Formaldehyde column density measurements as a suitable pathway to estimate near-surface ozone tendencies from space

Abstract

In support of future satellite missions that aim to address the current shortcomings in measuring air quality from space, NASA's Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field campaign was designed to enable exploration of relationships between column measurements of trace species relevant to air quality at high spatial and temporal resolution. In the DISCOVER-AQ data set, a modest correlation (r² = 0.45) between ozone (O₃) and formaldehyde (CH₂O) column densities was observed. Further analysis revealed regional variability in the O₃-CH₂O relationship, with Maryland having a strong relationship when data were viewed temporally and Houston having a strong relationship when data were viewed spatially. These differences in regional behavior are attributed to differences in volatile organic compound (VOC) emissions. In Maryland, biogenic VOCs were responsible for 28% of CH₂O formation within the boundary layer column, causing CH₂O to, in general, increase monotonically throughout the day. In Houston, persistent anthropogenic emissions dominated the local hydrocarbon environment, and no discernable diurnal trend in CH₂O was observed. Box model simulations suggested that ambient CH₂O mixing ratios have a weak diurnal trend (±20% throughout the day) due to photochemical effects, and that larger diurnal trends are associated with changes in hydrocarbon precursors. Finally, mathematical relationships were developed from first principles and were able to replicate the different behaviors seen in Maryland and Houston. While studies would be necessary to validate these results and determine the regional applicability of the O₃-CH₂O relationship, the results presented here provide compelling insight into the ability of future satellite missions to aid in monitoring near-surface air quality.