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

16Citations
Citations of this article
18Readers
Mendeley users who have this article in their library.

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.

Figures

  • Figure 1. Sketch of the SÆMS measurement configuration. A light beam is transmitted at TROPOS and direct to a retroreflector array mounted at tower 1 for several minutes. Afterwards the beam is moved to the second retroreflector array at tower 2 for several minutes, followed by the next round in which the beam is again directed to tower 1, and so on. Particle extinction is derived from the tower 1 and tower 2 long-path transmission observations, and thus is related to an almost horizontal path of 2840 m at a height of 30– 50 m above ground. The aerosol particle extinction measurements are set into context with meteorological observations of temperature (T ) and relative humidity (RH), which are measured at the roof of TROPOS (T3, RH3) and close to the retroreflectors at tower 1 (RH1, T1) and tower 2 (RH2, T2).
  • Figure 2. Measured particle extinction coefficients for the wavelengths of 390 nm (top), 550 nm (center) and 881 nm (bottom) as a function of relative humidity. The color scale indicates how frequently a given extinction coefficient was measured during the 2009–2012 period. Mean values (bold lines) of extinction coefficients and corresponding standard deviation (SD, vertical bars) are shown for 10 % humidity intervals.
  • Figure 3. Example of the three-point relative-humidity observation (over 9 days) with humidity sensors on top of the TROPOS building and at the two towers (see Fig. 1).
  • Figure 4. (a) Frequency distribution of ambient 550 nm particle extinction coefficient observed with SÆMS at Leipzig from 2009 to 2012; (b) same distribution after correction of the particle water uptake effect, i.e., after normalization of all values to 0 % relative humidity by means of Eq. (3) with the parameter for urban aerosol derived from the 4-year SÆMS study. The 2009–2012 mean value and respective SD are given as numbers.
  • Figure 5. SÆMS observations on (left) 20 August 2009 and (right) 27 August 2009. Almost constant horizontal transport of polluted air from westerly to southwesterly directions is indicated by 4-day HYSPLIT backward trajectories (a, d, arrival height of 500m). The temporal variation of the 550 nm particle extinction coefficient with relative humidity is shown in (b) for 20 August 2009 and in (e) for 27 August 2009, and the corresponding relationship between ambient extinction coefficient and relative humidity is presented in (c, f). The curves fitted to the data points in (c, f) are obtained with Eq. (3). The coefficient of determination R2 for each fit is given as number.
  • Figure 6. Mean value of the enhancement factor for the 550nm particle extinction coefficient (blue line; obtained with Eq. (3) for the mean value γ ). The upper and lower boundaries of the grayshaded area are obtained by using γ + δγ (upper boundary) and γ − δγ (lower boundary) in Eq. (3). The given mean values and SD of the parameter γ result from the evaluation of 143 observational cases collected in the years 2009–2012. The green curve is shown for comparison and represents urban haze conditions according to Hänel (1984) with γ = 0.44.
  • Figure 7. (a) Extinction coefficient for 550 nm (mean value, SD, number of measurements) for eight defined air-mass transport regimes based on SÆMS observations from 2009 to 2012, (b) same as (a), except prior to averaging all individual cases were normalized for dry conditions (RH= 0 %) by use of the derived cluster mean parameter γ (c) and (c) hygroscopic exponent γ for 550 nm (mean value and SD, computed with Eq. 3) for the eight air-mass transport regimes derived from SÆMS observations from 2009 to 2012. Numbers of available cases per cluster are given in addition, and (d) same as (b), but separately for dry conditions (RH= 0 %) for each year of the period from 2009 to 2012.
  • Figure 8. Particle extinction enhancement factors for 550 nm (80– 40 and 95–40 % RH enhancement) observed from days with occurring humidity variations between at least 40 and 80 % RH (a) and only from days with variations between at least 40 and 95 % RH (b) separated for the eight air-mass transport regimes. The 4-year mean values and SD are given.

References Powered by Scopus

Get full text
1107Citations
582Readers
Get full text
Get full text

Register to see more suggestions

Mendeley helps you to discover research relevant for your work.

Already have an account?

Cite

CITATION STYLE

APA

Skupin, A., Ansmann, A., Engelmann, R., Seifert, P., & Müller, T. (2016). Four-year long-path monitoring of ambient aerosol extinction at a central European urban site: Dependence on relative humidity. Atmospheric Chemistry and Physics, 16(4), 1863–1876. https://doi.org/10.5194/acp-16-1863-2016

Readers over time

‘15‘16‘17‘18‘19‘21‘22‘2302468

Readers' Seniority

Tooltip

PhD / Post grad / Masters / Doc 7

58%

Researcher 4

33%

Professor / Associate Prof. 1

8%

Readers' Discipline

Tooltip

Earth and Planetary Sciences 5

42%

Environmental Science 4

33%

Chemistry 2

17%

Physics and Astronomy 1

8%

Save time finding and organizing research with Mendeley

Sign up for free
0