Atmos. Meas. Tech., 11, 4883–4890, 2018 https://doi.org/10.5194/amt-11-4883-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. The influence of humidity on the performance of a low-cost air particle mass sensor and the effect of atmospheric fog Rohan Jayaratne1, Xiaoting Liu1, Phong Thai1, Matthew Dunbabin2, and Lidia Morawska1 1International Laboratory for Air Quality and Health, Queensland University of Technology, Brisbane, QLD 4001, Australia 2Institute for Future Environments, Queensland University of Technology, Brisbane, QLD 4001, Australia Correspondence: Lidia Morawska ([email protected]) Received: 29 March 2018 – Discussion started: 18 April 2018 Revised: 24 July 2018 – Accepted: 14 August 2018 – Published: 27 August 2018 Abstract. While low-cost particle sensors are increasingly 1 Introduction being used in numerous applications, most of them have no heater or dryer at the inlet to remove water from the sample The rapid technological advancements in the fields of ma- before measurement. Deliquescent growth of particles and terial science, digital electronics, and wireless communica- the formation of fog droplets in the atmosphere can lead to tion have given rise to a wide range of low-cost air quality significant increases in particle number concentration (PNC) sensors that are now readily available on the market. These and mass concentrations reported by such sensors. We car- sensors are increasingly being used in many applications that ried out a detailed study using a Plantower PMS1003 low- were previously not achievable with conventional expensive cost particle sensor, both in the laboratory and under ac- equipment (Kumar et al., 2015; Rai et al., 2017; Snyder et al., tual ambient field conditions, to investigate its response to 2013). Some of these applications are the monitoring of per- increasing humidity and the presence of fog in the air. We sonal exposure and indoor air pollution and the gathering of found significant increases in particle number and mass con- high-resolution spatio-temporal air pollution data by means centrations at relative humidity above about 75 %. During a of extensive sensor networks. The data thus derived are being period of fog, the total PNC increased by 28 %, while the utilised for a variety of air pollution management tasks such PNC larger than 2.5 µm increased by over 50 %. The PM10 as supplementing conventional air pollution monitoring, un- concentration reported by the PMS1003 was 46 % greater derstanding the link between pollutant exposure and human than that on the standard monitor with a charcoal dryer at health, emergency response management, hazardous leak de- the inlet. While there is a causal link between particle pol- tection and source compliance monitoring. In the process, lution and adverse health effects, the presence of water on they also serve to increase the community’s awareness and the particles is not harmful to humans. Therefore, air quality engagement towards air quality issues (Snyder et al., 2013; standards for particles are specifically limited to solid par- Jovasevic-Stojanovic et al., 2015; Rai et al., 2017). ticles and standard particle monitoring instruments are fit- However, there are many questions regarding the reliabil- ted with a heater or dryer at the inlet to remove all liquid ity and, in particular, the accuracy of these low-cost sensors material from the sample before the concentrations are mea- and their suitability in the applications that they are being sured. This study shows that it is important to understand that used (Lewis and Edwards, 2016). Many of these sensors have the results provided by low-cost particle sensors, such as the serious limitations. For example, while many particle sen- PMS1003, cannot be used to ascertain if air quality standards sors respond well to high concentrations, they fail to do so are being met. at lower levels such as typical ambient concentrations (Ja- yaratne et al., 2018; Rai et al., 2017; Kelly et al., 2017). Sin- gle gas sensors are very often affected by other interfering gases (Fine et al., 2010; Piedrahita et al., 2014), while en- vironmental parameters, such as temperature and humidity, can also affect the performance of these sensors under cer- Published by Copernicus Publications on behalf of the European Geosciences Union. 4884 R. Jayaratne et al.: Influence of humidity on a particle sensor tain conditions (Holstius et al., 2014; Rai et al., 2017; Crilley et al., 2018; Jayaratne et al., 2018). In this paper, we investigate the effect of atmospheric rel- ative humidity on the performance of a low-cost particulate matter sensor. Humid conditions can affect the performance of a sensor in several ways. For example, sensors that operate on the principle of light scattering are affected, as the particle refractive indices are dependent on relative humidity (Hänel, 1972; Hegg et al., 1993). High humidity can cause conden- sation to form on electrical components, leading to resistive bridges across components. In gas sensors, condensation on the sensor surfaces can affect the reactions that give rise to the measurable electric currents. Hygroscopic growth occurs when the relative humidity ex- ceeds the deliquescence point of a substance. There are many hygroscopic salts such as sodium chloride, that absorb wa- ter and grow at relative humidity as low as 70 %, present in Figure 1. Composition of particles in the atmosphere of Brisbane, the atmosphere, especially in marine environments (Hu et al., as derived from Harrison (2007). 