Quantifying the volatility of organic aerosol in the southeastern US
The volatility of organic aerosols (OA) has emerged as a property of primary importance in understanding their atmospheric life cycle, and thus abundance and transport. However, quantitative estimates of the thermodynamic (volatility, water solubility) and kinetic parameters dictating ambient-OA gas...
| Main Authors: | , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-01-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/501/2017/acp-17-501-2017.pdf |
| Summary: | The volatility of organic aerosols (OA) has emerged as a
property of primary importance in understanding their atmospheric life cycle,
and thus abundance and transport. However, quantitative estimates of the
thermodynamic (volatility, water solubility) and kinetic parameters
dictating ambient-OA gas-particle partitioning, such as saturation
concentrations (<i>C</i><sup>∗</sup>), enthalpy of evaporation (Δ<i>H</i><sub>vap</sub>),
and evaporation coefficient (<i>γ</i><sub>e</sub>), are highly uncertain. Here,
we present measurements of ambient-OA volatility at two sites in the
southeastern US, one at a rural setting in Alabama dominated by biogenic volatile organic compounds (BVOCs) as part of the Southern Oxidant
and Aerosol Study (SOAS) in June–July 2013, and another at a more
anthropogenically influenced urban location in North Carolina during
October–November 2013. These measurements applied a dual-thermodenuder (TD)
system, in which temperature and residence times are varied in parallel to
constrain equilibrium and kinetic aerosol volatility properties.
Gas-particle partitioning parameters were determined via evaporation kinetic
model fits to the dual-TD observations. OA volatility parameter values
derived from both datasets were similar despite the fact that measurements
were collected in distinct settings and seasons. The OA volatility
distributions also did not vary dramatically over the campaign period or
strongly correlate with OA components identified via positive matrix
factorization of aerosol mass spectrometer data. A large portion (40–70 %)
of measured ambient OA at both sites was composed of very-low-volatility
organics (<i>C</i><sup>∗</sup> ≤ 0.1 µg m<sup>−3</sup>). An effective Δ<i>H</i><sub>vap</sub> of bulk OA of ∼ 80–100 kJ mol<sup>−1</sup> and a <i>γ</i><sub>e</sub> value of ∼ 0.5 best describe the evaporation observed
in the TDs. This range of Δ<i>H</i><sub>vap</sub> values is substantially higher
than that typically assumed for simulating OA in atmospheric models (30–40 kJ mol<sup>−1</sup>). TD data indicate that <i>γ</i><sub>e</sub> is on the order of 0.1
to 0.5, indicating that repartitioning timescales for atmospheric OA are on
the order of several minutes to an hour under atmospheric conditions. The OA
volatility distributions resulting from fits were compared to those
simulated in the Weather, Research and Forecasting model with Chemistry
(WRF/Chem) with a current treatment of secondary organic aerosol (SOA) formation. The substantial
fraction of low-volatility material observed in our measurements is largely
missing from simulations, and OA mass concentrations are underestimated. The
large discrepancies between simulations and observations indicate a need to
treat low-volatility OA in atmospheric models. Volatility parameters
extracted from ambient measurements enable evaluation of emerging treatments
for OA (e.g., secondary OA using the volatility basis set or formed via
aqueous chemistry) in atmospheric models. |
|---|---|
| ISSN: | 1680-7316 1680-7324 |