RADIOCARBON CONCENTRATION IN SHORT-LIVED HERBACEOUS PLANTS AS INDICATOR OF FOSSIL CO2 EMISSIONS IN URBAN REGIONS

Atmospheric radiocarbon (¹⁴C) levels have been used to elucidate CO₂ sources and its exchange with other carbon reservoirs. Specifically, given that fossil fuels are ¹⁴C-free due to their age, radiocarbon observations in CO₂ from urban atmospheres can be used as direct tracer for fossil fuel-derived CO₂ to estimate anthropogenic emissions. This approach has been mainly applied by ¹⁴CO₂ analysis on instantaneous flask-collected air samples or by integrated samples obtained trapping CO₂ in alkaline solution during longer time periods (weeks or months). However, both sampling approaches have limitations; the first yields samples representative of only a moment in time; and the second involves pumping large amounts of air through a concentrated alkaline solution limiting the spatial distribution of sampling sites.

Plant biomass incorporates ¹⁴C from the atmospheric CO₂ so can potentially be used as indicators of the carbon isotopic composition of the local atmosphere at either high temporal resolution using short-lived plants, or at annual resolution using annual plants and tree-rings.

The aim of this study is to evaluate widely distributed short-lived plants as cheap and easy to sample bioindicators of atmospheric ¹⁴C variations related to fossil CO₂ emissions with a high temporal resolution. We present a comparison between ¹⁴C data from integrated CO₂ samples with two species of grass (Stenotaphrum secundatum and Lolium perenne) growing at the same sampling site inside Mexico City for the period 2008 – 2011. The main results suggest that both plant species do record variations in ¹⁴C atmospheric concentrations, showing similar trends to those found for integrated CO₂ samples. This indicates that the carbon contained in leafs of both studied plant species has been recently assimilated. It can be concluded that it is possible to estimate fossil CO₂ emissions with high temporal resolution from ¹⁴C concentrations in grasses growing in urban areas.