Urban surface-atmosphere fluxes of selected 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 mass spectral integration time and water vapor variation on flux estimates 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 daily deposition flux 4.6 µmol m −2  d −1 of 16 OOM was 16.3 % of HNO 3 deposition flux, but was higher than daily emission flux of OOMs by a factor of 2.2. Formic acid accounted for 66.2 % and 84.9 % of the summed deposition and emission fluxes of the 16 OOMs, respectively. Other appreciable contributors included isoprene-derived organonitrate C 4 H 7 NO 5 , peracetic acid C 2 H 4 O 3 , and nitrophenol C 6 H 5 NO 3 . OOM fluxes at this urban site were generally lower than those previously reported above forest canopies, although the differences were typically within one order of magnitude. This work provides key methodological guidance and observational constraints for surface-atmosphere exchange of underrepresented reactive carbons.