Airborne observations of aerosol extinction by in situ and remote‐sensing techniques: Evaluation of particle hygroscopicity

Abstract

Extensive profiling of aerosol optical, chemical, and microphysical properties was performed in the Washington DC/Baltimore MD region in July 2011 during NASA DISCOVER‐AQ. In situ extinction coefficient (σext,in‐situ) measurements were made aboard the NASA P3‐B aircraft coincident with remote‐sensing observations by the High‐Spectral Resolution Lidar (HSRL; σext,HSRL) aboard the NASA UC‐12 aircraft. A statistical comparison revealed good agreement within instrumental uncertainty (σext,in‐situ = 1.1 σext,HSRL − 3.2 Mm−1, r2 = 0.88) and demonstrated the robust nature of hygroscopicity measurements (f(RH)) necessary to correct observations at dry relative humidity (RH) to ambient conditions. The average liquid‐water contribution to ambient visible‐light extinction was as much as 43% in this urban region. f(RH) values were observed to vary significantly from 1.1 to 2.1 on a day‐to‐day basis suggesting influence from both local and transported sources. Results emphasize the importance of accounting for the RH dependence of optical‐ and mass‐based aerosol air‐quality measurements (e.g., of PM2.5), especially in relation to satellite and remote‐sensing retrievals.

Observations of rapid aerosol optical depth enhancements in the vicinity of polluted cumulus clouds

“…Computations of the median radius of the fine mode show an increase from 0.15 µm for the two morning retrievals vs. 0.17 µm for the two afternoon retrievals. These size distribution retrievals and 380-500 nm Ångström exponent both suggest that at least some of the increased AOD (0.54 at 440 nm in afternoon vs. 0.25 in morning) was associated with particle growth, likely from humidification (Twohy et al, 2009;Ziemba et al, 2013) and/or cloud processing. Another case where retrievals of aerosol size distributions were made before, during and after the cumulus cloud convection cycle occurred at the GSFC site, not during the DISCOVER-AQ campaign but on 15 July 2013.…”

“…The comparison of the σ scat,dry measurements to the modeled σ scat,dry calculated using Mie theory reveals good agreement (slope of 0.991 ± 0.004 and r 2 of 0.98) and gives confidence in both the σ scat,dry and dry size distribution measurements (Ziemba et al, 2013). An additional and parallel three-wavelength integrating nephelometer operating at a RH controlled at 80 ± 4 % (Ziemba et al, 2013) was used to measure the wet scattering coefficient. The sample flow routed to both nephelometers was actively dried using a Nafion dryer (Perma-Pure FC-125-240-10PP) which efficiently passed accumulation mode aerosol (> 90 % transmission).…”

“…The scattering coefficient has been corrected from angular truncation errors and illumination intensity issues based on Anderson and Ogren (1998). The comparison of the σ scat,dry measurements to the modeled σ scat,dry calculated using Mie theory reveals good agreement (slope of 0.991 ± 0.004 and r 2 of 0.98) and gives confidence in both the σ scat,dry and dry size distribution measurements (Ziemba et al, 2013). An additional and parallel three-wavelength integrating nephelometer operating at a RH controlled at 80 ± 4 % (Ziemba et al, 2013) was used to measure the wet scattering coefficient.…”

“…Ziemba et al (2013) compared in situ extinction coefficient measurements (adjusted to 532 nm) coincident with remote-sensing observations performed by the HSRL (measured at 532 nm). This comparison revealed good agreement (slope 1.11 and Atmos.…”

Profiling aerosol optical, microphysical and hygroscopic properties in ambient conditions by combining in situ and remote sensing

“…To date, such efforts focused mostly on low-humidity profiles, where there is no difference between the ambient remote sensing measurements and the in situ measurements performed under dry conditions inside the instruments (e.g., Weinzierl et al, 2009). High-humidity conditions have also been studied, but only for fine mode particle properties (e.g., Ziemba et al, 2013), as the coarse particle hygroscopic growth is not as easily constrained with in situ airborne techniques, mainly due to sampling inlet loses. The In situ/Remote sensing aerosol Retrieval Algorithm (IRRA) approach presented here addresses these limitations through the combination of in situ and active remote sensing measurements with hygroscopic modeling, making possible the vertical profiling of fine and coarse particles even for humid conditions.…”