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Measurements of O3, SO2, NO2 and HCHO column amounts using a Brewer spectrometer
Abstract
O3, SO2 and NO2 vertical column amounts and aerosol optical depths at 18 wavelengths from 303 to 363nm were measured daily for the past two years by using a Brewer MK3 double spectrometer in direct-sun mode. The algorithms used are described and compared to the standard algorithm. For O3 and SO2 the standard algorithm was modified by using all 6 measured wavelengths instead of only 4 or 5 and by including a correction for the diffuse irradiance entering the instrument's field of view. This reduces the statistical error of the retrievals by 40% and 50%, respectively, for O3 and SO2. The NO2 retrievals are based on a spectral fitting technique using wavelengths between 349 and 363nm. A 'Bootstrap method' has been developed to calibrate the Brewer for NO2 measurements. This method selects data with lowest NO2-amounts and uses them to derive the needed extraterrestrial solar spectrum. We discuss possible reasons why an intent to apply to same technique for deriving total HCHO columns failed. The main advantage of direct sun methods compared to Differential Optical Absorption Spectroscopy DOAS is that the uncertainty in the air mass factor is significantly smaller. We think that a better correction for the diffuse irradiance entering the instrument's field of view will further improve the retrievals, especially in the low wavelength range (303 to 320nm) used for O3 and SO2.
... To address this limitation, ground-based UV-visible remote sensing instruments able to operate in multiple observation modes (direct-sun, zenith-sky and off-axis spectroscopy techniques) are becoming increasingly common. These provide information on the column density of NO 2 as well as some limited vertical profiles [8][9][10][11]. More advanced algorithms can be used to further estimate surface NO 2 [12][13][14]. ...
We present tropospheric nitrogen dioxide (NO2) changes observed by the Canadian Pandora measurement program in the Greater Toronto Area (GTA), Canada, and compare the results with surface NO2 concentrations measured via in situ instruments to assess the local emission changes during the first two years of the COVID-19 pandemic. In the City of Toronto, the first lockdown period started on 15 March 2020, and continued until 24 June 2020. ECMWF Reanalysis v5 (ERA-5) wind information was used to facilitate the data analysis and reveal detailed local emission changes from different areas of the City of Toronto. Evaluating seven years of Pandora observations, a clear NO2 reduction was found, especially from the more polluted downtown Toronto and airport areas (e.g., declined by 35% to 40% in 2020 compared to the 5-year mean value from these areas) during the first two years of the pandemic. Compared to the sharp decline in NO2 emissions in 2020, the atmospheric NO2 levels in 2021 started to recover, but are still below the mean values in pre-pandemic time. For some sites, the pre-pandemic NO2 local morning rush hour peak has still not returned in 2021, indicating a change in local traffic and commuter patterns. The long-term (12 years) surface air quality record shows a statistically significant decline in NO2 with and without April to September 2020 observations (trend of −4.1%/yr and −3.9%/yr, respectively). Even considering this long-term negative trend in NO2, the observed NO2 reduction (from both Pandora and in situ) in the early stage of the pandemic is still statistically significant. By implementing the new wind-based validation method, the high-resolution satellite instrument (TROPOMI) can also capture the local NO2 emission pattern changes to a good level of agreement with the ground-based observations. The bias between ground-based and satellite observations during the pandemic was found to have a positive shift (5–12%) than the bias during the pre-pandemic period.
... The retrieved Pandora TCO amounts have also been successfully compared to a carefully calibrated double grating Brewer spectrometer #171 (Tzortziou et al., 2012) that uses a six-wavelength algorithm based on the BDM O 3 cross sections (an improvement over the standard four-wavelength method) as described by Cede and Herman (2005). The key results show good correlation between the Pandora and Brewer TCO amounts, even at high SZA, but with a clear seasonal difference caused by the assumption of a constant effective stratospheric temperature for the ozone absorption cross section, 225 • K, in the Pandora algorithm. ...
