Effects of Photolysis on the Chemical and Optical Properties of Secondary Organic Material Over Extended Time Scales

Abstract

As organic aerosol particles age in the atmosphere, their chemical and physical properties can change which impacts their loss rates and atmospheric lifetimes. Here we investigate 4 days of photolytic aging of α-pinene secondary organic aerosol particles collected on Teflon filters. Changes in the chemical and optical absorption properties of this secondary organic material were measured using UV/vis absorption spectroscopy, offline-aerosol mass spectrometry (AMS), electrospray ionization ultrahigh resolution mass spectrometry (UHR-MS), and attenuated total reflectance-Fourier transform infrared spectroscopy. For water-soluble extracts, our results show an initial decrease in the absorption cross section at ∼280 nm corresponding to the removal of carbonyls. The majority of this decrease occurs over the first 2 days with much smaller changes on days three and four. Offline-AMS shows a shift in ion intensity from C 2 H 3 O + to CO 2 + suggesting a corresponding increase in carboxylic acids or esters. Results from soft ionization UHR-MS indicate that oligomerization occurs concurrent with photolysis, leading to a shift toward the center of the mass distribution range (∼320 amu). Aged dimers and trimers are observed, generated from the photo products formed over the first 2 days. These results demonstrate that extended experimental time frames and complementary analytical techniques provide new insights into multigenerational aging processes of atmospheric aerosol particles.

UV/Vis and IRMPD Spectroscopic Analysis of the Absorption Properties of Methylglyoxal Brown Carbon

“…Data for absorption cross sections were collected from the dilute initial solutions for photolysis experiments with initial organic concentrations measured with the small volume nebulizer-aerosol mass spectrometer (SVN-AMS). 27,28 For analysis with the SVN-AMS, an internal standard (NH 4 15 NO 3 ) is mixed with the solution and nebulized into the AMS. The AMS provides quantification of both the total organic and the internal standard.…”

“…The wavelength-dependent mass absorption cross sections (MAC) for the bulk solutions were calculated using where A 10 (λ) is the absorbance (base-10), b is the path length (1 cm), and C org is the concentration of the photo-active organic in the solution (g cm –3 ). Data for absorption cross sections were collected from the dilute initial solutions for photolysis experiments with initial organic concentrations measured with the small volume nebulizer-aerosol mass spectrometer (SVN-AMS). , For analysis with the SVN-AMS, an internal standard (NH 4 15 NO 3 ) is mixed with the solution and nebulized into the AMS. The AMS provides quantification of both the total organic and the internal standard.…”

Photodegradation of Secondary Organic Aerosols by Long-Term Exposure to Solar Actinic Radiation

“…16,18,19 The C−C(O) bond in carbonyls can be broken by Norrish type-I and Norrish type-II photocleavage processes to form smaller products. 10,16,20 Peroxides break at the weak O−O bond, forming two O-centered radicals, which undergo secondary processes on a picosecond time scale. 21 However, not all photochemical processes result in fragmentation.…”

Emerging investigator series: chemical and physical properties of organic mixtures on indoor surfaces during HOMEChem

“…For offline AMS analysis, one third of the acetonitrile extract was dried and reconstituted with 50 mL Milli-Q water and atomized into a High Resolution-Time of Flight-Aerosol Mass Spectrometer (HR-AMS) 45,46 using the small volume nebulizer (SVN). 47 The spectrum for the eld blank was spectrally subtracted from the sample spectrum aer weighting the eld blank intensities by a factor of 0.2. The resulting electron ionization (EI) mass spectral data was analysed using soware packages SQUIRREL (v1.57I) and PIKA (v1.16I).…”

“…About one third of the dried extract samples were reconstituted with methanol ($300 mL), diluted by a factor of 200 with methanol, and sprayed in positive ion mode. Assignments were made using methods similar to previous work 47,51 with a mass accuracy window of 1 ppm, the molecular formula calculator (https://nationalmaglab.org/user-facilities/icr/icr-soware), and a range of values C 0-100 H 0-200 O 0-50 N 0-3 Na 0-1 . For the FT-ICR MS analysis, any peaks observed in the eld blank were removed from the nal sample peak list.…”