Alexander Cede

NO2 column amounts from ground‐based Pandora and MFDOAS spectrometers using the direct‐sun DOAS technique: Intercomparisons and application to OMI validation

[1] Vertical column amounts of nitrogen dioxide, C(NO 2 ), are derived from ground-based direct solar irradiance measurements using two new and independently developed spectrometer systems, Pandora (Goddard Space Flight Center) and MFDOAS (Washington State University). We discuss the advantages of C(NO 2 ) retrievals based on Direct Sun - Differential Optical Absorption Spectroscopy (DS-DOAS). The C(NO 2 ) data are presented from field campaigns using Pandora at Aristotle University (AUTH), Thessaloniki, Greece; a second field campaign involving both new instruments at Goddard Space Flight Center (GSFC), Greenbelt, Maryland; a Pandora time series from December 2006 to October 2008 at GSFC; and a MFDOAS time series for spring 2008 at Pacific Northwest National Laboratory (PNNL), Richland, Washington. Pandora and MFDOAS were compared at GFSC and found to closely agree, with both instruments having a clear-sky precision of 0.01 DU (1 DU = 2.67 × 10 16 molecules/cm 2 ) and a nominal accuracy of 0.1 DU. The high precision is obtained from careful laboratory characterization of the spectrometers (temperature sensitivity, slit function, pixel to pixel radiometric calibration, and wavelength calibration), and from sufficient measurement averaging to reduce instrument noise. The accuracy achieved depends on laboratory-measured absorption cross sections and on spectrometer laboratory and field calibration techniques used at each measurement site. The 0.01 DU precision is sufficient to track minute-by-minute changes in C(NO 2 ) throughout each day with typical daytime values ranging from 0.2 to 2 DU. The MFDOAS instrument has better noise characteristics for a single measurement, which permits MFDOAS to operate at higher time resolution than Pandora for the same precision. Because Pandora and MFDOAS direct-sun measurements can be made in the presence of light to moderate clouds, but with reduced precision (~0.2 DU for moderate cloud cover), a nearly continuous record can be obtained, which is important when matching OMI overpass times for satellite data validation. Comparisons between Pandora and MFDOAS with OMI are discussed for the moderately polluted GSFC site, between Pandora and OMI at the AUTH site, and between MFDOAS and OMI at the PNNL site. Validation of OMI measured C(NO 2 ) is essential for the scientific use of the satellite data for air quality, for atmospheric photolysis and chemistry, and for retrieval of other quantities (e.g., accurate atmospheric correction for satellite estimates of ocean reflectance and bio-optical properties). Changes in the diurnal variability of C(NO 2 ) with season and day of the week are presented based on the 2-year time series at GSFC measured by the Pandora instrument.

Validation of OMI tropospheric NO2 column densities using direct‐Sun mode Brewer measurements at NASA Goddard Space Flight Center

[1] This paper presents a comparison of NO 2 data measured with the Ozone Monitoring Instrument (OMI) on board the EOS-AURA satellite with ground-based direct-Sun Brewer measurement data. Since its deployment in July 2004, OMI has provided more than 2 years of daily high-resolution (∼13 x 24 km 2 at nadir) NO 2 vertical column density maps. We describe the retrieval, which includes an estimation of the stratospheric and tropospheric fraction of total NO 2 columns, the air mass factor (AMF) correction based on detected tropospheric NO 2 enhancements, and the generation of the gridded data product. We present a validation study of the gridded NO 2 data set using data from a Brewer MK3 double monochromator in direct-Sun mode located at NASA Goddard Space Flight Center in Greenbelt, Maryland, USA. Monthly averages of coinciding measurements correlate well (r = 0.9) but OMI data are about 25% lower than the Brewer measurement data (slope 0.75, intercept -0.38 x 10 15 molecules/cm 2 ). We present a detailed uncertainty analysis for both ground and satellite data and discuss the possible reasons for the observed differences.

Direct spectral measurements with a Brewer spectroradiometer: absolute calibration and aerosol optical depth retrieval.

We present three different methods for the absolute calibration of direct spectral irradiances measured with a Brewer spectroradiometer, which are shown to agree to within +/- 2%. Direct irradiance spectra derived by Brewer and Bentham spectroradiometers agree to within 4 +/- 3%. Good agreement was also found by a comparison of the aerosol optical depth and Angstrom exponent retrieved by the two instruments and a multifilter rotational shadowband radiometer. The spectral aerosol optical depth (300-365 nm) derived from six years of direct irradiance measurements at Thessaloniki shows a distinct seasonal variation, averaging to approximately 0.3 at 340 nm in winter and approximately 0.7 in summer.

