Clear‐column closure studies of aerosols and water vapor aboard the NCAR C‐130 during ACE‐Asia, 2001

Column closure studies are a tool to assess whether in situ and remote measurements of aerosol optical properties on a given aircraft are mutually consistent. In this paper we describe aerosol and water vapor column closure studies on the basis of instrumentation flown aboard the NCAR C‐130 aircraft in the ACE‐Asia field experiment in March–May 2001. For in situ observations, aerosol particles were sampled through a newly designed low‐turbulence inlet (LTI). In 28 profiles extending to altitudes of up to 8 km, the in situ observations of scattering and absorption were compared to measurements with the six‐channel NASA Ames Airborne Tracking Sun photometer (AATS‐6). The comparison of Sun photometer and in situ‐derived layer aerosol optical depth (AOD) at 550 nm showed agreement (closure) within the measurement uncertainties in 25 out of 28 case studies. The average difference in layer AOD derived from the two methods was 0.03, corresponding to an average difference of 11.5%. The uncertainties in AATS‐6‐derived layer AOD ranged between 5 and 59% (with a mean of 22%), and for the first time included an estimate for the uncertainty in layer AOD caused by possible horizontal variability in AOD encountered in the vertical profile. The average uncertainty in AATS‐6‐derived layer AOD due to possible horizontal variability alone was 19%. The uncertainties in in situ‐derived layer AOD were between 10 and 55% (with a mean of 19%). Stratification of the extinction closure data by ambient relative humidity (RH) revealed that in situ‐derived aerosol extinction at low ambient relative humidity (<20% RH) tended to be slightly less than Sun photometer‐derived aerosol extinction, while in situ‐derived aerosol extinction at higher relative humidity was slightly greater than the Sun photometer‐derived values. Stratification of the extinction closure data by the fine mode fraction of scattering indicated a modest enhancement of coarse mode extinction in the combined LTI/plumbing system. Analogous closure studies for layer water vapor and water vapor density showed that AATS‐6 measured these quantities with very high accuracy, with correlation coefficients of 0.989 and 0.955 (rms differences of 10% and 33%), respectively.