Stable carbon isotope ratios of nonmethane hydrocarbons and halocarbons in the atmosphere.

摘要

The stable carbon isotope ratio (13C/ 12C, or delta13C value) of nonmethane hydrocarbons (NMHCs) is shown to be a powerful new way to illuminate and quantify atmospheric processes. delta13C values can be used to quantify the extent of photochemical processing a compound has undergone since emission to the atmosphere. This allows us to characterize sources impacting a given site, to probe atmospheric mixing and transport, and to identify and quantify the impact of chemical reactions on atmospheric NMHC.; Compound specific delta13C values of NMHC and several halocarbons were measured in emissions to the atmosphere, and in urban, boreal, and remote air masses in both Hemispheres. For the first time the seasonal variation of isotope ratios of selected NMHCs was determined. Outlier delta 13C values observed for springtime ethane in the Arctic are compatible with Cl atom chemistry associated with tropospheric ozone depletion. Measurements of the delta13C of tropospheric ambient chloromethane allow additional constraints to be placed on the atmospheric budget of chloromethane.; In addition, a three-dimensional global chemical tracer model of the atmosphere was adapted to predict the temporal and spatial variation of the stable carbon isotope ratio of NMHCs and chloromethane. delta13C predictions allow the identification of two distinct seasonal regimes in the model, most visible at high latitudes and in locations remote from sources. From spring into summer the polar air mass behaves as an isolated system undergoing photochemical processing. In fall, model predictions can be described by relatively recent emissions progressively mixing with the summertime air mass.; To make the measurements, the analytical method (gas chromatography isotope ratio mass spectrometry, GC-IRMS) was extended to allow for the isotopic analysis of C2--C10 hydrocarbons and chloromethane in a single analysis at concentrations typically found in remote air masses, previously not achieved.