Measurements of nitrogen dioxide total column amounts using a Brewer double spectrophotometer in direct Sun mode

10 16 molecules cm � 2 ), which is more accurate than scattered light measurements for high NO2 amounts. Measured NO2 column amounts, ranging from 0 to 3 DU with a mean of 0.7 DU, show a pronounced daily course and a strong variability from day to day. The NO2 concentration typically increases from sunrise to noon. In the afternoon it decreases in summer and stays constant in winter. As expected from the anthropogenic nature of its source, NO2 amounts on weekends are significantly reduced. The measurements were compared to satellite retrievals from Scanning Image Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY). Satellite data give the same average NO2 column and show a seasonal cycle that is similar to the ground data in the afternoon. We show that NO2 must be considered when retrieving aerosol absorption properties, especially for situations with low aerosol optical depth.

Extreme solar total and UV irradiances due to cloud effect measured near the summer solstice at the high-altitude desertic plateau Puna of Atacama (Argentina)

Broadband data of solar total (300–3000nm) and ultraviolet (UV) (295–385nm) irradiances as far as spectrometric data measured near the Southern Hemisphere summer solstice at the high-altitude desertic plateau Puna of Atacama, Argentina, are presented. The simultaneous contributions of small solar zenith angle, high-altitude desertic intertropical region plus a cumulus cloud formation around the Suns angular position in the sky produced extreme high intensities. In particular, several measured total irradiances in a time interval of about one-and-a half hours near noon were higher than the extraterrestrial solar constant corrected for the actual Earth–Sun distance on these days (1412W/m2), reaching a maximum value of 1528W/m2 at 12:09 (Argentina local time, i.e. UT—3h) corresponding to a solar zenith angle of 16°. The highest UV irradiance measured was 69.5W/m2 at 13:29 (θz=2.4°). Mean percentage increases due to cloud effect of 6% for UV and 12% for total solar irradiances were estimated with respect to the situation of no-clouds near Sun. The mean UV over total irradiances percentage ratio was also determined giving 4.7% at the moment of cloud enhancement and 5% outside it, which shows that the UV component was also increased, but to a lesser extent. So situations with cumulus cloud formation in this region in relation to its biological and material degradation impacts must be taken into consideration. Photographs showing the particular configuration of the Sun—cumulus cloud system obtained at moments of very high solar intensities are also presented.

Attenuation of Erythemal Effective Irradiance by Cloudiness at Low and High Altitude in the Alpine Region

Abstract— The transmittance of cloudiness was examined for the daily totals of global erythemal effective irradiance and global total irradiance in Innsbruck (577 m above sea level [a.s.l.], Western Austria) for the periods 1981‐1988 and 1993‐1994 and at Jungfraujoch (3576 m a.s.l., Switzerland) for the period 1981‐1990. The influence of varying cloudiness on the daily totals of global erythemal effective irradiance is considerably greater than the influence of varying ozone. The lowest transmittance for daily totals of global erythemal effective irradiance is 9.4% in Innsbruck (577 m a.s.l.) and 23.5% at Jungfraujoch (3576 m a.s.l.); the median and the 25‐75 percentile range at 10/10 cloudiness are 41.8% (28.9‐56.7%) and 76.8% (63.5‐86.1%), respectively. The greater transmittance of global erythemal effective irradiance at Jungfraujoch originates from smaller thickness of the cloud layer in high mountains than in valleys. Similar transmittances were obtained for the daily totals of global total irradiance (300‐3000 nm) as a function of cloudiness.

Spatial and temporal variability of ozone and nitrogen dioxide over a major urban estuarine ecosystem

Spatial and temporal dynamics in trace gas pollutants were examined over a major urban estuarine ecosystem, using a new network of ground-based Pandora spectrometers deployed at strategic locations along the Washington-Baltimore corridor and the Chesapeake Bay. Total column ozone (TCO3) and nitrogen dioxide (TCNO2) were measured during NASA’s DISCOVER-AQ and GeoCAPE-CBODAQ campaigns in July 2011. The Pandora network provided high-resolution information on air-quality variability, local pollution conditions, large-scale meteorological influences, and interdependencies of ozone and its major precursor, NO2. Measurements were used to compare with air-quality model simulations (CMAQ), evaluate Aura-OMI satellite retrievals, and assess advantages and limitations of space-based observations under a range of conditions. During the campaign, TCNO2 varied by an order of magnitude, both spatially and temporally. Although fairly constant in rural regions, TCNO2 showed clear diurnal and weekly patterns in polluted urban areas caused by changes in near-surface emissions. With a coarse resolution and an overpass at around 13:30 local time, OMI cannot detect this strong variability in NO2, missing pollution peaks from industrial and rush hour activities. Not as highly variable as NO2, TCO3 was mostly affected by large-scale meteorological patterns as observed by OMI. A clear weekly cycle in TCO3, with minima over the weekend, was due to a combination of weekly weather patterns and changes in near-surface NOx emissions. A Pandora instrument intercomparison under the same conditions at GSFC showed excellent agreement, within ±4.8DU for TCO3 and ±0.07DU for TCNO2 with no air-mass-factor dependence, suggesting that observed variability during the campaign was real.

