Monitoring air quality in an urban area using remote sensing techniques and in situ measurements'

1
Monitoring air quality in an urban area using
remote sensing techniques and in situ
measurements.
Space Research Centre

• L. Kramer, R. Leigh, J. Remedios P. Monks.
• Leicester City Council

2
Satellites
In situ
Ground based remote sensing
3
In Situ Monitors
Run by Leicester City Council - Hourly averaged
NO2 concentrations (ppb) ? molybdenum converters.
4
CMAX-DOAS
Coated Glass
Plano -convex lens
Fibre-optic to Spectrometer
15o
10o
90o
5o
2o
5

• DOAS fitting window of 428-510nm includes NO2,
  O3, H2O and the oxygen dimer O4.
• Tropospheric DSCDs are produced from the
  subtraction of the concurrent zenith differential
  slant column for each measurement in the off axis
  views, removing the stratospheric signal.

• A box model is used to derive air mass factors
  (AMF) for NO2 for each of the viewing angles.

Data from the 5 deg view is used in all analyses
here.
6
CMAX-DOAS in situ comparisons

• Daily averaged NO2 concentrations for December
  2005 to March 2006 during DOAS measurement
  period.
• Group 1, defined as urban background, show no
  positive bias in the in situ measurements

7
OMI

• The Ozone Monitoring Instrument (OMI) was
  launched onboard the NASA EOS Aura satellite in
  July 2004.
• OMI is a Nadir viewing spectrometer that measures
  in the spectral range between 270 and 500 nm.
• Has a spectral resolution of 0.52 and 0.45 nm in
  the UV-1 and UV-2 channels and 0.63 nm in the
  visible channel.

• OMI has a large swath width of 2600 km, to obtain
  this viewing swath the viewing angle is 114
• In the normal operation mode, the OMI pixel size
  is 13 x 24 km2 making it suitable for comparisons
  with measurements on an urban scale.

8
OMI-DOAS comparison

• Black circles - all inner swath pixels
• Red crosses - cloud cleared (Cldfrlt0.2).
• Blue triangles - those measurements where OMI
  covers at least 90 of the area surrounding
  Leicester.
• This ensures that the sampling area of CMAX-DOAS
  is also sampled by the OMI pixel. The correlation
  is then greatly improved (R0.64).

9
OMI- in situ comparison

• The mean NO2 concentration and variability were
  calculated for the urban background monitoring
  stations and compared to OMI tropospheric NO2
  columns for 2005 and 2006.

Cloud cleared and inner swath pixels only.
10

• Near-surface measurements, particularly in urban
  areas are subject to variation due to spatial and
  temporal inhomogeneity of boundary layer NO2.
• The two different observation techniques also
  yield different samplings of the atmosphere on a
  spatial scale, which can introduce biases.
• A positive bias is observed in the near-surface
  concentrations due to the fact that OMI is
  measuring a larger area than what the in situ
  monitors measure.

11
To correct for the bias, background near-surface
NO2 measurements were included in the
analyses. The background near-surface NO2 data
was obtained from an in situ chemiluminescence
monitor in Market Harborough (52.55? N, 0.77 W?).
1-a
a
12
Spring red Summer blue Autumn black Winter -
green
13
Seasonal and Weekly cycles
Monthly averages of NO2 for cloud free days for
OMI (blue) and mean FOV-weighted NO2 from urban
background monitors (black) in 2005 and 2006.
The near-surface measurements demonstrate the
expected seasonal cycle of NO2 - low
concentrations in summer with an increase in the
winter months.
14
Seasonal and Weekly cycles
Urban background stations
Rural background stations

• For each day of the week the mean is calculated
  and normalised to the median weekly value (Beirle
  et al. 2003).
• Weekly cycles are similar for measurements from
  all instruments, with a noticeable decrease in
  NO2 at the weekend compared to the weekday levels
  
• A peak in NO2 levels on Monday is observed which
  may be due to an increase in traffic from
  commuters

FOV-weighted
OMI
15
CMAX- and in situ weekly cycles for Dec 2005 to
Mar 2006.
Much clearer cycle, with almost constant weekly
levels and a large reduction in NO2 on a Sunday.
This may show that the large OMI pixel is
dominated by background sources of NO2, which do
not display such a strong an anthropogenic cycle.
16
Summary

• Different observation techniques yield different
  samplings of the atmosphere on a spatial and
  temporal scale ? can introduce biases.
• The bias can be corrected for my introducing the
  background in situ NO2 in the correlation.
• The agreement now is very good for the spring and
  summer months with correlation coefficients of
  0.83 and 0.64 respectively. The correlation for
  autumn is also good (R0.60), however, during the
  winter OMI generally observes much higher
  concentrations of NO2 than what is represented by
  the near surface NO2 over the pixel This is also
  observed in the seasonal cycles.
• Weekly cycles reflects the ability of all three
  instruments to measure the anthropogenic cycles
  of air quality around Leicester.

17
Further work
OMI - in situ comparisons for other cities around
the UK
Manchester
Birmingham
18
LAMP

• Leicester Air Quality Measurement Project
• Monitoring techniques for NOx
• NOxy York
• BBCEAS Leic
• Mobile Monitor (Chemiluminescence) Council
• Chemiluminescence Monitors Council
• CMAX-DOAS Leic
• OMI

19
(No Transcript)
20
Airviro

• Airviro An Integrated System for Air Quality
  Management ? Developed by Swedish Meteorological
  and Hydrological Institute (SMHI)
• A geographical Emissions Database into which data
  can be entered from point sources
• 4 Different Models
• A street canyon model for investigating air
  pollution at a fine scale
• Gaussian plume and Eulerian grid models for
  investigating regional and local scale air
  pollution
• A Heavy Gas model for investigating accidental
  releases

21
(No Transcript)
22
(No Transcript)