In:
Atmospheric Chemistry and Physics, Copernicus GmbH, Vol. 20, No. 17 ( 2020-09-08), p. 10379-10400
Abstract:
Abstract. The detection and attribution of high background ozone (O3) events in
the southwestern US is challenging but relevant to the effective
implementation of the lowered National Ambient Air Quality Standard (NAAQS;
70 ppbv). Here we leverage intensive field measurements from the Fires,
Asian, and Stratospheric Transport−Las Vegas Ozone Study (FAST-LVOS) in
May–June 2017, alongside high-resolution simulations with two global
models (GFDL-AM4 and GEOS-Chem), to study the sources of O3 during
high-O3 events. We show possible stratospheric influence on 4 out of
the 10 events with daily maximum 8 h average (MDA8) surface O3
above 65 ppbv in the greater Las Vegas region. While O3 produced from
regional anthropogenic emissions dominates pollution events in the Las Vegas
Valley, stratospheric intrusions can mix with regional pollution to push
surface O3 above 70 ppbv. GFDL-AM4 captures the key characteristics of
deep stratospheric intrusions consistent with ozonesondes, lidar profiles,
and co-located measurements of O3, CO, and water vapor at Angel Peak,
whereas GEOS-Chem has difficulty simulating the observed features and
underestimates observed O3 by ∼20 ppbv at the surface.
On days when observed MDA8 O3 exceeds 65 ppbv and the AM4 stratospheric
ozone tracer shows 20–40 ppbv enhancements, GEOS-Chem simulates
∼15 ppbv lower US background O3 than GFDL-AM4. The two
models also differ substantially during a wildfire event, with GEOS-Chem
estimating ∼15 ppbv greater O3, in better agreement with
lidar observations. At the surface, the two models bracket the observed MDA8
O3 values during the wildfire event. Both models capture the
large-scale transport of Asian pollution, but neither resolves some
fine-scale pollution plumes, as evidenced by aerosol backscatter, aircraft,
and satellite measurements. US background O3 estimates from the two
models differ by 5 ppbv on average (greater in GFDL-AM4) and up to 15 ppbv
episodically. Uncertainties remain in the quantitative attribution of each
event. Nevertheless, our multi-model approach tied closely to observational
analysis yields some process insights, suggesting that elevated background
O3 may pose challenges to achieving a potentially lower NAAQS level
(e.g., 65 ppbv) in the southwestern US.
Type of Medium:
Online Resource
ISSN:
1680-7324
DOI:
10.5194/acp-20-10379-2020
DOI:
10.5194/acp-20-10379-2020-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2020
detail.hit.zdb_id:
2092549-9
detail.hit.zdb_id:
2069847-1
