A new method to quantify tracer dispersion in the upper troposphere

Public attention has been captured by urban pollution and wildfire smoke plumes and their adverse impacts on air quality and public health even far downstream of their origin. Downwind impacts depend on the rate at which these plumes dissipate. However, quantifying this rate using chemical transport models has proven difficult because numerical diffusion systematically overestimates plume dilution. This bias affects our understanding of non-linear chemistry, chemistry-climate coupling, as well as surface impacts. We therefore seek to constrain the rate at which an emitted mass of pollutant is diluted in the upper troposphere, by combining observations with model simulations.

The first step towards our goal is to track plumes in satellite retrievals. The task is daunting: plumes deform, split, and merge, and are at times obscured by clouds or simply out of satellites' sight. In addition, the standard practice of defining plumes as regions with pollutant concentrations greater than a preset threshold is rendered ineffective by the very dilution we aim to quantify: the threshold should change over time to reflect plume dilution, but at what rate?

To address these issues, we propose a new plume definition that incorporates both pollutant ('chemical') data and meteorological ('dynamical') data. Our approach views a plume as a collection of pollutant-enriched air masses, where each air mass is a region bounded by dynamical barriers. Because dilution is slow across such barriers, the envelope of the 'chemical-dynamical' plumes thus defined provides a spatial constraint on dilution processes. By tracking ‘chemical-dynamical’ plumes over time using Lagrangian tools, we aim to more accurately define the volume relevant to quantifying the dilution of the pollutant mass.

We present a proof of concept for our approach using simulated carbon monoxide and meteorological fields archived from the GEOS Chemical Forecast system. We show that our plume-tracking method a) reduces sensitivity to the choice of pollutant concentration threshold to define plumes, and b) can overcome the coverage limitations of pollutant data retrieved by satellite instruments. Overall, our new method represents a promising step towards quantifying tracer dilution using observations as a primary source of information.