A comparison of retrieved total column ozone (TCO) amounts between the Pandora
#34 spectrometer system and the Dobson #061 spectrophotometer from
direct-sun observations was performed on the roof of the Boulder, Colorado,
NOAA building. This paper, part of an ongoing study, covers a 1-year
period starting on 17 December 2013. Both the standard Dobson and Pandora
TCO retrievals required a correction, TCOcorr = TCO
(1 + C(T)), using a monthly varying effective ozone temperature,
TE,
derived from a temperature and ozone profile climatology. The correction is
used to remove a seasonal difference caused by using a fixed temperature in
each retrieval algorithm. The respective corrections C(TE) are
CPandora = 0.00333(TE-225) and CDobson
= -0.0013(TE-226.7) per degree K. After the applied corrections removed
most of the seasonal retrieval dependence on ozone temperature, TCO
agreement between the instruments was within 1 % for clear-sky conditions.
For clear-sky observations, both co-located instruments tracked the
day-to-day variation in total column ozone amounts with a correlation of
r2 = 0.97 and an average offset of 1.1 ± 5.8 DU. In addition,
the Pandora TCO data showed 0.3 % annual average agreement with satellite
overpass data from AURA/OMI (Ozone Monitoring Instrument) and 1 % annual
average offset with Suomi-NPP/OMPS (Suomi National Polar-orbiting
Partnership, the nadir viewing portion of the Ozone Mapper Profiler Suite).
... The algorithms and calibration techniques for the Dobson spectrophotometer (Komhyr and Evans, 2006) The retrieved Pandora TCO amounts have been successfully compared to a carefully calibrated double grating Brewer spectrometer #171 (Tzortziou et al., 2012) that uses a six-wavelength algorithm (an improvement over the standard 4-wavelength method) as described by Cede et al. (2005). The key results show good correlation between 10 the Pandora and Brewer TCO amounts, even at high SZA, but with a clear seasonal difference caused by the assumption of a constant effective stratospheric temperature for the ozone absorption cross section, 225 K, in the Pandora algorithm. ...
A comparison of retrieved total column ozone amounts TCO between the Pandora #34 spectrometer system and the Dobson #061 spectrophotometer from direct-sun observations was performed on the roof of the Boulder, Colorado NOAA building. This paper, part of an ongoing study, covers a one-year period starting on 17 December 2013. Both the standard Dobson and Pandora total column ozone TCO retrievals required a correction TCOcorr = TCO (1+C(T)) using the effective climatology derived ozone temperature T to remove a seasonal difference caused by using a fixed temperature in each retrieval algorithm. The respective corrections C(T) are CPandora = 0.00333(T−225) and CDobson = −0.0013 (T−226.7) per K. After the applied corrections removed the seasonal retrieval dependence on ozone temperature, TCO agreement between the instruments was within 1% for clear-sky conditions. For clear-sky observations, both co-located instruments tracked the day-to-day variation in total column ozone amounts with a correlation of r2 = 0.97 and an average offset of 1.1 ± 5.8 DU. In addition, the Pandora data showed 0.3% annual average agreement with satellite overpass data from AURA/OMI (Ozone Monitoring Instrument) and 1% annual average offset with Suomi-NPP/OMPS (Suomi National Polar-orbiting Partnership, the nadir viewing portion of the Ozone Mapper Profiler Suite).
This paper describes and applies a new technique for retrieving diurnal variability in tropospheric ozone vertical distribution using ground-based measurements of ultraviolet sky radiances. The measured radiances are obtained by a polarization-insensitive modified Brewer double spectrometer located at Goddard Space Flight Center, in Greenbelt, Maryland, USA. Results demonstrate that the Brewer angular (0-72° viewing zenith angle) and spectral (303-320 nm) measurements of sky radiance in the solar principal plane provide sufficient information to derive tropospheric ozone diurnal variability. In addition, the Brewer measurements provide stratospheric ozone vertical distributions at least twice per day near sunrise and sunset. Frequent measurements of total column ozone amounts from direct-sun observations are used as constraints in the retrieval. The vertical ozone profile resolution is shown in terms of averaging kernels to yield at least four points in the troposphere-low stratosphere, including good information in Umkehr layer 0 (0-5 km). The focus of this paper is on the derivation of stratospheric and tropospheric ozone profiles using both simulated and measured radiances. We briefly discuss the necessary modifications of the Brewer spectrometer that were used to eliminate instrumental polarization sensitivity so that accurate sky radiances can be obtained in the presence of strong Rayleigh scattering and aerosols. The results demonstrate that including a site-specific and time-dependent aerosol correction, based on Brewer direct-sun observations of aerosol optical thickness, is critical to minimize the sky radiance residuals as a function of observing angle in the optimal estimation inversion algorithm and improve the accuracy of the retrieved ozone profile.
