Online measurements of cycloalkanes based on NO+ chemical ionization in proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS)

Abstract. Cycloalkanes are important trace hydrocarbons existing in the atmosphere,
and they are considered a major class of intermediate volatile organic
compounds (IVOCs). Laboratory experiments showed that the yields of
secondary organic aerosols (SOAs) from oxidation of cycloalkanes are higher than acyclic alkanes with the same carbon number. However,
measurements of cycloalkanes in the atmosphere are still challenging at
present. In this study, we show that online measurements of cycloalkanes can
be achieved using proton transfer reaction time-of-flight mass spectrometry
with NO+ chemical ionization (NO+ PTR-ToF-MS). Cyclic and bicyclic
alkanes are ionized with NO+ via hydride ion transfer, leading to major
product ions of CnH2n-1+ and
CnH2n-3+, respectively. As isomers of cycloalkanes,
alkenes undergo association reactions with major product ions of
CnH2n ⚫ (NO)+, and concentrations of 1-alkenes and
trans-2-alkenes in the atmosphere are usually significantly lower than
cycloalkanes (about 25 % and <5 %, respectively), as a result
inducing little interference with cycloalkane detection in the atmosphere.
Calibrations of various cycloalkanes show similar sensitivities associated
with small humidity dependence. Applying this method, cycloalkanes were
successfully measured at an urban site in southern China and during a
chassis dynamometer study of vehicular emissions. Concentrations of both
cyclic and bicyclic alkanes are significant in urban air and vehicular
emissions, with comparable cyclic alkanes / acyclic alkanes ratios between
urban air and gasoline vehicles. These results demonstrate that NO+
PTR-ToF-MS provides a new complementary approach for the fast characterization
of cycloalkanes in both ambient air and emission sources, which can be
helpful to fill the gap in understanding the importance of cycloalkanes in
the atmosphere.
