Spatial and Temporal Variability of Carbon Dioxide using Structure Functions in Urban Areas: Insights for Future Active Remote CO2 Sensors

Abstract:

High resolution in-situ CO₂ measurements were recorded onboard the NASA P-3B during the DISCOVER-AQ (Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality) Field Campaigns during July 2011 over Washington DC/Baltimore, MD; January – February 2013 over the San Joaquin Valley, CA; September 2013 over Houston, TX; and July-August 2014 over Denver, CO. Each of these campaigns have approximately two hundred vertical soundings of CO₂ within the lower troposphere (surface to about 5 km) at 6-8 different sites in each of the urban area. In this study, we used structure function analysis, which are a useful way to quantify spatial and temporal variability, by displaying differences with average observations, to evaluate the variability of CO₂ in the 0-2 km range (representative of the planetary boundary layer). These results can then be used to provide guidance in the development of science requirements for the future ASCENDS (Active Sensing of CO₂ Emissions over Nights, Days, and Seasons) mission to measure near-surface CO₂ variability in different urban areas. We compare the observed in-situ CO₂ variability with the variability of the CO₂ column-averaged optical depths in the 0-1 km and 0-3.5 km altitude ranges in the four geographically different urban areas, using vertical weighting functions for potential future ASCENDS lidar CO₂ sensors operating in the 1.57 and 2.05 μm measurement regions. In addition to determining the natural variability of CO₂ near the surface and in the column, radiocarbon and anthropogenic pollution tracers are used to examine the variation of emission sources among these urban sites.