Due to the combined effect of two polarization sensitive elements, the entrance window and grating, the sensitivity of Brewer spectrophotometers for direct-sun measurements changes with solar zenith angle (SZA). We determined this SZA-polarization dependence with four independent methods, which agreed within ±1.5%. For SZA < 50° this effect is negligible. At SZA = 60°, 70°, and 80° the Brewer's sensitivity is reduced by 1%, 4%, and 10%, relative to SZA = 35°, when the direct solar irradiance is perpendicular to the entrance window. Differential absorption algorithms for retrieving trace gases (e.g., ozone) are unaffected since the polarization effect is almost wavelength independent. However, systematic errors are introduced in Langley extrapolations (2–4% overestimation of the zero air mass factor), retrievals of aerosol optical depth (overestimation of 0.01–0.04), and aerosol single scattering albedo. Therefore, Brewer direct irradiance measurements should be corrected for the SZA-polarization dependence. The effect in sky-radiance measurements can be removed only by hardware modifications.
The longest time series of UV-B and short wavelength of UV-A aerosol optical depth has been constructed for the period 1992-2002 using the direct Sun irradiances obtained during standard calculations of total ozone by the Brewer spectrophotometer at Belsk (51°84N, 20°78E), Poland. The long-term variations appear to be trendless with only small year-to-year variations. The statistical analysis of the 1-day change of aerosol optical depth in the UV range shows that in 30% of cases the scale of the aerosol change is important for the next day UV index forecast. In the analysis of dependence of the aerosol optical depth on wavelength we combine the CIMEL Sun photometer and the Brewer spectrophotometer optical depth data. The Ångström's exponent is calculated for the pairs of wavelengths in the visible, UV-A, and UV-A/UV-B ranges. It is found that Ångström's exponent decreases for the shorter wavelength pair, and it is possible that in the UV-B/UV-A range the exponent is actually negative, yielding an increase of aerosol optical depth with wavelength. The Mie code simulations corroborate the hypothesis of possible negative values for the Ångström's exponent in the UV-B/UV-A range.
The optical thickness of the atmosphere, τ, was deduced from
measurements of narrowband direct solar UV-B radiation. A measurement
campaign was organized to obtain the radiation at three different sites,
during the month of August 1999, using the same methods and instruments,
in order to deduce the atmospheric optical thickness in the spectral
UV-B range (280-320 nm). The three observation sites were chosen to
cover a wide range of measurement conditions; located near the tropical
Atlantic Ocean (Natal, 5.8°S, 35.2°W), on the Andes mountains
(La Paz, 16.5°S, 68.1°W), and in the biomass burning area of
central Brazil (Campo Grande, 19.2°S, 54.3°W). The UV-B
measurements were made with a Brewer spectrophotometer at each site.
Since the instrument measures atmospheric ozone and SO2
simultaneously, it is possible, from the total atmospheric optical
thickness τ, to deduce the aerosol optical thickness
τaerosol. The results have been combined in different
ways to compare with satellite data, showing good performance. Time
variations as short as about 10 min can be seen. On clear days the time
variations are relatively small, as expected. On the other hand, for the
biomass burning site, for conditions of mixed air masses (the instrument
is not looking directly at plumes) one can see very large variations in
τ in relatively short time intervals, for example, for one case,
from 3.5 to 7.0 in about 30 min. Absolute values for τ at Natal and
La Paz were near 2.0 and at Campo Grande, between 2.5 and 3.0, but with
occasional highs of about 4.5. For τaerosol, Natal and La
Paz had values between 0.0 and 0.4, whereas Campo Grande had most values
near 0.4, with occasional highs near 1.0, 1.2, and 2.2.
