Urban surface-atmosphere fluxes of pptv-level oxygenated organic molecules (OOMs) from eddy covariance observations

Abstract. Oxygenated Organic Molecules (OOMs) represent a substantial fraction of ambient reactive carbon and are potential precursors of secondary organic aerosols (SOA). Surface-atmosphere exchange flux modulates OOM budgets and subsequent SOA formation. This study presents the first urban eddy covariance measurements of pptv-level OOM surface-atmosphere fluxes, using an iodide-adduct chemical ionization mass spectrometer during the hottest month of the year in a central China megacity. We addressed the challenges of retrieving reliable fluxes for OOM species with low concentration signal-to-noise-ratios. Effects of data block averaging and water vapor dilution were investigated. We retrieved the fluxes of 16 OOMs, which displayed highly variable exchange behavior and fell into three categories: deposition-dominated, emission-dominated, and bidirectional exchange. Campaign-averaged total daily OOM deposition flux 1.64 μmol m^-2 d^-1 was 16.3% of HNO₃ deposition flux, but 4 times larger than the total daily OOM emission flux. Isoprene-derived organonitrate C₄H₇NO₅, IEPOX+ISOPOOH (C₅H₁₀O₃), formic acid, and nitrophenol C₆H₅NO₃ are identified as the dominant contributors to the total OOM fluxes. OOM fluxes at this urban site, however, were one to two orders of magnitude lower than previous flux observations above forest canopy. This work provides key methodological guidance and observational constraints for surface-atmosphere exchange of underrepresented reactive carbons.