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| Titel |
Examination of temporal and spatial variability of NO2 VCDs measured using mobile-MAX-DOAS in Toronto, Canada. |
| VerfasserIn |
Zoe Davis, Sabour Baray, Aida Khanbabkhani, William Fujs, Csilla Csukat, Robert McLaren |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250145832
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| Publikation (Nr.) |
 EGU/EGU2017-9805.pdf |
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| Zusammenfassung |
| Mobile-MAX-DOAS is an innovative technique used to estimate pollutant emission rates and
validate satellite measurements and air quality models. It is essential to identify
and examine factors that can significantly impact the accuracy of this developing
technique.
Mobile-MAX-DOAS measurements were conducted in Toronto, Canada with a
mini-MAX-DOAS instrument mounted (pointing backwards) on top of a car during August
and September, 2016. Scattered sunlight spectra were collected every 45 seconds in the
continuously repeated sequence of elevation angles of 30o, 30o, 30o, 30o, 40o, 30o, 90o.
Tropospheric VCDs were determined using the geometric approximation from
DSCDs fitted using a near-noon, low NO2 VCD FRS spectrum. The study goal was
to examine the validity of the assumption that VCDs remain relatively constant
at each measured location on a driving route encircling an urban area of interest
with typical time periods of 1.5-3 hours to estimate emissions and whether driving
direction significantly impacts results. NO2 VCD temporal variability was therefore
determined by repeating driving routes in both directions in quick succession on multiple
days.
Strong temporal variability in NO2 VCDs of up to a factor of two were observed for some
routes for the same vehicle locations under constant prevailing wind conditions
within <2 hours. These differences may be due to the effects of transport, changing
tropospheric chemistry and/or diurnal trends in emissions rates. Under these conditions
measurements along different portions of the encircled area in a large city may not be
representative of the entire measurement period, introducing error into the final emission
estimate.
Certain straight roads exhibited significantly different VCDs within < 30 minutes when
the instrument azimuth pointing direction was changed by 180o. The weighted average VCD
was ∼8(±3x) x1016molec. cm−1 from driving in one direction but ∼4 (± 1.5)
x1016molec. cm−1 from driving in the opposite direction. This indicates sufficient
horizontal inhomogeneity for the instrument to view significantly different NO2
regimes while at the same vehicle geographical location due to the different azimuth
direction.
NO2line fluxes were determined during weekday afternoon rush-hours by driving
repeatedly in both directions under tangential prevailing winds conditions on a road that is
8km downwind of Toronto and 4km downwind of a major highway. During one afternoon the
average NO2 VCD was 6(±2)x1016 molec. cm−2with a standard deviation of 3x1015 molec.
cm−2. This average value is consistent with NO2 VCDs retrieved using optimal estimation
methods from stationary MAX-DOAS measurements at nearby York University. Using a 10m
elevation measured wind-speed of 16km hr−1, the NO2 line flux was 3(± 9) x1018molec.
cm-1s-1, approximately 6 times that determined by Halla et al. (2011) for a line flux
measured in a rural area of southwestern Ontario, downwind of pollution sources in
Michigan and Ohio. Our resulting average NO2 flux of 84 (+/-25) mg m−2hr−1
is consistent with NOx fluxes measured in major European cities of up to 90 mg
m−2hr−1.
This work will be used as a baseline experiment to apply this method in other Canadian
cities. |
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