PM_2.5/PM₁₀ ratio characteristics over urban sites of India

Highlights

• PM_2.5/PM₁₀ ratio characteristics over India using multi-site and multi-year data.

• Suppressed PM_2.5/PM₁₀ ratio variations compared to that observed in PM concentrations.

• MERRA-2 derived PM_2.5/PM₁₀ ratio found to be overestimating during colder months.

• Weak relationship between the PM_2.5/PM₁₀ ratio and meteorological parameters.

Abstract

The PM_2.5/PM₁₀ ratio (PM_2.5 and PM₁₀ are defined as mass concentration of particles having aerodynamic diameter less than 2.5 and 10 µm respectively) is one of the important parameters in understanding the severity of the fine mode surface particulate matter pollution. The present study characterises PM_2.5/PM₁₀ ratio estimates from eight Indian urban sites with varying levels of urbanization. Five years (2015–2019) of collocated PM_2.5, PM₁₀, and meteorological (ambient temperature, relative humidity (RH), and wind speed) measurements are used to understand the spatial and temporal variability in the PM_2.5/PM₁₀ ratio at different scales and to investigate its relationship with meteorological parameters. Over the study sites, the seasonal mean PM_2.5/PM₁₀ ratio varied between 0.31 ± 0.08 (mean ± standard deviation) and 0.65 ± 0.13. Seasonally, the highest PM_2.5/PM₁₀ ratio was observed during winter and post-monsoon seasons. Sites in the Indo-Gangetic Plain (IGP) exhibited higher PM levels (PM_2.5 and PM₁₀) and higher PM_2.5/PM₁₀ ratios than the corresponding values recorded at other sites. The seasonal mean PM_2.5/PM₁₀ ratio estimated (over the study sites) using MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, version 2) ranged between 0.25 ± 0.08 and 0.77 ± 0.16, and exhibited consistent overestimation (when compared to values derived from measurements) during winter and pre-monsoon seasons. Grossly, the PM_2.5/PM₁₀ ratio exhibited a weak association with meteorological parameters. Interestingly, despite variations in geography, population, anthropogenic activities and PM concentrations across seasons and sites, the PM_2.5/PM₁₀ ratio showed low variability.

Introduction

Atmospheric particulate matter (PM) is a major air-pollutant responsible for the degradation of air-quality. PM_2.5 and PM₁₀ are routinely monitored as part of regulatory measurements by environmental protection agencies worldwide. Air-pollution can exacerbate several health issues such as allergenic reactions, lung dysfunction, cardiovascular diseases, cancers etc. (e.g., Pope et al., 2002, WHO, 2003, Harrison et al., 2010, Kim et al., 2015, Schraufnagel et al., 2019).

The relative dominance of fine-mode (or coarse-mode) particulates over a region is of prime importance in understanding sources of particulate matter, as PM emissions from different sources are characterised by particles with different size ranges. Fossil fuel combustion, vehicular emissions, industrial activities, gas to particle conversion (majority of these sources correspond to anthropogenic activities), etc. lead to the emission/formation of fine particulates. Coarse particles mostly consist of PM from fugitive emissions, wind erosion, desert dust, and emissions from activities such as mining, quarrying, and agriculture (majority of these sources correspond to natural activities, though not fully). PM_2.5 and PM₁₀ can be considered as measures of fine and coarse PM concentrations, respectively. The PM_2.5/PM₁₀ ratio quantifies the relative dominance of fine/coarse particles (in terms of their mass concentrations) and is mostly found useful in (i) identifying major sources of PM (e.g., Chan and Yao, 2008), thus assisting the regulatory bodies in formulation of guidelines to mitigate emissions from identified sources and (ii) estimating PM_2.5 when only the PM₁₀ measurement is available and vice versa (e.g., Chu et al., 2015).

Previous studies have reported a strong temporal variability in the PM_2.5/PM₁₀ ratio and a clear association with meteorological parameters such as temperature, wind speed, and relative humidity (Akinlade et al., 2015, Speranza et al., 2016). A trend analysis study (using data from 46 stations over the United Kingdom) reported an increasing trend in the PM_2.5/PM₁₀ ratio over the years (Munir et al., 2017), indicating an increasing dominance of anthropogenic emissions. Exhaustive studies reporting the PM_2.5/PM₁₀ ratio over the Indian subcontinent are fewer than required. Considering the vast geographical area of India, which is influenced by natural sources of PM (due to the long coastline, arid/desert areas etc.), and the country’s rapid development (leading to increasing anthropogenic emissions), large spatio-temporal variability is expected in the PM_2.5/PM₁₀ ratio. Further, long-term trends in the PM_2.5/PM₁₀ ratio need to be investigated to understand the progression of anthropogenic activities over time.

Studies reporting the PM_2.5/PM₁₀ ratio over India are available, but they mostly correspond to a single station/city (e.g., Shandilya et al., 2012, Fauzie and Venkataramana, 2017), or report a limited dataset (e.g. Ghosh et al., 2014) or considering PM_2.5/PM₁₀ ratio as secondary objectives of their studies (e.g. Sharma and Maloo, 2005, Iyer et al., 2018, Singh et al., 2019) or are event-based studies (e.g. Awasthi et al., 2011). Only a few exhaustive studies with multi-station and multi-year data exist (Nag et al., 2005, Kumar et al., 2014, Gupta et al., 2019). Some of the studies reporting the PM_2.5/PM₁₀ ratio over various Indian cities are listed in Table S1 of supplementary information (SI).

In the present study, PM_2.5 and PM₁₀ data measured by the central/state pollution control boards (CPCB/SPCBs) over various Indian cities are used. Sites with data available for at least five years were chosen for analysis to understand temporal patterns in PM_2.5, PM₁₀, and PM_2.5/PM₁₀ ratio. In the last decade, PCBs have established real-time ambient air-quality monitoring stations (Continuous Ambient Air Quality Monitoring Stations, CAAQMS), in most of the urban locations in India. In this paper, the analysis of data from the following eight sites (Fig. 1 and Table S2) has been presented: Zoo Park, Hyderabad (ZPO, 17.35°N, 78.45°E), Solapur (SPR, 17.66°N, 75.91°E); Airoli, Navi Mumbai (ARL, 19.16°N, 73.00°E); Chandrapur (CHR, 19.96°N, 79.30°E); GPO Civil Lines, Nagpur (GCL, 21.15°N, 79.09°E); Ardhali Bazar, Varanasi (ADB, 25.32°N, 82.98°E); R. K. Puram, Delhi (RKP, 28.57°N, 77.18°E); and Mandir Marg, Delhi (MDM, 28.63°N, 77.20°E). For these sites, simultaneous measurements of PM_2.5 and PM₁₀ for the period January 2015 to December 2019 are available. The objectives of the current study are (i) understanding temporal variability in the PM_2.5/PM₁₀ ratio over the eight study sites, (ii) investigating the relationship between the PM_2.5/PM₁₀ ratio and ambient meteorological parameters, and (iii) comparing the observed PM_2.5/PM₁₀ ratios with those retrieved from Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2). The geographical locations of the study sites are shown in Fig. 1. The study sites were spread across the length and breadth of the country (expect over the arid western India) representing various geographies of Indian sub-continent. The colour map in Fig. 1 depicts the gridded annual mean (for a representative year of 2019) MERRA-2 PM_2.5/PM₁₀ ratio. Details on MERRA-2 are provided in the next section.