Elsevier

Atmospheric Environment

Volume 246, 1 February 2021, 118076
Atmospheric Environment

Measurements of atmospheric aerosol hygroscopic growth based on multi-channel Raman-Mie lidar

https://doi.org/10.1016/j.atmosenv.2020.118076Get rights and content

Highlights

  • Acquiring relative humidity vertical profile exclusively from Raman- Mie lidar data.

  • The lidar-based results were compared with in situ measurements.

  • Air quality was explained with aerosol hygroscopicity.

  • Verified the feasibility of Raman-Mie lidar in studying aerosol hygroscopic growth.

  • Providing an effective method for studying the aerosol hygroscopic growth.

Abstract

In this study, an effective method with a multi-channel Raman-Mie lidar has been proposed to detect the hygroscopic growth of atmospheric aerosol in the boundary layer, and the effect of aerosol hygroscopic growth on the optical and microphysical properties of aerosol particles is studied. The vertical relative humidity profile is obtained by combining calibrated water vapor mixing ratio and temperature profiles from Raman-Mie lidar. Two cases were selected in Hefei to analyze the aerosol hygroscopic growth. Case studies show that the increase of the backscatter coefficient of aerosol is consistent with the increase of relative humidity, and the decrease of Ångstrom exponent indicates the increasing aerosol particle size due to the hygroscopic growth process. The Kasten model is used to fit the aerosol hygroscopic enhancement factor, and the b value from the Kasten parameterization for Case II is much larger than that for Case I, which shows the stronger hygroscopicity in Case II. The results indicate that the multi-channel Raman-Mie lidar has potential in the measurements of the hygroscopic growth of aerosols.

Introduction

Atmospheric aerosols play a crucial role in global climate (Charlson et al., 1992). On the one hand, aerosols have a direct impact on the radiation balance of atmospheric system by absorbing and scattering solar radiation (Vogelmann et al., 2003). On the other hand, aerosol particles can act as cloud condensation nuclei (CCN) and ice nuclei (IN), which will change the microphysical properties of clouds such as albedo and cloud droplet size distribution (Zieger et al., 2013; Twomey, 1977). Hygroscopicity is one of the main properties of aerosols and a key factor affecting atmospheric radiation. It refers to the fact that under the condition of high relative humidity (RH), aerosol's particle size may increase due to water uptake, thus changing its related optical and microphysical properties (Hänel, 1976). Many studies indicate that aerosol has an impact on human health (Araujo et al., 2008; Anenberg et al., 2010; Liao et al., 2015; Li et al., 2017). For example, exposure to fine airborne particulates is linked to increased respiratory and cardiovascular diseases (Hu et al., 2015). At the same time, the hygroscopicity of aerosol will also affect visibility (Jeong et al., 2007; Wang et al., 2014).

Several studies have been carried out over the past years to evaluate the hygroscopic growth of aerosols. Much of the recent research was performed by in-situ measurements, and one of the most commonly used instruments for in-situ measurement is the humidified tandem differential mobility analyzer (HTDMA), which measures the change of particle diameter due to water uptake under different relative humidity (Liu et al., 1978; Swietlicki et al., 2008; Wang et al., 2017). Additionally, humidified nephelometers are widely used to quantify the changes of optical properties in the growth of aerosol hygroscopicity, such as backscatter and extinction (Titos et al., 2016; Covert et al., 1972; Fierz-Schmidhauser et al., 2010). However, these instruments have some limitations. For example, it is difficult for them to provide an accurate RH value up to 85% (Wulfmeyer and Feingold, 2000). Moreover, they modify the ambient condition by drying and re-humidifying the sample of the air to a certain value, which changes the aerosol properties to some extent, and the aerosol particles will also be lost in the sampling line (Granados-Muñoz et al., 2015).

