A Review of Literature on the Usage of Low-Cost Sensors to Measure Particulate Matter

: With advances in technological sciences, individuals can utilize low-cost air monitoring sensors to record air quality at homes, schools, and businesses. Air quality data collected from LCSs are publicly accessible, informing the community of the air quality around them. It is important to measure local and regional particulate matter (PM) concentrations to keep the public involved, especially those with speciﬁc health concerns, such as asthma, wheezing, and seasonal allergies. The number of studies involving the use of LCSs to evaluate PM levels is increasing with more manufacturers producing ‘easy to use’ LCSs targeting the public. The goal of this review is to understand and incorporate the ﬁndings from studies using LCSs to analyze PM of various sizes, i.e., PM 1 , PM 2.5 , PM 4 , and PM 10 . This review integrates analyses from 51 different studies in 14 countries, including the U.S. The ﬁndings indicate spatial heterogeneity in the PM concentrations across a region. Some of the low-cost sensor manufacturers mentioned in these studies include Plantower, AQMesh, Alpha-sense, PurpleAir, E-MOTEs, and Shinyei. This review emphasizes the importance of LCSs in the ﬁeld of PM monitoring and its potential to inform the public about their exposure burden, and to aid state and federal decision makers in formulating policies for mitigating the effects of PM pollution in any urban or rural setting.


Introduction
Low-cost air quality sensors are manufactured to perform proper applications for ambient monitoring [1]. Low-cost sensors (LCSs) enable knowledge of the surrounding air by differentiating between good, moderate, and unhealthy air quality when visibility is impaired [2][3][4]. These consumer-grade sensors allow deployment in dense sensor networks, while improving the monitoring networks both spatially and temporally [2][3][4]. The capabilities of LCSs range according to the manufacturers' purpose of application, either for personal exposure, ambient, or mobile monitoring [1,2]. The increasing demand for the use of LCSs for ambient air monitoring can be explained by the enhanced interest of researchers and the public in air quality [2].
Various studies showing different LCSs manufacturers with their own calibrations, performance durations, and metrics have been covered in this study. Different reference instruments and methods are used to verify the reliability and accuracy of LCSs [2,5]. By verifying acquired data, said researcher can defend their methods of deployed sensors. Extended measures include a pre/post-deployment calibration from co-located analyses with regulatory-grade monitors [2,5]. Co-location tests are advised when working with LCSs to develop precision and accuracy amongst the measurements during deployment [2,5]. Quantification of measurement error can be deduced with the co-location data to identify any potential degradation or drift from the sensor or sensors over time [2].
Earth 2023, 4 170 measurement at each location. By calibrating devices at various sites, differences in the readings between the sites can be attributed to actual variations in air quality, rather than instrument defects or performance and measurement inaccuracies. This increased accuracy in measurement can lead to a more robust understanding of air quality across different sampled sites. Calibration at different sites, therefore, allows for necessary adjustments to be incorporated in the data analyses, further improving the accuracy of the results. Prior testing of LCSs will be best tested in the field under actual field conditions, rather than in a controlled setting to accelerate the sensor's "drift to harsher conditions in the field" [16].
Providing insight and awareness of ambient air quality levels to the public is very important and can be achieved by increasing the number of studies involving LCSs [8]. LCSs present high-resolution temporal and spatial indoor and/or outdoor data [1,8,9,[17][18][19][20]. There is potential for multiple studies to stem from the convenience of LCS, granting more opportunities and growth in air quality research [5].

