in June 2014 in Thailand, in France and in the United States of America.

First a laboratory study has been carried out to optimize the system before the measurement campaigns. One analyzer was dedicated to the monitoring of C2 to C6 (airmoVOC C2-C6, Chromatotec, France) and another for C6 to C12-i- (airmoVOC C6-C12, Chromatotec, France). The time, gas flow and temperature for the pre-concentration of the sample haw been adjusted independently for both analyzers. Columns, pressure and temperature gradient have been optimized to obtain a good separation for more than 100 VOCs. The identification of all compounds was confirmed using a Quadrupole Mass Spectrometer (DET QMS. Chromatotec. France) connected to the end of the chromatographic columns, in parallel to the FID detectors. The multiplexing system and software integrated within the DET QMS allow for indentifying and quantifying continuously VOCs and SMVOCs samples from airmoVOC C2-C6 and aimoVOC C6-C12 simultaneously Finally humid and dried samples were produced to check the efficiency of the drying systems and evaluate the impact of humidify on VOCs adsorption, pre-concentration and modification of retention times due to matrix effects.

Then the field deployable system has been installed in the different places where VOCs and SMVOCs have been monitored for one week. Ethylbenzene, m-xylene. p-xylene and o-xylene (EX) were found to be the most abundant molecules m the ambient air near cities. Benzene concentrations were much lower than EX. Hydrocarbons emitted by trees such as isoprene or terpenes were found to be insignificant at tins time of the year near the cities (less than 1% of the total emission) whereas they account for 80 % of emissions near the forests in France. For one measuring station, the VOC, SMVOCs as well as sulfur compound concentrations measured continuously were coupled to a weather station. Then data provided by analyzers and local weather tower feed a modeling software used to evaluate chemical and odor impact of the site on neighborhood. The web interface integrates in addition chemical data coming from other analyzers and odor complaints to establish correlation between forecasts and real perception of the vicinity to validate results.

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