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

Abstract

Abstract. During the ACRIDICON-CHUVA field project (September -October 2014; based in Manaus, Brazil) aircraftbased in-situ measurements of aerosol chemical composition were conducted in the tropical troposphere over the Amazon using the High Altitude and Long Range Research Aircraft (HALO), covering altitudes from the boundary layer height up to 14.4 km. The submicron non-refractory aerosol was characterized by flash-vaporization/electron impact-ionization aerosol particle mass spectrometry. The results show that significant secondary organic aerosol (SOA) formation by isoprene oxidation 5 products occurs in the upper troposphere, leading to increased organic aerosol mass concentrations above 10 km altitude.The median organic mass concentrations in the upper troposphere above 10 km range between 1.0 and 2.1 µg m −3 (referring to standard temperature and pressure; STP) with interquartile ranges of 0.6 to 3.0 µg m −3 (STP), representing 70 % of the total submicron non-refractory aerosol particle mass. The presence of isoprene epoxydiol-derived isoprene secondary organic aerosol (IEPOX-SOA) was confirmed by marker peaks in the mass spectra. We estimate the contribution of IEPOX-SOA to the 10 total organic aerosol in the upper troposphere to be about 20 %. After isoprene emission from vegetation, oxidation processes 1 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-232 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 9 April 2018 c Author(s) 2018. CC BY 4.0 License. occur at low altitudes and/or during transport to higher altitudes, which may lead to the formation of IEPOX (one oxidation product of isoprene). Reactive uptake or condensation of IEPOX on pre-existing particles leads to IEPOX-SOA formation and subsequently increasing organic mass in the upper troposphere. This organic mass increase was accompanied by an increase of the nitrate mass concentrations, most likely due to NO x production by lightning. We further found that the ammonium contained in the aerosol particles is not sufficient to neutralize the particulate sulfate and nitrate. Analysis of the ion ratio of 5 NO + to NO + 2 indicated that nitrate in the upper troposphere exists mainly in the form of organic nitrate. IEPOX-SOA and organic nitrates are coincident with each other, indicating that IEPOX-SOA forms in the upper troposphere either on acidic nitrate particles forming organic nitrates derived from IEPOX or on already neutralized organic nitrate aerosol particles.

Aircraft Observations of Aerosol in the Manaus Urban Plume and Surrounding Tropical Forest during GoAmazon 2014/15

“…Nitrate concentrations are significantly lower than sulfate concentrations, but tend to correlate more strongly with 25 organic concentrations than sulfate concentrations. There are indications that a portion of the nitrate mass exists as organic nitrates, consistent with measurements at the T3 site and with aircraft measurements (Allan et al, 2014;de Sá et al, 2018;Schulz et al, 2018 Figure 4 shows the fractional composition of the aerosol, which helps to illustrate the relative differences among the aerosol species. Organic species clearly dominate at all points, with sulfate the next most important species.…”

Aerosol characteristics and particle production in the upper troposphere over the Amazon Basin

“…Most of the UT organics are aged and oxidized, but in some of the CN-enriched layers, younger and less oxidized OA was evidenced by much lower f 44 /f 43 ratios. A detailed discussion of the AMS measurements during ACRIDICON-CHUVA will be presented in Schulz et al (2018).…”

African volcanic emissions influencing atmospheric aerosols over the Amazon rain forest

“…To investigate the probability of the volcanic sulfate plume reaching the ATTO site, trajectories were calculated using the National Oceanic and Atmospheric Administration (NOAA) hybrid single-particle Lagrangian integrated trajectory HYSPLIT model Hess, 1997, 1998;Stein et al, 2015). NOAA Global Data Assimilation System (GDAS; Kleist et al, 2009) data at 1 • × 1 • resolution were used as meteorological input for HYSPLIT.…”

“…As discussed above, the locations along the trajectory should be interpreted as the centers of a plume, which had dispersed both vertically and horizontally. This dispersion is not considered in the trajectory mode of HYSPLIT (Stein et al, 2015), but of course takes place in reality by turbulent and convective processes. Consequently, a trajectory "parcel" moving at 1.8 km of alti- tude will certainly be dispersed to the surface in the highly convective Amazonian lower troposphere .…”