Fig. 3: Multiphase production of formic acid in the SAPHIR chamber.
From: Ubiquitous atmospheric production of organic acids mediated by cloud droplets
The formaldehyde (HCHO) mixing ratio was measured (in parts per billion by volume, ppbv) by differential optical absorption spectroscopy (black), whereas the sum of HCHO and methanediol (HOCH2OH) was measured using the Hantzsch method. The difference between the Hantzsch and differential optical absorption spectroscopy signals enables visualization of HOCH2OH (blue). Formic acid (HCOOH) was monitored by using proton-transfer reaction time-of-flight mass spectrometry (red). The instrument uncertainties (shading) are 5% for HCHO, 12% for HOCH2OH and 20% for HCOOH. On injection of the formalin (stabilized formaldehyde) solution into the Teflon chamber, HOCH2OH immediately outgasses from the droplets. The chamber roof is initially closed (stage I). The gas-phase HCHO mixing ratio is initially very low, but increases to be as abundant as HOCH2OH just before the start of the photo-oxidation when the roof is opened (stage II). The decay of the HCHO and HOCH2OH signals is concurrent with an additional production of HCOOH. Finally, addition of carbon monoxide (CO) as an OH scavenger enabled quantification of the wall effects (stage III). Experimental details are provided in Supplementary Information, sections 1 and 4.
