Abstract
Abstract. We report on the retrieval of aerosol extinction profiles at four wavelengths from ground-based multi-axis differential absorption spectroscopy (MAXDOAS) measurements performed in Beijing, China. Measurements were made over a 10-month time period (June 2008 to April 2009) using a newly developed MAXDOAS instrument. A retrieval algorithm, based on an on-line implementation of the radiative transfer code LIDORT and the optimal estimation technique, has been designed to provide near real time information on aerosol extinction vertical profiles. The algorithm was applied to O4 measurements at four wavelengths (360, 477, 577, and 630 nm). The total aerosol optical depths (AODs) calculated from the retrieved profiles exhibit higher values in spring and summer and lower values in autumn and winter. Comparison of the retrieved total AODs with values from a co-located CIMEL sunphotometer revealed a good correlation. The best results are obtained for the UV region with a correlation coefficient (R) of 0.91 and a slope of the linear regression fit of 1.1. At the longest wavelength, R drops down to 0.67 and the slope increases to 1.5. The results confirm that good quality O4 slant column measurements are essential for the success of the retrievals. A method is presented to determine a correction factor to account for systematic errors. It is demonstrated that the algorithm is capable of reliably retrieving aerosol extinction profiles for a wide range of atmospheric conditions (total AODs at 360 nm ranging from about 0.1 to 3). The results open up new perspectives for the extension of the algorithm for the near real time retrieval of trace gas vertical profiles.
Highlights
China’s rapid industrial development, urbanization, and the consequent large increase in energy consumption – primarily provided by coal-fired power plants – have resulted in an alarming deterioration of air quality
We report on the application of a new retrieval algorithm developed for near real time automated retrieval of aerosol extinction vertical profiles on multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements made in Beijing during the June 2008–April 2009 period
The aerosol extinction vertical profile is retrieved from a set of measurements, which in our case is a set of fitted O4 differential slant column densities (DSCDs) for different LOS directions in one DOAS scan
Summary
China’s rapid industrial development, urbanization, and the consequent large increase in energy consumption – primarily provided by coal-fired power plants – have resulted in an alarming deterioration of air quality. Long-term ground-based measurements providing information on the total column and on the vertical distribution of tropospheric aerosol and trace gases are indispensable. Such time series can be used to monitor the evolution of air pollution, to evaluate the direct effect of measures taken to improve air quality, and to validate air quality models and satellite observations. The aerosol influence the length of the light path and the observed trace gas absorptions The latter dependency makes MAXDOAS measurements suitable for the retrieval of aerosol properties. We report on the application of a new retrieval algorithm developed for near real time automated retrieval of aerosol extinction vertical profiles on MAXDOAS measurements made in Beijing during the June 2008–April 2009 period. The second section describes the retrieval algorithm and the specific settings ap-
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