A brief introduction
The effects of aerosol particles (solid or liquid particles suspended in the atmosphere) are manifold for climate since they interact with solar radiation (e.g. via light scattering and absorption) and influence cloud properties by serving as cloud condensation nuclei. All these phenomena affect the global energy budget and, despite the increased number of studies in the last decades, the various aerosol effects remain to contribute to the largest uncertainty in climate predictions.
Aerosol optical properties and especially aerosol light scattering is strongly dependent on ambient relative humidity (see e.g. Zieger et al., 2013 or Titos et al., 2016). Depending on their size, composition and the ambient humidity, atmospheric particles will take up varying amounts of water, thereby altering their optical properties. Along with particle size, this humidity dependence also plays an important role in the life cycle of atmospheric particles by influencing their growth (activation) into cloud droplets or by influencing wet and dry deposition processes (which eventually removes particles from the atmosphere).
Earth System Models (ESM’s) use a variety of schemes to account for aerosol hygroscopic growth. These schemes need to be evaluated and tested against observations to improve predictions by climate models in general. However, until now, the ability of ESM’s to predict hygroscopic growth has not been rigorously evaluated against in-situ measurements on a larger and more comprehensive way.
Within this project, we make use of a wide range of measurements of particle light scattering coefficients at elevated relative humidity using so called humidified nephelometers, which were conducted during the last twenty years at various sites around the globe. This data has been brought together from numerous individual field campaigns and monitoring activities and has been re-processed and re-analyzed to eventually compare these observations to predictions of ESM’s.
The new benchmark data set
In the first part of this project, we have compiled a large global data set of in-situ measurements of aerosol hygroscopicity made both at long-term DOE/ARM sites and during many of the ARM Mobile Facility (AMF) deployments, as well as measurements from the NOAA collaborative network and the European ACTRIS network. These measurements represent a variety of aerosol types (e.g., clean marine, polluted continental, biomass burning, and desert dust) and have been harmonized and processed under a standardized manner. Figure 1 shows as an example the median values for the scattering enhancement factor, f(RH=85%, 550nm). This is a unique data set which covers most aerosol types around the globe and will be extremely valuable to evaluate model performance with respect to aerosol hygroscopic growth and their effect on particle light scattering.
The re-processed and quality assured data set of aerosol hygroscopic growth is now openly available on EBAS and on the ACTRIS data center (see Figure 2) as well as on the Bolin Centre for Climate Research 0database. In our first paper, we describe the sites, the harmonization process, the resulting data set, and show some first results.
For the second part of this project, modelers involved in the AeroCom project (Aerosol Comparisons between Observations and Models; http://aerocom.met.no/), including the NCAR/DOE CAM5 model, have been requested to generate model output of aerosol optical properties and composition as a function of relative humidity. The models have utilized identical anthropogenic emissions and were run with constrained meteorology, thus minimizing differences in the anthropogenic sources and large scale atmospheric transport. The model output is given for the location of the in-situ measurements. The model data is also available through the Bolin Centre for Climate Research Database.
As a major goal, models and measurements are evaluated to determine (i) how well model simulations represent the observations of aerosol water uptake; (ii) whether differences between the models and measurements can be explained by the model parameterizations of hygroscopic growth; (ii) if there are biases which may be related to region or aerosol type. We found a large variety among models with a tendency to overestimate light scattering at elevated relative humidities (RH). Some factors are crucial such as model parameterization of hygroscopicity and chemistry. In our paper we give some recommendations for models to update hygroscopic growth parameterization for sea salt. It is of interest to compare models and measurements at similar RH conditions, paying attention to the difficulties that this arises. We also recommend that models provide scattering values at specified RH values for pure components separately. Further variables are important for this investigation, so it is relevant to asses model simulations of aerosol chemistry and size where enough measurements are available, and their impact on model scattering simulation. We recommend that the AeroCom community carries out a new multi-model experiment with a common hygroscopicity scheme.
The work is published in Atmospheric Chemistry and Physics.
Outreach, news and conference presentations
We list below information about workshops, papers and the most recent international conferences in which this project has been presented:
- Our model-measurement evaluation “A global model-measurement evaluation of particle light scattering coefficients at elevated relative humidity” is finally published in ACP.
- eGMAC session: Monitoring and Understanding Trends in Surface Radiation, Clouds, and Aerosols. Maria presents the model-measurement evaluation at eGMAC (online conference). The presentation can be found here.
