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Joint research activity 2:

Building a new generation of humidity-controlled aerosol monitors

Objectives and expected impact

The goal of this joint research activity is to build a new generation of standard, cost-effective humidity-controlled aerosol monitors for long-term monitoring activities. This activity is organized with 2 main objectives:

Objective 1
Building of a Humidity Tandem Differential Mobility Analyzer (H-TDMA):
None of the operational H-TDMAs in Europe can be used, at present, for long-term monitoring activities. There is, therefore, an urgent need for a new generation of cost-effective H-TDMAs that can be used for monitoring purposes. Such a new generation of instruments can only be built by capitalizing the experience of the JRA2 partners.

Objective 2
Building of a humidity-controlled nephelometer(RH-nephelometer):
This new generation of RH-nephelometers will allow for scattering coefficient measurements at various specified RH. It will be an improved, easy to use, continuously operated, easily transportable, and cost-effective RH-nephelometer. It will serve as a basis to upgrade current nephelometers at selected stations and as a standard instrument for future intercomparisons. Designs of the new generation of RH-controlled aerosol monitors will be made available to the entire scientific community.

Description of work

The chemical mixing state of the aerosol is the most important parameter in controlling whether the models describe the atmospheric processes correctly. Measurements of aerosol hygroscopic growth factors (HGF) can be used to separate fresh (nearly hydrophobic) combustion-related aerosol particles from those that are more aged (and thus more hygroscopic).

The most common instrumentation to measure water uptake on particles in ambient air is the so called Humidity Tandem Differential Mobility Analyzer (H-TDMA) where a defined monodisperse aerosol is selected under dry conditions, and the new size is measured after conditioning the particles at a specified high RH. This instrumentation has been custom-built by different research groups and is used as research instruments in various field experiments. Within the proposed network, 7 different H-TDMAs are available. These are all different in their design that is often research-driven.

A common feature to all available H-TDMAs is that they have to be operated by highly skilled personnel. For this reason, none of the operational H-TDMAs in Europe can be used, at present, for long-term monitoring activities.

A less common instrumentation but used in continuous monitoring is a humidity controlled nephelometer showing how the scattering depends on the RH. Most in situ aerosol parameters are measured without RH control (see NA4). A comparison with remote sensing or column integrating instruments is only possible after transformation of these dry values into the ambient values. This clearly shows the need to know this RH enhancement factor of the scattering coefficient. As for H-TDMAs, RH control in nephelometers is not a trivial task. A few controlled RH nephelometers are currently used for monitoring activities at the US National Oceanic and Atmospheric Administration (NOAA). Capitalizing the experience of the partners and the partnership with NOAA, the second objective of this Joint Research activity is, therefore, the construction of a new generation of RH-nephelometers that allow for scattering coefficient measurements at various specified RH.

To meet these objectives, the following activities will be implemented:

Task 1: Development of harmonized procedures among the 7 H-TDMAs existing in the network and a prototype of a new generation of a simple, stand-alone H-TDMA. At an early stage of the project, an intercomparison workshop of H-TDMAs and controlled RH DMPS will take place in cooperation with the FP6 project ACCENT. This workshop will focus on all aspects of instrumental design, including calibration of RH, residence time under humidified conditions, etc. Based on the experience of the workshop, the individual designs will be adapted. In a second workshop, the improvement will be assessed. All partners participating in this JRA will participate. Based on the experience of all participating groups and the results of the first workshop, a new H-TDMA will be built, designed for unattended long-term operation. Under the lead of SU, the best designs of the various instruments will be combined to reach this goal. The instrument will first be tested with different laboratory aerosols and then in the field under various conditions.

Task 2: Development of a RH controlled nephelometer. In close collaboration with NOAA, a nephelometer with controlled RH will be built (lead: PSI). The instrument will first be tested with different laboratory aerosols and then in the field under various conditions as well as in the second intercomparison of the H-TDMAs. The instrument will then be operated at various sites together with an H-TDMA to test if a hygroscopic closure between the increase in size and increase in the scattering coefficient can be reached. This will provide the framework required for transformation of the nephelometer data obtained within NA4 from dry to ambient conditions. Based on the experience, CNRS will build a nephelometer that is capable to switch between dry and high RH conditions alternatingly. This is envisaged to be the prototype for future dual-purpose (low and high RH) nephelometers in the network.