2 research outputs found
Estimation of secondary organic aerosol formation parameters for the volatility basis set combining thermodenuder, isothermal dilution, and yield measurements
Secondary organic aerosol (SOA) is a major fraction of the total organic
aerosol (OA) in the atmosphere. SOA is formed by the partitioning onto
pre-existent particles of low-vapor-pressure products of the oxidation of
volatile, intermediate-volatility, and semivolatile organic compounds.
Oxidation of the precursor molecules results in a myriad of organic products,
making the detailed analysis of smog chamber experiments difficult and the
incorporation of the corresponding results into chemical transport models
(CTMs) challenging. The volatility basis set (VBS) is a framework that has
been designed to help bridge the gap between laboratory measurements and
CTMs. The parametrization of SOA formation for the VBS has been
traditionally based on fitting yield measurements of smog chamber
experiments. To reduce the uncertainty in this approach, we developed an
algorithm to estimate the SOA product volatility distribution, effective
vaporization enthalpy, and effective accommodation coefficient combining SOA yield measurements with thermograms (from thermodenuders) and areograms
(from isothermal dilution chambers) from different experiments and
laboratories. The algorithm is evaluated with “pseudo-data” produced from
the simulation of the corresponding processes, assuming SOA with known
properties and introducing experimental error. One of the novel features of
our approach is that the proposed algorithm estimates the uncertainty in the predicted yields for different atmospheric conditions (temperature, SOA
concentration levels, etc.). The uncertainty in these predicted yields is
significantly smaller than that of the estimated volatility distributions
for all conditions tested.</p
Positive matrix factorization of organic aerosol: insights from a chemical transport model
Factor analysis of aerosol mass spectrometer measurements
(organic aerosol mass spectra) is often used to determine the sources of
organic aerosol (OA). In this study we aim to gain insights regarding the
ability of positive matrix factorization (PMF) to identify and quantify the
OA sources accurately. We performed PMF and multilinear engine (ME-2)
analysis on the predictions of a state-of-the-art chemical transport model
(PMCAMx-SR, Particulate Matter Comprehensive Air Quality Model with
extensions – source resolved) during a photochemically active period for
specific sites in Europe in an effort to interpret the diverse factors
usually identified by PMF analysis of field measurements. Our analysis used
the predicted concentrations of 27 OA components, assuming that each of them
is “chemically different” from the others.
The PMF results based on the chemical transport model predictions are quite
consistent (same number of factors and source types) with those of the
analysis of AMS measurements. The estimated uncertainty of the contribution
of fresh biomass burning is less than 30 % and of the other primary
sources less than 40 %, when these sources contribute more than 20 % to
the total OA. The PMF uncertainty increases for smaller source
contributions, reaching a factor of 2 or even 3 for sources which
contribute less than 10 % to the OA.
One of the major questions in PMF analysis of AMS measurements concerns the
sources of the two or more oxygenated OA (OOA) factors often reported in
field studies. Our analysis suggests that these factors include secondary OA
compounds from a variety of anthropogenic and biogenic sources and do not
correspond to specific sources. Their characterization in the literature as
low- and high-volatility factors is probably misleading, because they have
overlapping volatility distributions. However, the average volatility of the
one often characterized as a low-volatility factor is indeed lower than that
of the other (high-volatility factor). Based on the analysis of the
PMCAMx-SR predictions, the first oxygenated OA factor includes mainly
highly aged OA transported from outside Europe, but also highly aged
secondary OA from precursors emitted in Europe. The second oxygenated OA
factor contains fresher secondary organic aerosol from volatile, semivolatile, and intermediate
volatility anthropogenic and biogenic organic compounds. The exact
contribution of these OA components to each OA factor depends on the site
and the prevailing meteorology during the analysis period.</p