The optical thickness of the atmosphere, τat, was deduced
from measurements of narrowband direct solar UV-B (280-320 nm)
radiation. This is the radiation that is strongly absorbed in the
stratosphere by ozone, especially near the lower limit wavelength of
these measurements, 306.3 nm. Measurement campaigns were organized to
obtain radiation data at different sites, for different kinds of
aerosol, using the same methods and instruments, in order to deduce the
atmospheric optical thickness for different atmospheric conditions. The
measurements were made with a Brewer spectrophotometer at each site.
This instrument measures also the atmospheric ozone and sulphur dioxide
columns, and therefore it is possible to deduce the aerosol optical
thickness, τae. The possibility of additional absorptions
in the UV-B band, especially formaldehydes, is investigated. Two data
sets are examined: C. Grande and S. Paulo. A large percentage of the
data show that the wavelength dependence of the aerosol optical
thickness is one that appears to increase with wavelength. In other
words, the difference between the absolute values at 320.1 and 306.3 nm,
is positive for the majority of cases. However, this difference is not
always larger than the uncertainty/error of the measurement. This
intriguing result opposes the Angstrom law, applicable to a much larger
dynamic range in wavelength. For the C. Grande data (with generally
larger values of optical thickness), the difference is positive in 16
out of 19 cases, but there are only 4 cases when the difference is
larger than the error. For the S. Paulo morning data, however, the
opposite is true. The difference is positive with only one exception,
and there are 13 cases out of 17 where the difference is larger than the
error.
Ground-based NO2 total column measurements have been collected since 1992 using Brewer spectrophotometry at Rome, an urban site, and Ispra (semi-rural). These are the only ground-based stations regularly monitoring NO2 in Italy. The methodology of measurement together with the procedure to control its quality is described. From the analysis of the time series it was found that the mean value of the NO2 column is 1.63 DU at Rome and 1.60 DU at Ispra. A first attempt to compare NO2 vertical column densities (VCD) from GOME with those derived from ground-based Brewer measurements, under different atmospheric conditions and measurement time lags is here presented. The results of this pilot study showed unsatisfactory agreement because different atmospheres are probed by GOME and Brewer instruments. The GOME space resolution resulted insufficient to fully characterize the Rome and Ispra highly localized polluted areas.
The first Iberian UV radiation intercompari-son was held at "El Arenosillo"-Huelva station of the Insti-tuto Nacional de Técnica Aeroespatial (INTA) from Septem-ber 1 to 10, 1999. During this campaign, seven Brewer spectrophotometers and one Li-Cor spectrophotometer mea-sured the total column aerosol optical depth (AOD) at 306, 310, 313.5, 316.75 and 320 nm. The AOD calibration of one Brewer was transferred to all other Brewers using one day of intensive measurements. The remaining days were used to observe the stability and reproducibility of the AOD measurements by the different instruments. All Brewer spectrophotometers agreed to within an AOD of 0.03 dur-ing the whole measurement campaign. The differences in AOD between the Li-Cor spectrophotometer and the Brewer spectrophotometers were between -0.07 and +0.02 at 313.5, 316.75, and 320 nm. This investigation demonstrates the possibility of using the existing worldwide Brewer network as a global UV aerosol network for AOD monitoring.
For the first time tropospheric NO2 columns from GOME were
compared to tropospheric columns derived from in situ measurements with
the DLR research aircraft Falcon on a clear day above Austria. Under
these conditions the agreement between the two methods is very good. The
in situ measurements yield a tropospheric NO2 column of (4.2
+/- 1.7).1015 molec./cm2, whereas the GOME data
result in columns of (3.5 +/- 0.9).1015 molec./cm2
and (4.1 +/- 1.0).1015 molec./cm2 for
near-real-time and dedicated analyses, respectively. The most important
uncertainty of the aircraft measurements is caused by the lack of data
in the lower boundary layer. The GOME uncertainties in this particular
case are dominated by the assumptions made for the airmass factor
calculation. This work is the first independent validation of
tropospheric NO2 columns from satellite instrumentation.