Particulate Matter Sources
Particulate matter is liquid and solid particles in the air and can be classified as primary or secondary [21]. Primary PM is released right into the atmosphere, whereas secondary PM is the product of chemical reactions in the atmosphere [22]. Chemical constituents of particulate matter primarily consist of inorganic ions that can be expanded further with organic and elemental carbon, metals, and particle-bound water [23]. Organics from VOCs, sulfates, and nitrates are all examples of secondary PM [22]. Common anthropogenic sources are highly variable, including biomass and fuel combustion [23]. The majority of PM stems from fuel combustion derived from vehicular and traffic emissions; therefore, PM levels and traffic emissions are heavily correlated [22]. Similarly, industrialized urban areas with fossil fuel combustions are commonly found with higher levels of PM [22]. Urban air quality is generally affected by traffic-related emissions and indirectly affected by photochemical reactions [24]. Urban air pollution gradients are correlated with the increase in human population and traffic emissions [24]. Urban areas consist of intersections, busy streets, and high buildings, resulting in poor ventilation buildup of primary air pollutants. Therefore, the spatial variability and exposure to primary PM is high in urban areas, whereas secondary pollutants are regional in scale and driven by meteorology [24].

Particulate Matter Classification
Particulate matter is categorized by size, or the aerodynamic equivalent diameter (AED), to determine the transportability in the atmosphere and the ability to be inhaled through the respiratory tract [23]. Fine particulate matter (PM 2.5 ) has an AED of 2.5 µm and can travel 100-1000 km [21,23]. Common sources for PM 2.5 draw from the transformation of NO x , SO 2 , and organics, as well as the combustion of oil, coal, and gasoline. Coarse particulate matter (PM 10 ) has an AED of 10µm and can travel 1-10 km [21][22][23]. While, during dust storms, medium-sized particles (16-31 µm) can travel up to 1600 km, the smallest dust particles of size less than 16 µm have the ability to spread globally [25]. PM 10 is composed of the suspension and resuspension of soils and industrial dust [22]. The distribution of particulates depends on the amount and size (or mass) of the particles. The classification of particles from the number distribution would depend on the count of particles, whereas the mass distribution relies on the mass of the particles at each size [22]. Particulate matter in urban environments tends to mostly distribute fine aerosols [22].

Human Health Concerns
Indoor activities (i.e., cooking, heating with natural gas, and the use of office equipment) can generate a number of PM concentrated particles [20,23,26]. Most of someone's day (80-90%) is spent indoors, exposing them to indoor generated PM and ambient infiltrated PM [26]. Finer particles suspend in the air for longer periods of time, weeks, or months, due to their size [23]. Thus, finer particles between 1 µm and 5 µm are able to be inhaled deeper and deposit themselves further into the respiratory system, likely in the respiratory bronchioles and alveoli [23]. Larger aerodynamic diameters between 5 µm and 10 µm can be filtered easier and thus, are settled in the upper throat, trachea, or bronchi [23]. Due to the collection of particulates in the respiratory system, the body counter reacts with coughing and sneezing [21,23]. Particulate matter has proven to have several adverse health effects, due to both chronic and acute exposures. Significant health problems occurring after acute exposure will increase visits to the hospital, with a risk of cardiovascular and/or respiratory diseases, respiratory symptoms, premature mortality, heart attacks, and lung inflammation [22,23,26]. Chronic exposures to particulate matter result in a decrease in lung function, stunt lung growth, development of asthma, and overall damage to the lungs [22]. Finer particles have a stronger correlation with short-term exposures and hospital visits for respiratory disease. On the other hand, coarse particles have a weaker correlation with short-term exposures and hospital visits for respiratory disease [22,26].

Materials and Methods
Recent studies have used a variety of LCSs to analyze data of PM levels to monitor pollutant exposures, indoors and outdoors. More precisely, this review focuses on LCSs' performance while collecting PM real-time data/non-real-time data. Such literature will be analyzed and noted.

Study Section Criteria
This review considered articles published after January 2000 till the present year, 2022. As the use of LCSs is increasing with time, the majority of the studies are after the year 2015 [27]. All studies involving characterizing, assessing, and analyzing particulate matter with LCSs were included in the search. Indoor and outdoor studies from different parts of the world were included to compare the low-cost sensor manufacturers. Literature measuring other aerosol species was not included. The focus is only on particulate matter. Literature evaluating and comparing different LCSs was also included in the review [26,28,29]. The search did find a review evaluating LCSs against reference measurements [30]. Another review study focused on the research of low-cost PM sensors in southeast Asian countries [5].

