Inorganic Salt Interference on CO₂⁺ in Aerodyne AMS and ACSM Organic Aerosol Composition Studies

Abstract

Aerodyne aerosol mass spectrometer (AMS) and Aerodyne aerosol chemical speciation monitor (ACSM) mass spectra are widely used to quantify organic aerosol (OA) elemental composition, oxidation state, and major environmental sources. The OA CO fragment is among the most important measurements for such analyses. Here, we show that a non-OA CO signal can arise from reactions on the particle vaporizer, ion chamber, or both, induced by thermal decomposition products of inorganic salts. In our tests (eight instruments, n = 29), ammonium nitrate (NHNO) causes a median CO interference signal of +3.4% relative to nitrate. This interference is highly variable between instruments and with measurement history (percentiles P = +0.4 to +10.2%). Other semi-refractory nitrate salts showed 2-10 times enhanced interference compared to that of NHNO, while the ammonium sulfate ((NH)SO) induced interference was 3-10 times lower. Propagation of the CO interference to other ions during standard AMS and ACSM data analysis affects the calculated OA mass, mass spectra, molecular oxygen-to-carbon ratio (O/C), and f. The resulting bias may be trivial for most ambient data sets but can be significant for aerosol with higher inorganic fractions (>50%), e.g., for low ambient temperatures, or laboratory experiments. The large variation between instruments makes it imperative to regularly quantify this effect on individual AMS and ACSM systems.

Long-term chemical analysis and organic aerosol source apportionment at nine sites in central Europe: source identification and uncertainty assessment

“…However, Bozzetti et al (2016) assessed a shorter time period with diurnal resolution, instead of one sample per month as in the present work. Cellulose concentrations from other European sites during other years are consistent with the results in this study (Sanchez-Ochoa et al, 2007;Yttri et al, 2011). In general, the background cellulose concentrations at the southern alpine sites are higher and also the temporal behaviour deviates from that observed at the northern sites: the maximal concentrations are not reached in July-August but rather in May or October-November.…”

“…The correction factor CO 2,meas NO 3,meas NH 4 NO 3 ,pure was determined based on measurements of aqueous NH 4 NO 3 conducted regularly during the entire measurement period and varied between ∼ 1 and ∼ 5 % (Pieber et al, 2016).…”

Advanced source apportionment of carbonaceous aerosols by coupling offline AMS and radiocarbon size-segregated measurements over a nearly 2-year period

“…To correct for the interference of NH 4 NO 3 on the CO + 2 signal as described in Pieber et al (2016) …”

Gas phase composition and secondary organic aerosol formation from gasoline direct injection vehicles investigated in batch and flow reactors: effects of prototype gasoline particle filters

“…HR-ToF-AMS data were corrected for background gas-phase CO 2 in the emissions by subtracting a CO 2 -signal measured in a particle-free sample. The interaction of inorganic salts with pre-deposited carbon on the tungsten vaporizer can lead to the generation of CO 2 + signal in the open minus closed HR-ToF-AMS mass spectra (Pieber et al, 2016). Here photochemical aging of the 5 exhaust resulted in significant NH 4 NO 3 formation, reaching NO 3 /OA ratios of 5.…”

“…Here photochemical aging of the 5 exhaust resulted in significant NH 4 NO 3 formation, reaching NO 3 /OA ratios of 5. A CO 2 + signal at 3.5% to NO 3 was determined by calibration (see Figure S3, Supporting Information) and corrected according to (Pieber et al, 2016). exceeded all other phases of cold-and hot-started WLTC by 2 to 4 orders of magnitude (median, GDI1-3).…”