Four-year long-path monitoring of ambient aerosol extinction at a central European urban site: dependence on relative humidity

摘要

pstrongAbstract./strong The ambient aerosol particle extinction coefficient is measured with the Spectral Aerosol Extinction Monitoring System (S??MS) along a 2.84span class="thinspace"/spankm horizontal path at 30a??50span class="thinspace"/spanm height above ground in the urban environment of Leipzig (51.3?°span class="thinspace"/spanN, 12.4?°span class="thinspace"/spanE), Germany, since 2009. The dependence of the particle extinction coefficient (wavelength range from 300 to 1000span class="thinspace"/spannm) on relative humidity up to almost 100span class="thinspace"/span% was investigated. The main results are presented. For the wavelength of 550span class="thinspace"/spannm, the mean extinction enhancement factor was found to be 1.75a???±a??0.4 for an increase of relative humidity from 40 to 80span class="thinspace"/span%. The respective 4-year mean extinction enhancement factor is 2.8a???±a??0.6 for a relative-humidity increase from 40 to 95span class="thinspace"/span%. 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 2009a??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 S??MS 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 S??MS extinction values (for 40span class="thinspace"/span% 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 (??ngstr??m exponent) revealed an increase of the 390a??881span class="thinspace"/spannm ??ngstr??m exponent from, on average, 0.3 (at 30span class="thinspace"/span% relative humidity) to 1.3 (at 95span class="thinspace"/span% relative humidity) for the 4-year period./p.