- February 2020: Project workshop at Stockholm University.
- European Geoscience Union (EGU), Maria presents the model-measurement evaluation at EGU given this year as an online conference. More details can be found here.
- European Aerosol Conference 2019 (EAC), A Global Model-Measurement Evaluation of Particle Light Scattering Coefficients at Elevated Relative Humidity, Poster: Maria B. (August 2019, Gothenburg, Sweden).
- Iberian Meeting on Aerosol Science and Technology 2019 (RICTA), A global picture of hygroscopicity related aerosol light scattering enhancement factors, Plenary talk: Gloria T. (July 2019, Lisbon, Portugal).
- Joint Atmospheric Radiation Measurement (ARM) User Facility/Atmospheric System Research (ASR) Principal Investigators Meeting 2019, Evaluation and Improvement of the Parameterization of Aerosol Hygroscopicity in Global Climate Models Using In-situ Surface Measurements, Talk & poster: Paul Z. (June 2019, Rockville, USA).
- European Geophysical Union G.A. 2019 (EGU), Comparison between in-situ surface measurements and global climate model outputs of particle light scattering coefficient as a function of relative humidity, Talk: Maria B. (April 2019, Vienna, Austria).
- European Geophysical Union G.A. 2019 (EGU), Climatology of aerosol light scattering enhancement factors, Poster: Maria B. (April 2019, Vienna, Austria).
- Aerosols, Clouds, and Trace Gases Research Infrastructure G.A. 2019 (ACTRIS), An overview of the effect of water uptake on aerosol particle light scattering: climatology, evaluation of proxies and comparison with global models, Talk: Gloria T. (April 2019, Darmstadt, Germany).
- NOAA Global Monitoring Division Annual Conference , An overview of the effect of water uptake on aerosol particle light scattering: observations, evaluation of proxies and comparison with global models, Poster: Betsy A. (May 2019, Boulder, CO USA).
- European Geophysical Union G.A. 2018 (EGU), The effect of aerosol water on particle light scattering at low relative humidity, Poster: Elisabeth A. (April 2018, Vienna, Austria).
- Nordic Society for Aerosol Research G.A. 2018 (NOSA), A global overview of the effect of water uptake on aerosol particle light scattering using in-situ surface measurements, Poster: Maria B. (March 2018, Helsinki, Finland).
- 17th AeroCom workshop, Evaluation and improvement of the parameterization of aerosol hygroscopicity in global climate models using in-situ surface measurements, Talk: Maria B. (College Park, Maryland, USA, 15 – 19 October, 2018).
- 17th AeroCom workshop, What is “dry”?: The effect of aerosol water on particle light scattering at low relative humidity, Poster: Betsy A. (College Park, Maryland, USA, 15 – 19 October, 2018).
- Joint Atmospheric Radiation Measurement (ARM) User Facility/Atmospheric System Research (ASR) Principal Investigators Meeting 2018, A global overview of the effect of water uptake on aerosol particle light scattering using in-situ surface measurements, Poster: Gloria T. (March 2018, Vienna, VA, USA).
- Aerosols, Clouds, and Trace Gases Research Infrastructure G.A. 2017 (ACTRIS), Evaluation and improvement of the parameterization of aerosol hygroscopicity in global climate models using in-situ surface measurements, Talk: Paul Z. (March 2017, Granada, Spain).
- 16th AeroCom workshop, Evaluation and improvement of the parameterization of aerosol hygroscopicity in global climate models using in-situ surface measurements, Talk: Maria B. (Helsinki, Finland, 9 – 13 October, 2017).
- 16th Aerocom workshop, Linking recent findings from the Stockholm sea spray chamber to global climate models, Poster: Paul Z. (Helsinki, Finland, 9 – 13 October, 2017).
- CAS-TWAS-WMO Forum on Climate Science (CTWF) & AeroCom/AeroSat workshop 2016, Evaluation and improvement of the parameterization of aerosol hygroscopicity in global climate models using in-situ surface measurements, Poster: Paul Z. (19-24 September 2016, Beijing, China).
Two one-week workshops have been held in Stockholm University, in October 2017 and April 2018 with the participation of all the members involved in the project.
Betsy Andrews (NOAA, USA)
Gloria Titos (University of Granada, Spain)
Kai Zhang (PNNL, USA)
This work is financed by the Department of Energy (USA) under the project DE-SC0016541.