An atmospheric chemistry interpretation of mass scans obtained from a proton transfer mass spectrometer flown over the tropical rainforest of Surinam

Abstract

Data on a variety of organic gases are presented, obtained with a proton transfer mass spectrometer (PTR-MS) operated during the March 1998 LBA/CLAIRE airborne measurement campaign, between 60 and 12500 m over the rainforest in Surinam (2° N-5° N, 54° W-57° W). The instrument can detect molecules with a proton affinity greater than water, including alkenes, dialkenes, carbonyls, alcohols, and nitriles. Many such molecules are emitted from the rainforest (e.g., isoprene) or formed from the oxidation of primary emissions (e.g., methylvinylketone (MVK) and methacrolein (MACR)). From a comparison with modelled data; the variation with altitude; previously reported biogenic emissions and the time and location of the measurement, possible and probable identities for the significant masses encountered in the range 33-140 amu have been deduced. The main observed protonated masses, postulated identities and observed average boundary layer mixing ratios over the rainforest were: 33 methanol (1.1 nmol/mol); 42 acetonitrile (190 pmol/mol); 43 multiple possibilities (5.9 nmol/mol), 45 acetaldehyde (1.7 nmol/mol), 47 formic acid (not quantified); 59 acetone (2.9 nmol/mol), 61 acetic acid (not quantified), 63 dimethyl sulphide (DMS) (289 pmol/mol), 69 isoprene (1.7 nmol/mol), 71 MVK + MACR (1.3 nmol/mol), 73 methyl ethyl ketone (1.8 nmol/mol), 75 hydroxyacetone (606 pmol/mol), 83 C5 isoprene hydroxy carbonyls C5H8O2, methyl furan, and cis 3-hexen-1-ol (732 pmol/mol), 87 C5 carbonyls and methacrylic acid, 95 possibly 2-vinyl furan (656 pmol/mol), 97 unknown (305 pmol/mol), 99 cis hexenal (512 pmol/mol) and 101 isoprene C5 hydroperoxides (575 pmol/mol). Some species agreed well with those derived from an isoprene only photochemical model (e.g., mass 71 MVK + MACR) while others did not and were observed at higher than previously reported mixing ratios (e.g., mass 59 acetone, mass 63 DMS). Monoterpenes were not detected above the detection limit of 300 pmol/mol. Several species postulated are potentially important sources of HOx in the free troposphere, e.g., methanol, acetone, methyl ethyl ketone, methyl vinyl ketone and methacrolein.