Water and Organic Nitrate Detection in an AMS: Laboratory Characterization and Application to Ambient Measurements
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Beschreibung
Atmospheric aerosols were studied by three different means. Laboratory
experiments determined the relative ionization efficiency of water (RIEH2O) in an Aerodyne
Aerosol Mass Spectrometers (AMS), simulation chamber experiments gave insight to the
reaction products of biogenic volatile organic compounds (BVOC) oxidation products, and
the findings were applied to two field campaign measurements at Cabauw, NL, in May
2008 and February 2009. Knowing the liquid water content of aerosol particles is vital for
the assessment of their climate forcing potential. A value of 2 for RIEH2O was determined
by studying oxalate salts with different amounts of crystal water. BVOCs contribute much
more to the global budget of VOCs than anthropogenic ones but oxidation products in
terms of secondary organic aerosol often correlate to anthropogenic tracers such as NOx
from fossil fuel burning. In atmospheric simulation chamber experiments, organic nitrates
from BVOC-NO3 oxidation showed higher vapor pressures than pure organic compounds
produced in the same reactions. Organic nitrates comprised up to approx. 41 % of the
particulate phase. A specific fragmentation ratio of nitrate (NO2
+/NO+) of 0.1 was found by
high resolution AMS analysis differing strongly from the value of 0.4 known for the most
abundant ambient NO3 specie (NH4NO3). Ambient average particulate mass loadings were
9.72 μg/m3 dominated by organics (40 %) in 2008 and 5.62 μg/m3 dominated by nitrate
(42 %) in 2009. Data comparison to collocated instruments showed good agreement.
Positive Matrix Factorization analysis of the particulate organic fraction distinguished semi
and low volatile oxygenated organic aerosol (OOA) as well as hydrocarbon like organic
aerosol (HOA) in both campaigns. An additional highly oxygenated OA with a mass
spectrum very similar to fulvic acid was found in 2008. The average contribution of
organic nitrate to the particulate nitrate mass was 34.9 % in 2008 and 5.2 % in 2009.
Autor*in
Amewu A. Mensah