A Closure Study of Total Scattering Using Airborne In Situ Measurements from the Winter Phase of TCAP
We examine the performance of our approach for calculating the total scattering coefficient of both non-absorbing and absorbing aerosol at ambient conditions from aircraft data. Our extended examination involves airborne in situ data collected by the U.S. Department of Energy's (DOE) Gulf Stream 1 aircraft during winter over Cape Cod and the western North Atlantic Ocean as part of the Two-Column Aerosol Project (TCAP). The particle population represented by the winter dataset, in contrast with
... , in contrast with its summer counterpart, contains more hygroscopic particles and particles with an enhanced ability to absorb sunlight due to the larger fraction of black carbon. Moreover, the winter observations are characterized by more frequent clouds and a larger fraction of super-micron particles. We calculate model total scattering coefficient at ambient conditions using size spectra measured by optical particle counters (OPCs) and ambient complex refractive index (RI) estimated from measured chemical composition and relative humidity (RH). We demonstrate that reasonable agreement (~20% on average) between the observed and calculated scattering can be obtained under subsaturated ambient conditions (RH < 80%) by applying both screening for clouds and chemical composition data for the RI-based correction of the OPC-derived size spectra. Keywords: aircraft measurements of aerosol microphysical; chemical, and optical components and ambient relative humidity; ultra-high sensitivity aerosol spectrometer (UHSAS); passive cavity aerosol spectrometer (PCASP); cloud and aerosol spectrometer (CAS); aerosol mass spectrometer (AMS); single particle soot photometer (SP2); single particle mass spectrometer (miniSPLAT); integrating nephelometer; humidification system; winter phase of Two-Column Aerosol Project (TCAP) coefficients from approximately one year of measurements made at Cape Cod, Massachusetts as part of the Two-Column Aerosol Project (TCAP) campaign [10, 11] and demonstrated that there were three major groups of air masses. These groups have been classified as "anthropogenic", "marine", and "hybrid" air-masses in terms of scattering Angstrom exponent (AE). The "anthropogenic" and "marine" air-masses have large (1.8 ± 0.5) and small (0.9 ± 0.3) values of AE, respectively  . Since the AE is related inversely to particle size , the "anthropogenic" and "marine" air-masses are dominated by relatively small and large particles, respectively. It is quite interesting that the "anthropogenic" air-mass patterns have been more frequently observed during summer, while their "marine" counterparts were common during winter [10, 11] . The TCAP provided a unique opportunity to characterize aerosol physical, chemical and optical properties at the site during summer and winter, and thus to document strong seasonal changes of these properties [10, 11] . These properties have been obtained from integrated data provided by a suite of TCAP ground-based and airborne instruments with different design and sensitivity to particle size [10, 11] . Thus, successful data integration is a difficult task with far-reaching impacts on further process-oriented model evaluations. This is especially true for airborne measurements mainly due to well-known stringent requirements for the high-resolution data acquisition by a compact and multiple-variable aircraft system [13, 14] . Typically, a special kind of quantitative comparison experiment-as it traditionally referred to as a closure study-is performed to assess the consistency and reasonableness of integrated data     . We have previously performed a closure study of ambient total scattering coefficient using integrated TCAP data collected by the U.S. Department of Energy's (DOE) Gulfstream 1 (G-1) aircraft over Cape Cod and the western North Atlantic Ocean during summer and obtained good agreement (~10% on average) between the measured and calculated total scattering coefficients for mostly clear-sky conditions with high concentration of small particles  . In comparison with summer, winter represents more challenging observational conditions for such a comparison because: (1) more frequent cloudy days, which increases the variability of the observed scattering coefficients; and (2) increased fraction of large particles, which have often been ignored in the closure studies due to limited information on their chemical composition. With this challenge in mind, we attempt to address the following question: What level of agreement between the measured and calculated total scattering coefficients can be achieved for challenging wintertime conditions? To answer this question, we first describe the airborne in situ data collected during winter campaign that provide information on aerosol chemical, microphysical, and optical properties. We do that with a particular focus on the cloud contamination issue and the required cloud screening application (Section 2). Also, our study aims to illustrate the differences between the airborne in situ data collected during summer  and winter (this study), which in turn defines seasonal diversity between the corresponding microphysical and optical aerosol properties (Section 3) in the context of the closure study (Section 4). Ambient size distribution and complex refractive index are examples of key aerosol properties for the closure study. The latter are required to calculate the ambient total scattering coefficient over the variety of observational conditions (Section 4). Comparison of the calculated and observed ambient total scattering coefficients is considered as part of the closure study (Section 4) with an overall goal to illustrate the consistency and reasonableness of the wintertime airborne in situ data. To ease comparison of the outlined seasonal changes of the microphysical, chemical, and optical aerosol properties (winter versus summer), we use the same format for our winter plots (Sections 2-4) identical to that for the corresponding summer plots  . Our main findings are summarized in the last section.