Inversion of tropospheric profiles of aerosol extinction and HCHO and NO 2 mixing ratios from MAX-DOAS observations in Milano during the summer of 2003 and comparison with independent data sets

Abstract. We present aerosol and trace gas profiles derived from MAX-DOAS observations. Our inversion scheme is based on simple profile parameterisations used as input for an atmospheric radiative transfer model (forward model). From a least squares fit of the forward model to the MAX-DOAS measurements, two profile parameters are retrieved including integrated quantities (aerosol optical depth or trace gas vertical column density), and parameters describing the height and shape of the respective profiles. From these results, the aerosol extinction and trace gas mixing ratios can also be calculated. We apply the profile inversion to MAX-DOAS observations during a measurement campaign in Milano, Italy, September 2003, which allowed simultaneous observations from three telescopes (directed to north, west, south). Profile inversions for aerosols and trace gases were possible on 23 days. Especially in the middle of the campaign (17–20 September 2003), enhanced values of aerosol optical depth and NO2 and HCHO mixing ratios were found. The retrieved layer heights were typically similar for HCHO and aerosols. For NO2, lower layer heights were found, which increased during the day. The MAX-DOAS inversion results are compared to independent measurements: (1) aerosol optical depth measured at an AERONET station at Ispra; (2) near-surface NO2 and HCHO (formaldehyde) mixing ratios measured by long path DOAS and Hantzsch instruments at Bresso; (3) vertical profiles of HCHO and aerosols measured by an ultra light aircraft. Depending on the viewing direction, the aerosol optical depths from MAX-DOAS are either smaller or larger than those from AERONET observations. Similar comparison results are found for the MAX-DOAS NO2 mixing ratios versus long path DOAS measurements. In contrast, the MAX-DOAS HCHO mixing ratios are generally higher than those from long path DOAS or Hantzsch instruments. The comparison of the HCHO and aerosol profiles from the aircraft showed reasonable agreement with the respective MAX-DOAS layer heights. From the comparison of the results for the different telescopes, it was possible to investigate the internal consistency of the MAX-DOAS observations. As part of our study, a cloud classification algorithm was developed (based on the MAX-DOAS zenith viewing directions), and the effects of clouds on the profile inversion were investigated. Different effects of clouds on aerosols and trace gas retrievals were found: while the aerosol optical depth is systematically underestimated and the HCHO mixing ratio is systematically overestimated under cloudy conditions, the NO2 mixing ratios are only slightly affected. These findings are in basic agreement with radiative transfer simulations.

[1]  S. Reimann,et al.  Volatile Organic Compounds in the Po Basin. Part A: Anthropogenic VOCs , 2005 .

[2]  T. Eck,et al.  Variability of Absorption and Optical Properties of Key Aerosol Types Observed in Worldwide Locations , 2002 .

[3]  J. Burrows,et al.  Enhanced O3 and NO2 in thunderstorm clouds: Convection or production? , 1999 .

[4]  Ulrich Platt,et al.  Observations of BrO and its vertical distribution during surface ozone depletion at Alert , 2002 .

[5]  Steffen Beirle,et al.  Comparison of ambient aerosol extinction coefficients obtained from in-situ, MAX-DOAS and LIDAR measurements at Cabauw , 2010 .

[6]  Yugo Kanaya,et al.  First retrieval of tropospheric aerosol profiles using MAX-DOAS and comparison with lidar and sky radiometer measurements , 2008 .

[7]  Yugo Kanaya,et al.  Dual-wavelength aerosol vertical profile measurements by MAX-DOAS at Tsukuba, Japan , 2008 .

[8]  Stefan Reimann,et al.  A photochemical modeling study of ozone and formaldehyde generation and budget in the Po basin , 2007 .

[9]  Klaus Pfeilsticker,et al.  The Monte Carlo atmospheric radiative transfer model McArtim: Introduction and validation of Jacobians and 3D features , 2011 .

[10]  J. Lambert,et al.  Retrieval of Tropospheric BrO and NO2 from UV-Visible Observations , 2004 .

