Abstract
Abstract. During the ACRIDICON-CHUVA field project (September–October 2014; based in Manaus, Brazil) aircraft-based 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 (BL) 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 products occurs in the upper troposphere (UT), leading to increased organic aerosol mass concentrations above 10 km altitude. The median organic mass concentrations in the UT above 10 km range between 1.0 and 2.5 µg m−3 (referring to standard temperature and pressure; STP) with interquartile ranges of 0.6 to 3.2 µg m−3 (STP), representing 78 % of the total submicron non-refractory aerosol particle mass. The presence of isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA) was confirmed by marker peaks in the mass spectra. We estimate the contribution of IEPOX-SOA to the total organic aerosol in the UT to be about 20 %. After isoprene emission from vegetation, oxidation processes 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 preexisting particles leads to IEPOX-SOA formation and subsequently increasing organic mass in the UT. This organic mass increase was accompanied by an increase in the nitrate mass concentrations, most likely due to NOx production by lightning. Analysis of the ion ratio of NO+ to NO2+ indicated that nitrate in the UT 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 UT either on acidic nitrate particles forming organic nitrates derived from IEPOX or on already neutralized organic nitrate aerosol particles.
Highlights
Volatile organic compounds (VOCs) emitted by vegetation can lead to the formation of secondary organic aerosol (SOA) through atmospheric oxidation and further chemical processes (e.g., Zhang et al, 2007; Jimenez et al, 2009; Hallquist et al, 2009; Shrivastava et al, 2017)
This leads to the conclusion that the third possibility, in situ SOA formation with subsequent growth of the aerosol particles to large enough sizes that they can be detected by the C-ToF-AMS, is the dominant process in the upper troposphere (UT)
We presented results from airborne aerosol measurements with a C-ToF-AMS conducted during the ACRIDICONCHUVA campaign in September and October 2014 in the tropical lower, middle, and upper troposphere over the Amazon region
Summary
Volatile organic compounds (VOCs) emitted by vegetation can lead to the formation of secondary organic aerosol (SOA) through atmospheric oxidation and further chemical processes (e.g., Zhang et al, 2007; Jimenez et al, 2009; Hallquist et al, 2009; Shrivastava et al, 2017). Laboratory studies demonstrate that around 50 % of isoprene-derived particulate matter is associated with IEPOX production and uptake through the HO2 pathway and in the presence of acidic aerosol particles (Liu et al, 2015). Despite the enhancing effect of increased sulfate concentrations for IEPOX-SOA production, the NO effect is dominating (de Sá et al, 2017) During another airborne measurement campaign in the Amazon rainforest (SAMBBA), the highest f82 values were around 9 ‰, measured at the top of the boundary layer with a maximum flight altitude of 5 km (Allan et al, 2014). It has been suggested that aerosol formation in the upper troposphere can provide a source for cloud condensation nuclei for lower altitudes in the Amazon region (Wang et al, 2016; Andreae et al, 2018) Another interesting aspect is the presence of organic nitrates. A comparison of these parameters for different altitudes is presented
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