Summertime contributions of isoprene, monoterpenes, and sesquiterpene oxidation to the formation of secondary organic aerosol in the troposphere over Mt. Tai, Central East China during MTX2006

Abstract. To better understand the contribution of biogenic volatile organic compounds to the formation of secondary organic aerosol (SOA) in high mountain regions, ambient aerosols were collected at the summit of Mt. Tai (1534 m, a.s.l.), Central East China (CEC) during the Mount Tai eXperiment 2006 campaign (MTX2006) in early summer. Biogenic SOA tracers of isoprene, monoterpenes, and β-caryophyllene oxidation products were measured using gas chromatography/mass spectrometry. All the biogenic SOA tracers showed no clear diurnal variations, suggesting that they are formed during long-range atmospheric transport. Although isoprene- and monoterpene-derived SOA tracers did not correlate with levoglucosan (a biomass burning tracer), β-caryophyllinic acid showed a good correlation with levoglucosan, indicating that biomass burning may be a source for this compound. Total concentrations of isoprene oxidation products are much higher than those of monoterpene and β-caryophyllene oxidation products. The ratio of isoprene to monoterpene oxidation products (R_iso/mono) was found to co-vary with ozone and NO_x during the summer campaign. The average R_iso/mono value was 6.94 at daytime and 10.0 at nighttime. These values are among the highest in the aerosols studied in different regions, which may be due to the large isoprene fluxes, high O₃ and NO_x levels and relatively high OH concentrations in CEC. Using a tracer-based method, we estimated the average concentrations of secondary organic carbon (SOC) derived from isoprene, monoterpenes, and β-caryophyllene to be 1.76 μgC m⁻³ at daytime and 1.85 μgC m⁻³ at nighttime. These values correspond to 11.2% and 11.0% of the total OC concentrations, in which isoprene-derived SOC are 7.4% and 8.0% at day- and night-time, respectively. This study suggests that isoprene is a more significant precursor for biogenic SOA than monoterpenes and β-caryophyllene in high altitude in CEC.