Abstract
Abstract. The ambient aerosol particle extinction coefficient is measured with the Spectral Aerosol Extinction Monitoring System (SÆMS) along a 2.84 km horizontal path at 30–50 m height above ground in the urban environment of Leipzig (51.3° N, 12.4° E), Germany, since 2009. The dependence of the particle extinction coefficient (wavelength range from 300 to 1000 nm) on relative humidity up to almost 100 % was investigated. The main results are presented. For the wavelength of 550 nm, the mean extinction enhancement factor was found to be 1.75 ± 0.4 for an increase of relative humidity from 40 to 80 %. The respective 4-year mean extinction enhancement factor is 2.8 ± 0.6 for a relative-humidity increase from 40 to 95 %. A parameterization of the dependency of the urban particle extinction coefficient on relative humidity is presented. A mean hygroscopic exponent of 0.46 for the 2009–2012 period was determined. Based on a backward trajectory cluster analysis, the dependence of several aerosol optical properties for eight air flow regimes was investigated. Large differences were not found, indicating that local pollution sources widely control the aerosol conditions over the urban site. The comparison of the SÆMS extinction coefficient statistics with respective statistics from ambient AERONET sun photometer observations yields good agreement. Also, time series of the particle extinction coefficient computed from in situ-measured dry particle size distributions and humidity-corrected SÆMS extinction values (for 40 % relative humidity) were found in good overall consistency, which verifies the applicability of the developed humidity parameterization scheme. The analysis of the spectral dependence of particle extinction (Ångström exponent) revealed an increase of the 390–881 nm Ångström exponent from, on average, 0.3 (at 30 % relative humidity) to 1.3 (at 95 % relative humidity) for the 4-year period.
Highlights
The importance of atmospheric aerosols in the global climate system due to scattering and absorption of radiation and the influence on the formation of clouds is well known (Charlson and Heintzenberg, 1995; Heintzenberg and Charlson, 2009)
Besides the study of the dependence of particle extinction on relative humidity, we provide a general overview of the 4-year statistics of particle extinction coefficients
The particle water content is responsible for roughly 50 % of particle extinction in the lowermost part of the troposphere at this urban site
Summary
The importance of atmospheric aerosols in the global climate system due to scattering and absorption of radiation and the influence on the formation of clouds is well known (Charlson and Heintzenberg, 1995; Heintzenberg and Charlson, 2009). Skupin et al.: Relative-humidity dependence of particle extinction to almost 100 %, before cloud droplet activation begins (Arnulf et al, 1957; Goes, 1963, 1964; Elterman, 1964; Badayev et al, 1975; Stratmann et al, 2010; Liu et al, 2011; Chen et al, 2014; Zieger et al, 2014) These efforts were partly based on controlled laboratory studies. Motivated by the need for more aerosol field observations with emphasis on undisturbed, but complex aerosol mixtures at ambient humidity conditions, we designed and setup the Spectral Aerosol Extinction Monitoring System (SÆMS) (Skupin et al, 2014), which allows us to continuously monitor the wavelength spectrum of the particle extinction coefficient at a height of 30–50 m above ground between two towers, which are 2.84 km apart from each other.
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