Physical and chemical properties of PM₁ in Delhi: A comparison between clean and polluted days

Considering the significance of PM₁ aerosol in assessing health impacts of air pollution, an extensive analysis of PM₁ samples collected at an urban site in Delhi is presented in this study. Overall, PM₁ contributed to about 50 % of PM_2.5 mass which is alarming especially in Delhi where particle mass loadings are usually higher than prescribed limits. Major portion of PM₁ consisted of organic matter (OM) that formed nearly 47 % of PM₁ mass. Elemental carbon (EC) contributed to about 13 % of PM₁ mass, whereas SO₄^2- (16 %), NH₄⁺ (10 %), NO₃^- (4 %) and Cl^- (3 %) were the major inorganic ions present. Sampling was performed in two distinctive campaign periods (in terms of meteorological conditions and heating (fire) activities), during the year 2019, each spanning two-week time, i.e. (i) September 3^rd-16^th (clean days), and (ii) November 22^nd-December 5^th (polluted days). Additionally, PM_2.5 and black carbon (BC) were measured simultaneously for subsequent analysis. The 24-h averaged mean concentrations of PM_2.5 and BC during clean days (polluted days) were 70.6 ± 26.9 and 3.9 ± 1.0 μg m^-3 (196 ± 104 and 7.6 ± 4.1 μg m^-3), respectively, which were systematically lower (higher) than that of the annual mean (taken from studies conducted at same site in 2019) of 142 and 5.7 μg m^-3, respectively. Changes in characteristic ratios (i.e., organic carbon (OC)/elemental carbon (EC) and K⁺/EC) of chemical species detected in PM₁ show an increase in biomass emissions during polluted days. Increase in biomass emission can be attributed to increase in heating practices (burning of biofuels such as wood logs, straw, and cow-dung cake) in- and around- Delhi because of fall in temperature during second campaign. Furthermore, a significant increase in NO₃^- fraction of PM₁ is observed during second campaign which shows fog processing of NO_X due to conducive meteorological conditions in winters. Also, comparatively stronger correlation of NO₃^- with K⁺ during second campaign (r = 0.98 as compared to r = 0.5 during first campaign) suggests the increased heating practices to be a contributing factor for increased fraction of NO₃^- in PM₁. We observed that during polluted days, meteorological parameters such as dispersion rate also played a major role in intensifying the impact of increased local emissions due to heating activities. Apart from this, change in the direction of regional emission transport to study site and the topology of Delhi are the possible reasons for the elevated pollution level, especially PM₁ during winter in Delhi. This study also suggests that black carbon measurement techniques used in current study (optical absorbance with heated inlet and evolved carbon techniques) can be used as reference techniques to determine the site-specific calibration constant of optical photometers for urban aerosol.