2010). Jamriska et al. (2008) found a significant effect of rel- ative humidity on traffic emission particles in the size range 150–880 nm and attributed it to hygroscopic particle growth. struments, such as the TEOM, have built-in drying features Crilley et al. (2018) demonstrated a significantly large pos- at the sample inlets, it is not standard on low-cost sensors itive artefact in measured particle mass by an Alphasense and even in many other mid-cost monitors such as the TSI OPC-N2 sensor during times of high ambient relative hu- DustTrak (Kingham et al., 2006). midity. Manikonda et al. (2016) cautioned against using PM There have been very few studies of the effect of relative sensors in outdoor locations at high humidity due to hygro- humidity on the performance of low cost sensors. Wang et scopic growth of particles. In circumstances where the rel- al. (2015) investigated the performance of three low cost par- ative humidity approaches 100 %, there is the possibility of ticle sensors based on light scattering and concluded that the mist or fog droplets that are detected as particles. While there absorption of infrared radiation by a film of water on a parti- is a causal link between particle pollution and adverse human cle can cause an overestimation of the derived particle mass health effects, the presence of water on the particles plays no concentration due to the reduced intensity of light received part in it. Therefore, air quality standards for particles are by the phototransistor. Hojaiji et al. (2017) showed that the based on the dry, solid material only, and stipulate that the particle mass concentration reported by a Sharp PM sensor liquid portion must be eliminated when measuring particle increased when the humidity was increasing but not when it mass for regulatory purposes. In order to achieve this, many was decreasing. While several studies have drawn attention conventional particle mass monitors, such as the standard ta- to a possible effect of humidity on the performance of low pered element oscillating microbalance (TEOM), employ a cost sensors, no study has reliably quantified the effect. This charcoal heater at its inlet to remove all liquids from the par- study was carried out to investigate and to assess the magni- ticles that are being measured (Charron et al., 2004; Alexan- tude of the effect of relative humidity on the performance of drova et al., 2003). Thus, sensors with no drying facility at a low-cost particle sensor and to understand the mechanisms the inlet measure what is actually present in the environment involved. rather than what is required under regulatory protocols. The composition of particles in the atmosphere of Bris- 2 Method bane, as derived from Harrison (2007), is shown in Fig. 1. The subtropical, near-coastal environment is characterised In this study, we focussed on the effect of relative humidity by the presence of several hygroscopic salts such as sodium on the performance of a low-cost particle sensor in the labo- chloride, ammonium sulfate, and ammonium nitrate that ratory and under real world conditions in an outdoor location have deliquescence relative humidities in the range of 70– at an air quality monitoring station with standard instrumen- 80 % (Hu et al., 2010). Many particles in the air in Bris- tation. bane contain these salts in varying concentrations. Once the relative humidity exceeds the respective deliquescence val- 2.1 The test sensor ues, those salts begin to absorb water, resulting in particle growth and the excess water registered by PM sensors, un- Prior to commencing this study we tested a range of low- less they are removed at the instrument inlets by heating or cost particle sensors, including the Sharp GP2Y, Shinyei drying (Alexandrova et al., 2003). While more expensive in- PPD42NS, Plantower PMS1003, Innociple PSM305 and the Atmos. Meas. Tech., 11, 4883–4890, 2018 www.atmos-meas-tech.net/11/4883/2018/ R. Jayaratne et al.: Influence of humidity on a particle sensor 4885 Nova SDS011 (Jayaratne et al., 2018). All of them were The station also included a nephelometer to monitor atmo- found to be affected to some degree by humidity with the spheric visibility in terms of the particle back-scatter (BSP) Sharp and Shinyei being affected at relative humidity as low coefficient, reported in units of Mm−1.