At present, numerous studies have investigated the aerosol hygroscopic growth effect by using lidar system (Lv et al., 2017; Fernández et al., 2015). Compared with in-situ measurements, lidar has the advantage that it can carry on the measurement even when the humidity is close to saturation. Another advantage is that the measurement is conducted in the unmodified ambient atmospheric conditions (Lv et al., 2017). However, since the aerosol detected by lidar is not controlled in any way, the atmosphere vertical homogeneity in the analyzed layers needs to be ensured, which means that the same aerosol type or mixture must exist along the analyzed height range, and there is almost no change in the aerosol load. With the increase of environmental relative humidity, the optical properties of aerosol change only due to the increase of aerosol size caused by water uptake, but not the change of aerosol load or type (Bedoya-Velásquez et al., 2017). When the assumption is established, the actual aerosol properties are unaffected. Relative humidity is an important parameter in the aerosol hygroscopic growth. The common method to obtain RH profiles are by radiosonde (RS). However, RS measurements have low temporal sampling and they could be drifted away from the vertical atmosphere probed by the lidar systems (Bedoya-Velásquez et al., 2017). In order to overcome the disadvantage of RS measurements, some researchers propose the methodology for retrieving RH profiles by the combination of calibrated water vapor mixing ratio r (z) profiles from a Raman lidar water vapor channel with temperature profiles obtained from microwave radiometer (MWR) measurements (Navas-Guzmán et al., 2014; Barrera-Verdejo et al., 2016), but the detection accuracy can be affected by the change of meteorological elements (Xu et al., 2014), while lidar can avoid the above problems and is able to measure temperature and water vapor continuously more reliable and accurate.

Thus, for more accurate and effective detection of aerosol hygroscopic growth, a method of aerosol hygroscopic growth was proposed based exclusively on multi-channel Raman-Mie lidar system, then it was applied to the cases studies of Hefei, and the relationship between the aerosol hygroscopic growth and the air pollution was discussed. The results of this study will provide an effective method to obtain the RH profile and measure the hygroscopic growth of aerosols. This paper is organized as follows. The description of instruments and methodology is presented in Sect. 2. The results and discussion are introduced in Sect. 3. Finally, conclusions are given in Sect. 4.

Section snippets

Instruments

In this study, a multi-channel Raman-Mie lidar system was used to measure atmospheric aerosol hygroscopic growth, which mainly composed of laser transmitting system, receiving optical system, subsequent optical system, and signal and data acquisition system. The system parameters are shown in Table 1. The lidar system emitted two laser beams simultaneously to the atmosphere with the wavelength of 532 nm and 355 nm. After the telescope received the backscatter light, the subsequent optical path

Retrieval of relative humidity profiles

Fig. 1 shows the profiles of atmospheric temperature, relative humidity from Raman-Mie lidar and relative humidity bias between the radiosonde and Raman-Mie lidar. The profiles of temperature show that the results obtained by lidar is in good agreement with the results measured by radiosonde. The temperature measurement error below 4 km is less than 2 k. The relative humidity profiles show similar trends below 3 km through the comparison. However, the large differences are mainly concentrated

Conclusion

In this study, a multi-channel Raman-Mie lidar was used to detect the hygroscopic growth of aerosols in a well-mixed boundary layer. Water vapor profile and temperature profile obtained from Raman-Mie lidar are combined to obtain the atmospheric relative humidity. This approach enabled us to acquire vertical profiles of relative humidity exclusively from Raman- Mie lidar data.

In order to verify the effectiveness of this method, experimental observation was carried out in Hefei, Anhui Province.

CRediT authorship contribution statement

Yuefeng Zhao: Data curation. Xu Wang: Methodology, Visualization, Writing - original draft, preparation. Yangjian Cai: Supervision. Jie Pan: Writing - review & editing. Weiwei Yue: Conceptualization. Huaqiang Xu: Validation. Jingjng Wang: Investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully acknowledge the support of National Natural Science foundation of China (No 81400285) and Key Technology Research and Development Program of Shandong Province (No 2016GGX101016).

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