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The CU 2-D-MAX-DOAS Instrument – Part 2: Raman Scattering Probability Measurements and Retrieval of Aerosol Optical Properties

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The CU 2-D-MAX-DOAS Instrument – Part 2: Raman Scattering Probability Measurements and Retrieval of Aerosol Optical Properties Atmos. Meas. Tech., 9, 3893–3910, 2016 www.atmos-meas-tech.net/9/3893/2016/ doi:10.5194/amt-9-3893-2016 © Author(s) 2016. CC Attribution 3.0 License. The CU 2-D-MAX-DOAS instrument – Part 2: Raman scattering probability measurements and retrieval of aerosol optical properties Ivan Ortega1,2, Sean Coburn1,2, Larry K. Berg3, Kathy Lantz2,4, Joseph Michalsky2,4, Richard A. Ferrare5, Johnathan W. Hair5, Chris A. Hostetler5, and Rainer Volkamer1,2 1Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA 2Cooperative Institute for Research in Environmental Sciences (CIRES), Boulder, CO, USA 3Pacific Northwest National Laboratory, Richland, WA, USA 4Global Monitoring Division, Earth System Research Laboratory, NOAA, Boulder, CO, USA 5NASA Langley Research Center, Hampton, VA, USA Correspondence to: Rainer Volkamer ([email protected]) Received: 8 December 2015 – Published in Atmos. Meas. Tech. Discuss.: 18 January 2016 Revised: 12 July 2016 – Accepted: 13 July 2016 – Published: 23 August 2016 Abstract. The multiannual global mean of aerosol optical retrievals of AOD430 and g with data from a co-located depth at 550 nm (AOD550/ over land is ∼ 0.19, and that over CIMEL sun photometer, Multi-Filter Rotating Shadowband oceans is ∼ 0.13. About 45 % of the Earth surface shows Radiometer (MFRSR), and an airborne High Spectral AOD550 smaller than 0.1. There is a need for measurement Resolution Lidar (HSRL-2). The average difference (rela- techniques that are optimized to measure aerosol optical tive to DOAS) for AOD430 is C0:012 ± 0:023 (CIMEL), properties under low AOD conditions. We present an −0:012 ± 0:024 (MFRSR), −0:011 ± 0:014 (HSRL- inherently calibrated retrieval (i.e., no need for radiance 2), and C0:023 ± 0:013 (CIMELAOD − MFRSRAOD/ calibration) to simultaneously measure AOD and the aerosol and yields the following expressions for correla- phase function parameter, g, based on measurements of tions between different instruments: DOASAOD D azimuth distributions of the Raman scattering probability −.0:019 ± 0:006/ C .1:03 ± 0:02/× CIMELAOD 2 (RSP), the near-absolute rotational Raman scattering (RRS) (R D 0:98), DOASAOD D −.0:006 ± 0:005/ C .1:08 ± 2 intensity. We employ radiative transfer model simulations 0:02/× MFRSRAOD (R D 0:98), and CIMELAOD D 2 to show that for solar azimuth RSP measurements at solar .0:013 ± 0:004/C.1:05 ± 0:01/× MFRSRAOD (R D 0:99). elevation and solar zenith angle (SZA) smaller than 80◦, The average g measured by DOAS on both days was RSP is insensitive to the vertical distribution of aerosols and 0:66 ± 0:03, with a difference of 0:014 ± 0:05 compared to maximally sensitive to changes in AOD and g under near- CIMEL. Active steps to minimize the error in the RSP help molecular scattering conditions. The University of Colorado to reduce the uncertainty in retrievals of AOD and g. As two-dimensional Multi-AXis Differential Optical Absorp- AOD decreases and SZA increases, the RSP signal-to-noise tion Spectroscopy (CU 2-D-MAX-DOAS) instrument was ratio increases. At AOD430 ∼ 0.4 and 0.10 the absolute deployed as part of the Two Column Aerosol Project (TCAP) AOD errors are ∼ 0.014 and 0.003 at 70◦ SZA and 0.02 at Cape Cod, MA, during the summer of 2012 to measure di- and 0.004 at 35◦ SZA. Inherently calibrated, precise AOD rect sun spectra and RSP from scattered light spectra at solar and g measurements are useful to better characterize the relative azimuth angles (SRAAs) between 5 and 170◦. Dur- aerosol direct effect in urban polluted and remote pristine ing two case study days with (1) high aerosol load (17 July, environments. 0.3 < AOD430 < 0.6) and (2) near-molecular scattering con- ditions (22 July, AOD430 < 0:13) we compare RSP-based Published by Copernicus Publications on behalf of the European Geosciences Union. 3894 I. Ortega et al.: The CU 2-D-MAX-DOAS instrument – Part 2 1 Introduction the zenith. Measurements at low EA have maximum sensitiv- ity in the lowermost part of the atmosphere. More recently, Atmospheric aerosol particles play a key role in the energy two-dimensional MAX-DOAS (2-D-MAX-DOAS) has been balance of Earth’s atmosphere (IPCC, 2013). The aerosol op- shown to be a promising technique to measure the trace gas tical depth (AOD), defined as a vertical integral of the aerosol variability around the measurement site from scattered light extinction coefficient from the Earth surface to the top of the spectra at different azimuth angles (AAs), defined relative atmosphere, is an important input to assessments of how the to north (Wang et al., 2014; Ortega et al., 2015). The Uni- atmospheric aerosol burden affects the budget of incoming versity of Colorado (CU) 2-D-MAX-DOAS instrument has solar radiation in global climate models (Hansen et al., 2002; demonstrated range-resolved measurements of NO2 and oxy- Chung et al., 2005; McComiskey et al., 2008). McComiskey genated hydrocarbons from azimuth scans at low EA. The et al. (2008) studied the sensitivity of aerosol direct radia- spatial scale probed by 2-D-MAX-DOAS closely resembles tive forcing using representative uncertainties in currently es- the grid-cell size of atmospheric models and satellite pixels tablished methods to measure aerosol optical properties. For and can be used to systematically characterize chemical gra- a typical AOD uncertainty of 0.01 (best case scenario ex- dients under inhomogeneous conditions (Ortega et al., 2015). pected for newly calibrated ground-based radiometric instru- In this paper we exploit solar azimuth scattered light and ment in the visible spectra range; Eck et al., 1999; Holben direct sun measurements to assess aerosol column prop- et al., 1998), the error in the aerosol direct forcing is about erties using solar almucantar measurements. The informa- 0.6 W m−2 (top of the atmosphere) and 1.3 W m−2 (surface) tion content regarding aerosol properties using this geome- for a solar zenith angle (SZA) of 45◦ (McComiskey et al., try has been discussed in detail for radiance measurements 2008). The multiannual global mean AOD550 estimated from with single wavelength channel detectors elsewhere (Box satellites finds that about 28 and 43 % of the land surface and and Deepak, 1979; Nakajima et al., 1983; Kaufman et al., 15 and 46 % of the ocean surface have AOD ≤ 0.05 and ≤ 0.1 1994; Bohren and Huffman, 1998; Dubovik et al., 2000). We (Remer et al., 2008); current ground-based networks capture use solar almucantar scans in combination with hyperspec- frequent AOD values below 0.15 (Holben et al., 2001; Au- tral measurements and describe a new retrieval scheme to es- gustine et al., 2008; Michalsky et al., 2010; Mao et al., 2014). timate AOD430 and aerosol phase functions (simplified by Low AOD conditions are projected to be more prevalent in g, Henyey–Greenstein (HG) approximation) based on quan- the future (Westervelt et al., 2015). Under these conditions, titative analysis of the rotational Raman scattering (RRS) measurements of AOD with higher accuracy and precision by atmospheric molecules (Ring effect) (Grainger and Ring, are even more desirable. 1962; Chance and Spurr, 1997). RRS causes “filling-in” of Traditional AOD measurements often employ radiometric the solar Fraunhofer lines and has to be taken into account calibrated instruments, e.g., CIMEL sun photometer (Holben to accurately estimate absorption of trace gases using pas- et al., 1998) and Multifilter Rotating Shadowband Radiome- sive DOAS techniques (Platt and Stutz, 2008). Several stud- ter (MFRSR) (Harrison et al., 1994). In general, the retrieval ies have described the quantitative analysis of RRS and its of AOD is estimated based on the extinction of the direct effect in solar scattering UV–vis observations (Vountas et al., sun irradiance measurements. The quality of such measure- 1998, 2003; de Beek et al., 2001; Langford et al., 2007). The ments is improved under high AOD and cloud-free condi- quantitative analysis of RRS by DOAS was introduced by tions. In contrast, under molecular scattering conditions, i.e., Wagner et al. (2004, 2009a) with the so-called Raman scat- AOD430 < 0:13 (Rayleigh scattering extinction under over- tering probability (RSP) (the probability that a detected pho- head sun conditions), the measurements become subject to ton has undergone a rotational Raman scattering event). Un- higher relative uncertainties (Holben et al., 1998). Holben et der cloud-free conditions the AOD has a strong effect on the al. (1998) pointed out that the error in AOD by means of RSP, which further exhibits a high dependency on the so- solar sky brightness (scattering) in the solar aureole region lar relative azimuth angle (Wagner et al., 2009b, 2014). To may be lower than traditional direct sun extinction methods. the best of our knowledge, there has been no previous mea- However, to our knowledge, this has not previously been ex- surement of AOD and g using almucantar scans of RSP by ploited in measurements to date. MAX-DOAS. Multi-axis differential optical absorption spectroscopy (MAX-DOAS) can simultaneously retrieve trace gases and aerosol optical properties (Hönninger et al., 2004; Wagner et 2 Experimental al., 2004; Frieß et al., 2006; Clémer et al., 2010). The MAX- DOAS technique relies on spectrally resolved solar scattered 2.1 The Two-Column Aerosol Project (TCAP) field light measurements at several elevation angles (EAs), defined campaign between the horizon and zenith (Hönninger et al., 2004). The retrieval approach does not require radiometric calibration, The first phase of the Department of Energy (DOE) TCAP and the trace gases and aerosol optical properties are mea- field campaign took place at Cape Cod, MA, during the sum- sured relative to a reference spectrum, typically recorded in mer of 2012 (Berg et al., 2016).
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