Hyperspectral aerosol optical depths from TCAP flights

Abstract

[1] The 4STAR (Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research), a hyperspectral airborne Sun photometer, acquired aerosol optical depths (AOD) at 1 Hz during all July 2012 flights of the Two-Column Aerosol Project. Root-mean-square differences from Aerosol Robotic Network ground-based observations were 0.01 at wavelengths between 500-1020 nm, 0.02 at 380 and 1640 nm, and 0.03 at 440 nm in four clear-sky fly-over events, and similar in ground side-by-side comparisons. Changes in the above-aircraft AOD across 3 km deep spirals were typically consistent with integrals of coincident in situ (on Department of Energy Gulfstream 1 with 4STAR) and lidar (on NASA B200) extinction measurements within 0.01, 0.03, 0.01, 0.02, 0.02, and 0.02 at 355, 450, 532, 550, 700, and 1064 nm, respectively, despite atmospheric variations and combined measurement uncertainties. Finer vertical differentials of the 4STAR measurements matched the in situ ambient extinction profile within 14% for one homogeneous column. For the AOD observed between 350 and 1660 nm, excluding strong water vapor and oxygen absorption bands, estimated uncertainties were~0.01 and dominated by (then) unpredictable throughput changes, up to ±0.8%, of the fiber optic rotary joint. The favorable intercomparisons herald 4STAR's spatially resolved high-frequency hyperspectral products as a reliable tool for climate studies and satellite validation.

Airborne Aerosol in Situ Measurements during TCAP: A Closure Study of Total Scattering

“…The total scattering coefficient at three wavelengths (0.45, 0.55, 0.7 µm) was measured at dry (RH < 20%) conditions using a TSI integrating nephelometer (Figure 4), while the light scattering hygroscopic growth, known as f(RH), was measured using a humidification system at three defined RHs (near 45%, 65% and 90%) at a single wavelength (0.525 µm) [51]. Similar to Shinozuka et al [52], we adjust the f(RH) obtained at the 0.525 µm wavelength to the nephelometer wavelengths (0.45, 0.55, 0.7 µm) by multiplying the obtained f(RH) by 0.98, 1.01 and 1.04, respectively. The conventional truncation error correction [53] has been applied to the total scattering measured by the integrating nephelometer.…”

NO&lt;sub&gt;2&lt;/sub&gt; and HCHO measurements in Korea from 2012 to 2016 from Pandora spectrometer instruments compared with OMI retrievals and with aircraft measurements during the KORUS-AQ campaign

“…11). The 4STAR is an airborne sunphotometer, capable of measuring total C(NO 2 ), C(O 3 ), water vapor and AOD columns in its direct-sun mode (SegalRozenhaimer et al, 2014;Shinozuka et al, 2013), which is similar to the mode used by the PSI network.…”

Aerosol light-scattering enhancement due to water uptake during the TCAP campaign

“…The mean ± standard deviations of the temperature and relative humidity within the dry nephelometer for the whole measurement campaign were T = 26 ± 4 • C and RH = 30 ± 13 %. In order to minimize losses of volatile compounds, the temperature of the sampled air was kept below 35 • C (Bergin et al, 1997;ten Brink et al, 2000). Only 0.5 % of the 1 min observations occurred at temperatures above this value and these data were not further considered in the study.…”

“…These measurements at low RH can differ from what would be observed at ambient conditions and thus difficult to relate to observations of the radiative energy budget. Therefore, knowledge of the scattering enhancement due to water uptake is of great importance in order to transform dry measurements into more relevant ambient measurements, especially when comparing in situ with remote sensing measurements (e.g., Zieger et al, 2011Zieger et al, , 2012Esteve et al, 2012;Shinozuka et al, 2013) or for satellite retrievals (e.g., Wang and Martin, 2007).…”