Search Methods
A complete search of all literature was centered on multiple key terms, similar to low-cost sensors (low-cost air sensor) or (low-cost sensor) or (low-cost air quality Monitors) or (lowcost monitors) and Particulate Matter (PM 2.5 ) or (Particulate Matter) or (PM). The databases used to search for relevant literature included PubMed, ProQuest, and ScienceDirect.

Selection of Studies
The key terms related to LCSs and PM were limited to title fields and/or abstracts during searching. Studies that were not published in English were excluded. All literature identified as applicable for the purpose of the review was added to a reference manager (EndNote X9). Literature not pertaining relevance to this review was excluded. Databases and search terms used are summarized in Table 1.

Database Search Filters Results
PubMed (low-cost air sensor) OR (low-cost sensor) OR (low-cost air quality monitors) OR (low-cost monitors) AND (PM) AND (PM 2.5 ) 48 ProQuest "Low-cost sensor" and "PM" and "PM 2.5 " 948 ScienceDirect "Low-cost sensor" and "low-cost air sensor" and "PM" and "PM 2.5 " 580 Total 1576

Literature Retrieval and Study Characteristics
Initially, the combined search comprised 1576 articles. Titles and abstracts from the articles were skimmed to be further considered. Overall, 68 articles were reviewed, and 51 articles underwent full text review to analyze low-cost monitor studies with PM concentrations [1,2,4,[6][7][8][9][10][11][12][15][16][17][18][19][20][21][22]24,26,28,. Figure 1 shows the number of papers reviewed and their year of publication. There was a surge in the number of studies from 2018 to 2019 and 2020. The possibility of a correlation between PM and the spread of the COVID-19 pandemic has resulted in the growth of interest in this subject in recent years [60].

Literature Retrieval and Study Characteristics
Initially, the combined search comprised 1576 articles. Titles and abstracts from the articles were skimmed to be further considered. Overall, 68 articles were reviewed, and 51 articles underwent full text review to analyze low-cost monitor studies with PM concentrations [1,2,4,[6][7][8][9][10][11][12][15][16][17][18][19][20][21][22]24,26,28,. Figure 1 shows the number of papers reviewed and their year of publication. There was a surge in the number of studies from 2018 to 2019 and 2020. The possibility of a correlation between PM and the spread of the COVID-19 pandemic has resulted in the growth of interest in this subject in recent years [60].   . Studies compared different manufactured LCSs with other sensors and/or with publicly available data from EPA's station monitoring air quality, near the area of such study, for further validation. Results of the selected studies, including emission sources, type of site, PM type, sampling region, sampling period, LCS's manufacture, and number of sensors, were summarized in the study.

Results
The summarization of characteristics for LCSs evaluating particulate matter is demonstrated in Table 2. Overall, there were 51 different studies from 14 countries, comprised of Australia, China, Norway, the United Kingdom, Oman, Portugal, Greece, India, Spain, Taiwan, South Korea, Japan, Canada, and the U.S. All studies focused on LCSs that measure any type of PM (PM 1 , PM 2.5 , PM 4 , PM 10 ). Out of the fifty-one studies, all of them employed air quality monitoring systems that measured PM 2.5 , and twelve of them measured PM 1 , two studies measured PM 4 , nineteen measured PM 10 , and two studies measured all sizes of PM (Table 2).    The sensor shows greater sensitivity to bigger particles than smaller particles when exposed to the same concentration of mass.
During the studies, the sampling period varied from long-term to short-term, depending on the desired results of the researcher. The long-term may be considered 6 months and older, while the short-term would be classified as any duration under 6 months. As a result, there are 21 long-term studies and 30 short-term studies.

