Intercomparison of multiaxis and long‐path differential optical absorption spectroscopy measurements in the marine boundary layer

Multiaxis differential optical absorption spectroscopy (MAX‐DOAS) has many advantages that make it ideally suited for trace gas monitoring. MAX‐DOAS instruments can be relatively small, are easy to operate, and allow long‐term automated operation. The MAX‐DOAS technique also allows derivation of vertical aerosol and trace gas profiles in the troposphere and thus the monitoring of pollution aloft. However, this relatively new technique still requires validation to determine uncertainties, for example, introduced by the radiative transfer modeling needed to derive trace gas concentrations. Here we present MAX‐DOAS measurements of NO2 and HCHO performed in the Gulf of Maine during the ICARTT campaign in summer 2004. O4 measurements were performed to gain information on the vertical distribution of aerosols, which were used in radiative transfer calculations to convert the measured trace gas slant column densities to concentrations. The successful identification of an aerosol layer between 1 and 2 km altitude illustrates the potential for aerosol remote sensing. The comparison with LP DOAS measurements performed simultaneously shows that MAX‐DOAS accurately measures trace gases that are well mixed within the boundary layer. MAX‐DOAS measurements also provide valuable information on the vertical distribution of pollutants in and above the boundary layer, as demonstrated by the identification of elevated HCHO levels in a layer between 1 km and 2 km altitude. This phenomenon, which is associated with outflow of continental air from the American northeast, was not detected by in situ ground‐based instruments.