[11]  Stanley C. Solomon,et al.  On the interpretation of zenith sky absorption measurements , 1987 .

[12]  Clive D Rodgers,et al.  Inverse Methods for Atmospheric Sounding: Theory and Practice , 2000 .

[13]  K. Pfeilsticker,et al.  Cloudy sky optical paths as derived from differential optical absorption spectroscopy observations , 1998 .

[14]  Johannes Orphal,et al.  The temperature and pressure dependence of the absorption cross-sections of NO2 in the 250–800 nm region measured by Fourier-transform spectroscopy , 2002 .

[15]  Tong Zhu,et al.  Spatial and temporal variations of aerosols around Beijing in summer 2006: 2. Local and column aerosol optical properties , 2010 .

[16]  Klaus Pfeilsticker,et al.  On the influence of tropospheric clouds on zenith‐scattered‐light measurements of stratospheric species , 1995 .

[17]  T. Eck,et al.  An emerging ground-based aerosol climatology: Aerosol optical depth from AERONET , 2001 .

[18]  Ulrich Platt,et al.  MAX‐DOAS O4 measurements: A new technique to derive information on atmospheric aerosols: 2. Modeling studies , 2006 .

[19]  R. L. Curier,et al.  The 2005 and 2006 DANDELIONS NO2 and aerosol intercomparison campaigns , 2008 .

[20]  I. Pundt,et al.  Multibeam long-path differential optical absorption spectroscopy instrument: a device for simultaneous measurements along multiple light paths. , 2005, Applied optics.

[21]  John P. Burrows,et al.  MAX-DOAS measurements of atmospheric trace gases in Ny- ˚ Alesund - Radiative transfer studies and their application , 2004 .

[22]  F. Hendrick,et al.  Multiple wavelength retrieval of tropospheric aerosol optical properties from MAXDOAS measurements in Beijing , 2010 .

[23]  S. Reimann,et al.  Volatile Organic Compounds in the Po Basin. Part B: Biogenic VOCs , 2005 .

[24]  Christoph Kern,et al.  Intercomparison of four different in-situ techniques for ambient formaldehyde measurements in urban air , 2005 .

[25]  Min Shao,et al.  MAX-DOAS measurements in southern China: retrieval of aerosol extinctions and validation using ground-based in-situ data , 2010 .

[26]  Ulrich Platt,et al.  Determination of aerosol properties from MAX-DOAS observations of the Ring effect , 2009 .

[27]  Geert K. Moortgat,et al.  Temperature dependence of the absorption cross sections of formaldehyde between 223 and 323 K in the wavelength range 225–375 nm , 2000 .

[28]  Yugo Kanaya,et al.  Intercomparison of slant column measurements of NO 2 and O 4 by MAX-DOAS and zenith-sky UV and visible spectrometers , 2010 .

[29]  W. Junkermann,et al.  A New Portable Instrument for Continuous Measurement of Formaldehyde in Ambient Air , 2006 .

[30]  John P. Burrows,et al.  MAX-DOAS measurements of formaldehyde in the Po-Valley , 2004 .

[31]  Christopher A. Cantrell,et al.  Technical Note: Review of methods for linear least-squares fitting of data and application to atmospheric chemistry problems , 2008 .

[32]  W. Junkermann.  On the distribution of formaldehyde in the western Po-Valley, Italy, during FORMAT 2002/2003 , 2009 .

[33]  Ulrich Platt,et al.  Differential optical absorption spectroscopy , 2008 .

[34]  T. Kelly,et al.  Continuous Monitoring of Gaseous Formaldehyde Using an Improved Fluorescence Approach , 1994 .

[35]  James B. Burkholder,et al.  Absorption measurements of oxygen between 330 and 1140 nm , 1990 .

[36]  Ulrich Platt,et al.  MAX‐DOAS O4 measurements: A new technique to derive information on atmospheric aerosols—Principles and information content , 2004 .

[37]  T. Wagner,et al.  Multi axis differential optical absorption spectroscopy (MAX-DOAS) of gas and aerosol distributions. , 2005, Faraday discussions.