Comparison of atmospheric spectral radiance measurements from five independently calibrated systems

A variety of instruments have been developed over the past 50 years to measure spectral radiance in absolute units at UV and visible wavelengths with high spectral resolution. While there is considerable experience in the measurement of spectral irradiance, less emphasis has been given to the reliable measurement of spectral radiance from ground observations. We discuss the methodology and calibration procedures for five instruments capable of making such measurements. Four of these instruments are based on double monochromators that scan each wavelength in turn, and one is based on a single monochromator with a charged coupled device (CCD) allowing the recording of all wavelengths simultaneously. The measured spectral radiance deviates between 3% and about 35% depending on the instruments. The results are compared with radiative transfer calculations when the aerosol characteristics of the atmosphere are known.

Direct Sun measurements of NO2 column abundances from Table Mountain, California: Intercomparison of low- and high-resolution spectrometers

The NO_2 total column abundance, C_(NO_2) was measured with a direct Sun viewing technique using three different instruments at NASA Jet Propulsion Laboratorys (JPL) Table Mountain Facility in California during an instrument intercomparison campaign in July 2007. The instruments are a high‐resolution (∼0.001 nm) Fourier transform ultraviolet spectrometer (FTUVS) from JPL and two moderate‐resolution grating spectrometers, multifunction differential optical absorption spectroscopy (MF‐DOAS) (∼0.8 nm) from Washington State University and Pandora (∼0.4 nm) from NASA Goddard Space Flight Center. FTUVS uses high spectral resolution to determine the absolute NO_2 column abundance independently from the exoatmospheric solar irradiance using rovibrational NO_2 absorption lines. The NO_2 total column is retrieved after removing the solar background using Doppler‐shifted spectra from the east and west limbs of the Sun. The FTUVS measurements were used to validate the independently calibrated measurements of multifunction differential optical absorption spectroscopy (MF‐DOAS) and Pandora. The latter two instruments start with measured high‐Sun spectra as solar references to retrieve relative NO_2 columns and then apply modified Langley or “bootstrap” methods to determine the amounts of NO_2 in the references to obtain the absolute NO_2 columns. The calibration offset derived from the FTUVS measurements is consistent with the values derived from Langley and bootstrap calibration plots of the NO_2 slant column measured by the grating spectrometers. The calibrated total vertical column abundances of NO_2, C_(NO_2) from all three instruments are compared showing that MF‐DOAS and Pandora data agree well with each other, and both data sets agree with FTUVS data to within (1.5 ± 4.1)% and (6.0 ± 6.0)%, respectively.

Atmospheric NO2 dynamics and impact on ocean color retrievals in urban nearshore regions

Urban nearshore regions are characterized by strong variability in atmospheric composition, associated with anthropogenic emissions and meteorological processes that influence the circulation and accumulation of atmospheric pollutants at the land-water interface. If not adequately corrected in satellite retrievals of ocean color, this atmospheric variability can impose a false impression of diurnal and seasonal changes in nearshore water quality and biogeochemical processes. Consideration of these errors is important for measurements from polar orbiting ocean color sensors but becomes critical for geostationary satellite missions having the capability for higher frequency and higher spatial resolution observations of coastal ocean dynamics. We examined variability in atmospheric NO2 over urban nearshore environments in the Eastern US, Europe, and Korea, using a new network of ground-based Pandora spectrometers and Aura-OMI satellite observations. Our measurements in the US and in Europe revealed clear diurnal and day-of-the-week patterns in total column NO2 (TCNO2), temporal changes as large as 0.8 DU within 4 h, and spatial variability as large as 0.7 DU within an area often covered by just a single OMI pixel. TCNO2 gradients were considerably stronger over the coastal cities of Korea. With a coarse resolution and an overpass at around 13:30 local time, OMI cannot detect this strong variability in NO2, missing pollution peaks from industrial and rush hour activities. Observations were combined with air quality model simulations and radiative transfer calculations to estimate the impact of atmospheric NO2 variability on satellite retrievals of coastal ocean remote sensing reflectance and biogeochemical variables (i.e., chlorophyll and CDOM).