Further validation at other seasons, and regions, including a more
comprehensive sampling of the boundary layer is needed.
The wavelength accuracy of the Brewer spectrophotometer in the 280–360-nm spectral range is improved by a new grating-drive mechanism and a new dispersion function derived from the Brewer geometry. With the new mechanism, the reproducibility of wavelength settings for spectral emission lines is better than 0.3 pm (0.0003 nm) and, with a thermally compensated version, the effect of temperature is less than 0.4 pm K⁻¹. The new dispersion function fits spectral line positions better than the standard function and is less prone to extrapolation error. Applying the new function to data from four new and ten standard drives shows that the new drives perform as well as the best of the standard ones and much better than the majority.
A Brewer MKIII double spectrophotometer has been modified to measure direct sun and sky radiance from 303nm to 363nm for the purpose of measuring aerosol optical depth, Angstrom parameter, and single scattering albedo. Results from a detailed instrument calibration showed that there is a temperature dependence of 0.3% per degree Celsius, the field of view was 2.6° full width half maximum, and the wavelength calibration was accurately determined using a dye-LASER. Using both integrating sphere and lamp-diffuser plate combinations, absolute diffuse radiometric calibration was performed and converted into direct calibration using the measured field of view. Aerosol optical depth and Angstrom parameter were measured on 4 clear sky days in June 2003 at Greenbelt, Maryland and compared to AERONET-data at the same location. The average difference in the aerosol optical depth at 340nm was smaller than 0.02. A depolarizing element was inserted in the Brewer's optical path to reduce the very pronounced polarization sensitivity, and additional polarized filters were added to explore the possibility to obtain additional aerosol information. Because of a defect in the depolarizer, the current residual polarization is 5%, which has to be reduced to less than 1% to derive additional aerosol parameters from sky radiance measurements.
The polarization sensitivity of a Brewer MKIII double spectrophotometer was measured in the laboratory. We found two major sources of polarization sensitivity. 1) The flat quartz plate as the first optical element alters the polarization state of the transmitted light by Fresnel reflection at oblique incident angles. 2) The internal grating produces almost 100% polarization of the incident light perpendicular to the direction of the ruled grating. The combination of both effects results in a zenith angle (ZA) dependence of the instrument"s sensitivity for unpolarized input such as from Direct Sun measurements. The Brewer is 2% more sensitive at ZA=0° and 10% less sensitive at ZA=80° with respect to normal incidence (ZA=35°). Since the ZA-dependence is independent of wavelength this effect cancels out when calculating wavelength-ratios as used for total ozone retrieval. However the ZA-dependence causes errors when absolute signals at single wavelengths are needed as for aerosol optical depth (AOD) retrievals. Based on our laboratory measurements an overestimation of the Langley extrapolation between 3% and 5% is estimated even at best atmospheric conditions. The ZA-dependence causes 0.025-0.045 overestimation of AOD and an underestimation of the Angstrom exponent. We believe that this effect has not been detected from Brewer AOD-measurements since it is masked by larger uncertainty sources of other nature and AOD-comparisons to other instruments in the short UV-region are rare. Knowing the ZA-dependence it is possible to correct for the ZA-effect. We modified our Brewer by incorporating a depolarizer in its optical path and replacing the flat quartz window by a curved one, so that the input is always at normal incidence, which reduces the ZA-effect.
In this work, the new double-monochromator version of the Brewer Ozone Spectrophotometer, especially designed for spectral UV measurements, is briefly described and its measurements are compared with those obtained by a conventional, single-monochromator Brewer instrument. The comparison shows that the performance of the new instrument is much superior in the low UVB wavelengths (lambda<305 nm), because of the much higher stray-light rejection capability of the instrument. The conventional Brewer spectrophotometer is capable of recording accurately solar UVB spectra at wavelengths greater than about 300 nm. The error in UVB dose calculations made using data from the conventional Brewer is estimated to be of the order of +1%, while the absolute irradiances at wavelengths below 300 nm may be overestimated by more than 10%, depending on the measured wavelength and the solar zenith angle.