A multi-scale approach to monitor urban carbon-dioxide emissions in the atmosphere over Vancouver, Canada

Abstract:

Measurements of long-lived greenhouse gases in the urban atmosphere are potentially useful to constrain and validate urban emission inventories, or space-borne remote-sensing products. We summarize and compare three different approaches, operating at different scales, that directly or indirectly identify, attribute and quantify emissions (and uptake) of carbon dioxide (CO₂) in urban environments. All three approaches are illustrated using in-situ measurements in the atmosphere in and over Vancouver, Canada.

Mobile sensing may be a promising way to quantify and map CO₂ mixing ratios at fine scales across heterogenous and complex urban environments. We developed a system for monitoring CO₂ mixing ratios at street level using a network of mobile CO₂ sensors deployable on vehicles and bikes. A total of 5 prototype sensors were built and simultaneously used in a measurement campaign across a range of urban land use types and densities within a short time frame (3 hours). The dataset is used to aid in fine scale emission mapping in combination with simultaneous tower-based flux measurements. Overall, calculated CO₂ emissions are realistic when compared against a spatially disaggregated scale emission inventory.

The second approach is based on mass flux measurements of CO₂ using a tower-based eddy covariance (EC) system. We present a continuous 7-year long dataset of CO₂ fluxes measured by EC at the 28m tall flux tower ‘Vancouver-Sunset’. We show how this dataset can be combined with turbulent source area models to quantify and partition different emission processes at the neighborhood-scale. The long-term EC measurements are within 10% of a spatially disaggregated scale emission inventory.

Thirdly, at the urban scale, we present a dataset of CO₂ mixing ratios measured using a tethered balloon system in the urban boundary layer above Vancouver. Using a simple box model, net city-scale CO₂ emissions can be determined using measured rate of change of CO₂ mixing ratios, estimated CO₂ advection and entrainment fluxes. Daily city-scale emissions totals predicted by the model are within 32% of a spatially scaled municipal greenhouse gas inventory.

In summary, combining information from different approaches and scales is a promising approach to establish long-term emission monitoring networks in cities.