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. During the ACRIDICON-CHUVA field project(September–October 2014; based in Manaus, Brazil) aircraft-based in situmeasurements of aerosol chemical composition were conducted in the tropicaltroposphere over the Amazon using the High Altitude and Long Range ResearchAircraft (HALO), covering altitudes from the boundary layer (BL) height up to14.4  km . The submicron non-refractory aerosol was characterized byflash-vaporization/electron impact-ionization aerosol particle massspectrometry. The results show that significant secondary organic aerosol(SOA) formation by isoprene oxidation products occurs in the uppertroposphere (UT), leading to increased organic aerosol mass concentrationsabove 10  km altitude. The median organic mass concentrations in theUT above 10  km range between 1.0 and2.5  µ g m −3 (referring to standard temperature and pressure;STP) with interquartile ranges of 0.6 to 3.2  µ g m −3 (STP),representing 78 % of the total submicron non-refractory aerosol particlemass. The presence of isoprene-epoxydiol-derived secondary organic aerosol(IEPOX-SOA) was confirmed by marker peaks in the mass spectra. We estimatethe contribution of IEPOX-SOA to the total organic aerosol in the UT to be about20 %. After isoprene emission from vegetation, oxidation processes occurat low altitudes and/or during transport to higher altitudes, which may leadto the formation of IEPOX (one oxidation product of isoprene). Reactiveuptake or condensation of IEPOX on preexisting particles leads to IEPOX-SOAformation and subsequently increasing organic mass in the UT.This organic mass increase was accompanied by an increase in the nitrate massconcentrations, most likely due to NOx production bylightning. Analysis of the ion ratio of NO+ to NO 2 + indicated that nitrate in the UT exists mainly in the form oforganic nitrate. IEPOX-SOA and organic nitrates are coincident with eachother, indicating that IEPOX-SOA forms in the UT either onacidic nitrate particles forming organic nitrates derived from IEPOX or onalready neutralized organic nitrate aerosol particles.

atmospheric chemistry and physics

Aircraft-based observations of isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA) in the tropical upper troposphere over the Amazon region