Simulating secondary organic aerosol in a regional air quality model using the statistical oxidation model – Part 2: Assessing the influence of vapor wall losses
The influence of losses of organic vapors to chamber walls during secondary organic aerosol (SOA) formation experiments has recently been established. Here, the influence of such losses on simulated ambient SOA concentrations and properties is assessed in the University of California at Davis / C...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/3041/2016/acp-16-3041-2016.pdf |
| Summary: | The influence of losses of organic vapors to chamber walls during secondary
organic aerosol (SOA) formation experiments has recently been established.
Here, the influence of such losses on simulated ambient SOA concentrations
and properties is assessed in the University of California at Davis / California Institute of Technology (UCD/CIT) regional air quality model using
the statistical oxidation model (SOM) for SOA. The SOM was fit to laboratory
chamber data both with and without accounting for vapor wall losses
following the approach of Zhang et al. (2014). Two
vapor wall-loss scenarios are considered when fitting of SOM to chamber data
to determine best-fit SOM parameters, one with “low” and one with “high”
vapor wall-loss rates to approximately account for the current range of
uncertainty in this process. Simulations were run using these different
parameterizations (scenarios) for both the southern California/South Coast
Air Basin (SoCAB) and the eastern United States (US). Accounting for vapor
wall losses leads to substantial increases in the simulated SOA
concentrations from volatile organic compounds (VOCs) in both domains, by factors of ∼ 2–5
for the low and ∼ 5–10 for the high scenarios. The magnitude of
the increase scales approximately inversely with the absolute SOA
concentration of the no loss scenario. In SoCAB, the predicted SOA fraction
of total organic aerosol (OA) increases from ∼ 0.2 (no) to ∼ 0.5
(low) and to ∼ 0.7 (high), with the high vapor wall-loss
simulations providing best general agreement with observations. In the
eastern US, the SOA fraction is large in all cases but increases further
when vapor wall losses are accounted for. The total OA ∕ ΔCO ratio
captures the influence of dilution on SOA concentrations. The simulated
OA ∕ ΔCO in SoCAB (specifically, at Riverside, CA) is found to
increase substantially during the day only for the high vapor wall-loss
scenario, which is consistent with observations and indicative of
photochemical production of SOA. Simulated O : C atomic ratios for both SOA
and for total OA increase when vapor wall losses are accounted for, while
simulated H : C atomic ratios decrease. The agreement between simulations and
observations of both the absolute values and the diurnal profile of the O : C
and H : C atomic ratios for total OA was greatly improved when vapor
wall-losses were accounted for. These results overall demonstrate that vapor
wall losses in chambers have the potential to exert a large influence on
simulated ambient SOA concentrations, and further suggest that accounting
for such effects in models can explain a number of different observations
and model–measurement discrepancies. |
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| ISSN: | 1680-7316 1680-7324 |