Abstract
Abstract. In the field, aerosol in-situ measurements are often performed under dry conditions (relative humidity RH<30–40%). Since ambient aerosol particles experience hygroscopic growth at enhanced RH, their microphysical and optical properties – especially the aerosol light scattering – are also strongly dependent on RH. The knowledge of this RH effect is of crucial importance for climate forcing calculations or for the comparison of remote sensing with in-situ measurements. Here, we will present results from a four-month campaign which took place in summer 2009 in Cabauw, The Netherlands. The aerosol scattering coefficient σsp(λ) was measured dry and at various, predefined RH conditions between 20 and 95% with a humidified nephelometer. The scattering enhancement factor f(RH,λ) is the key parameter to describe the effect of RH on σsp(λ) and is defined as σsp(RH,λ) measured at a certain RH divided by the dry σsp(dry,λ). The measurement of f(RH,λ) together with the dry absorption measurement (assumed not to change with RH) allows the determination of the actual extinction coefficient σep(RH,λ) at ambient RH. In addition, a wide range of other aerosol properties were measured in parallel. The measurements were used to characterize the effects of RH on the aerosol optical properties. A closure study showed the consistency of the aerosol in-situ measurements. Due to the large variability of air mass origin (and thus aerosol composition) a simple parameterization of f(RH,λ) could not be established. If f(RH,λ) needs to be predicted, the chemical composition and size distribution need to be known. Measurements of four MAX-DOAS (multi-axis differential optical absorption spectroscopy) instruments were used to retrieve vertical profiles of σep(λ). The values of the lowest layer were compared to the in-situ values after conversion of the latter ones to ambient RH. The comparison showed a good correlation of R2 = 0.62–0.78, but the extinction coefficients from MAX-DOAS were a factor of 1.5–3.4 larger than the in-situ values. Best agreement is achieved for a few cases characterized by low aerosol optical depths and low planetary boundary layer heights. Differences were shown to be dependent on the applied MAX-DOAS retrieval algorithm. The comparison of the in-situ extinction data to a Raman LIDAR (light detection and ranging) showed a good correlation and higher values measured by the LIDAR (R2 = 0.82−0.85, slope of 1.69–1.76) if the Raman retrieved profile was used to extrapolate the directly measured extinction coefficient to the ground. The comparison improved if only nighttime measurements were used in the comparison (R2 = 0.96, slope of 1.12).
Highlights
Atmospheric aerosol particles change in size due to water uptake which is determined by their chemical composition and the ambient relative humidity (RH)
Max-PlanckInstitute for Chemistry (MPI) uses a more simplified approach following the technique of Li et al (2010): the aerosol extinction profile is described by only two parameters which are determined by fitting the measured O4 optical depths to the radiative transfer simulations using a least squares method
The humidograms were parameterized with an empirical equation, which has been used in previous studies (Clarke et al, 2002; Carrico et al, 2003) and has been found to best describe the individual branches: f (RH) = a(1 − RH)−γ, (8)
Summary
Atmospheric aerosol particles change in size due to water uptake which is determined by their chemical composition and the ambient relative humidity (RH) As a result their optical properties – especially the aerosol light scattering – strongly depend on RH. The first comparison of the extinction coefficient (measured at Ghuangzhou, China) with a single MAX-DOAS instrument (similar retrieval as for the instrument by the Max-Planck-Institute for Chemistry (MPI), see below) to nephelometer data was made by Li et al (2010) using a single parameterization from a different station (60 km further away) to calculate the ambient aerosol extinction coefficients from the dry nephelometer data They only used ground based RH measurements and differences between indoor and ambient RH and temperature conditions were not accounted for. The vertical profiles of the aerosol extinction coefficient obtained from MAX-DOAS and their comparison to LIDAR measurements are discussed in an upcoming publication (Frieß et al, 2011)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
