Zhaoyang Meng

Coupling of comprehensive two-dimensional gas chromatography with quadrupole mass spectrometry: application to the identification of atmospheric volatile organic compounds.

Observation data of atmospheric volatile organic compounds (VOCs) are highly needed in air quality assessment, photochemical mechanism study, and emission control policy-making, while it has been a challenge to accurately and comprehensively measure them. Comprehensive two-dimensional gas chromatography (GC×GC) is one of the advanced techniques in analysis of complex mixtures, providing a good choice for measurement of VOCs. However, the requirement for a fast detector limits the application of quadrupole mass spectrometry (qMS) in GC×GC analysis. This paper presents a method of a common qMS detector coupled with GC×GC to the identification of atmospheric VOCs. About 125 VOCs including alkanes, alkenes, aromatics, oxygenated hydrocarbons, and halocarbons were identified in the measurement of standard gas mixtures and/or urban air samples from Beijing. The results were applied to the analysis of GC×GC-FID by one to one correspondence of peaks of the equivalent compounds between the GC×GC-FID and GC×GC-qMS chromatograms, and the retention times of the identified components in GC×GC-FID in turn undertake the qualitative analysis without the further help of MS. The wrap-around phenomenon which may confuse the match of peaks was discussed in detail. The cooperation of GC×GC-FID and GC×GC-qMS which assists the identification makes the GC×GC exploration more affordable and yet practical on both qualitative and quantitative analysis. The method and the identified results can be expanded to analyze other volatiles.

Summertime ambient ammonia and its effects on ammonium aerosol in urban Beijing, China

To improve the understanding of NH3 variation in urban Beijing, high temporal resolution (1-min averaged) NH3 data derived during the summer of 2009 were analyzed. The results indicated that after rain there was generally an increase in NH3 concentration. The analysis of the 1-min data revealed that a weak but significant linear correlation existed between NH3 and NOx in August, indicating a connection between NH3 concentrations and traffic levels during summer. This was further supported by peak NH3 levels approximately coinciding with morning rush hours. The daily NH3 concentrations were weakly correlated with NH4+ concentrations, suggesting that NH3 played an important precursor role in NH4+ in PM2.5 formation. The mean mass ratio of NH3/NHx was 0.76±0.13. This revealed that NHx (NH3+NH4+) was influenced by local sources during the summer period and that NH3 dry deposition could contribute substantially to NHx deposition. A high temperature, relative humidity, and degree of oxidization could increase photochemical reactions and aqueous processing, having the important effect of SO2 to SO42- and NH3 to NH4+ conversion in summer. The back trajectory analysis indicates that the transport of air masses from the North China Plain region contributed to the atmospheric NH3 and NH4+ aerosol variations in Beijing.