Discussion
The goal of this review was to summarize the studies that included LCSs measuring PM concentrations. The studies were conducted in both indoor and outdoor microenvironments, in a variety of countries and states, during specific sampling periods. Additionally, the review was able to take note of the reference instruments being used to evaluate and calibrate the overall performance of LCSs in PM ambient monitoring.
Literature analyzing PM ambient data generally used regression models for statistical calibration, as seen in various studies [12,35,48]. For instance, the generalized additive model (GAM) is a non-parametric regression method used in a couple of studies [4,35], while data from other studies was referenced back to AQ-SPEC and their methods [3,11].
Based on the search, a limitation throughout the studies was associated with the shortterm duration, where sampling was a couple of hours [7,22,[42][43][44], while some were a little less than an hour [46,48]. Depending on the research objective to study long-term or shortterm exposures, the length of the LCS performance varies. Additionally, various studies have demonstrated that weather conditions, like temperature, relative humidity, and other environmental factors, can impact the output of low-cost PM sensors. Temperature and relative humidity had variable impacts on the sensor response in a study from Hong Kong [61]. In another Chilean study, a weak correlation was observed between low-cost sensors and traditional instruments at study sites where PM 10 dominated. Therefore, the size and composition of particles can also impact the accuracy of PM sensors [62]. Hence, these meteorological factors are considered, and algorithms are implemented to reduce the interference [20,26]. The growing development of LCSs and calibration techniques has improved data quality from the sensors [5].
Using LCSs enables the characterization of real-time measurements at a reduced expense, thereby extending the spatial coverage of air pollutant measurements beyond what is achievable with existing federal reference methods [30]. The selection of LCS for any study should consider multiple factors, such as the sampling period, cost and size of Earth 2023, 4 183 sensor, study site, emission sources to be studied, and meteorological factors at the site for the sampling period. Studies have reported that the LCSs are more accurate in specific environments. A study, which used the Plantower sensor for the assessment of cooking emissions on indoor air quality, concluded that the sensor produces data acceptable for residential air [54]. Koala monitor is powered by a small solar panel and a rechargeable Li-ion battery [16]. It operates effectively with as little as 5 h of sunlight each day and can continue to function for a few days, even in the absence of direct sunlight, once it has been fully charged. This sensor can be very useful in studies which use locations with little or no electricity supply. A study used a sensor manufactured by Dylos Co. to examine the personal exposure monitoring of PM 2.5 in indoor and outdoor microenvironments. It documented that the sensor is not specifically designed for mobile measurements [43]. In a 5-day field evaluation and calibration study of AirBeam2 by Huang et al., it was suggested that the accuracy of PM concentration measurement might be affected by high levels of relative humidity and hydroscopic salts [20]. As a result, the sensor may not be suitable for use in studies conducted near the seashore. In a 320 day study by Sayahi et al. [36], which evaluated the Plantower particulate matter sensors in the field, the raw data collected by the sensors had a high correlation of R 2 > 0.858 in the winter season. Hence, multiple factors need to be taken into consideration to finalize a suitable sensor for an air pollution study.

Conclusions
In the last few years, low-cost sensor usage to measure various air pollutants, such as PM, has increased substantially throughout the world. These sensors are crucial in assessing the air pollution exposure burden of the local population in any urban or rural environment. Federally monitored Continuous Ambient Monitoring Sites are not ubiquitous in any landscape; therefore, air pollution exposure misclassification is a major issue in this field. This review paper provides a thorough and succinct presentation of low-cost sensors to measure particulate matter pollution across the globe.
There is a growing demand for LCSs due to their practical features, such as smaller size and lower cost, compared to traditional instruments used in the field of air quality monitoring. The quality and uses of LCSs is increasing rapidly across all corners of the world. This review paper highlighted studies determining LCSs performances to assess ambient PM. The study also highlighted the merits and limitations of the sensors. This evaluation is relevant today in addressing human respiratory health issues after the global COVID-19 pandemic. Results from this and other such reviews can assist with future studies for LCSs and assist with other research related to PM assessment.