Thermodynamical framework for effective mitigation of high aerosol loading in the Indo-Gangetic Plain during winter.

The Indo-Gangetic Plain (IGP) experiences severe air pollution every winter, with ammonium chloride and ammonium nitrate as the major inorganic fractions of fine aerosols. Many past attempts to tackle air pollution in the IGP were inadequate, as they targeted a subset of the primary pollutants in an environment where the majority of the particulate matter burden is secondary in nature. Here, we provide new mechanistic insight into aerosol mitigation by integrating the ISORROPIA-II thermodynamical model with high-resolution simultaneous measurements of precursor gases and aerosols. A mathematical framework is explored to investigate the complex interaction between hydrochloric acid (HCl), nitrogen oxides (NOx), ammonia (NH3), and aerosol liquid water content (ALWC). Aerosol acidity (pH) and ALWC emerge as governing factors that modulate the gas-to-particle phase partitioning and mass loading of fine aerosols. Six "sensitivity regimes" were defined, where PM1 and PM2.5 fall in the "HCl and HNO3 sensitive regime", emphasizing that HCl and HNO3 reductions would be the most effective pathway for aerosol mitigation in the IGP, which is ammonia-rich during winter. This study provides evidence that precursor abatement for aerosol mitigation should not be based on their descending mass concentrations but instead on their sensitivity to high aerosol loading.

Enhanced secondary aerosol formation driven by excess ammonia during fog episodes in Delhi, India.

The Indo-Gangetic Plain (IGP) has high wintertime fine aerosol loadings that significantly modulate the widespread fog formation and sustenance. Here, we investigate the potential formation of secondary inorganic aerosol driven by excess ammonia during winter fog. Physicochemical properties of fine aerosols (PM1 and PM2.5) and trace gases (HCl, HONO, HNO3, SO2, and NH3) were simultaneously monitored at hourly resolution using Monitor for AeRosols and Gases in Ambient air (MARGA-2S) for the first time in India. Results showed that four major ions, i.e., Cl-, NO3-, SO42-, and NH4+ contributed approximately 97% of the total measured inorganic ionic mass. The atmosphere was ammonia-rich in winter and ammonium was the dominant neutralizer with aerosol neutralization ratio (ANR) close to unity. The correlation between ammonium and chloride was ≥0.8, implying the significant formation of ammonium chloride during fog in Delhi. Thermodynamical model ISORROPIA-II showed the predicted PM1 and PM2.5 pH to be 4.49 ± 0.53, and 4.58 ± 0.48 respectively which were in good agreement with measurements. The ALWC increased from non-foggy to foggy periods and a considerable fraction of fine aerosol mass existed in the supermicron size range of 1-2.5 µm. The sulfur oxidation ratio (SOR) of PM1, PM2.5 reached up to 0.60, 0.75 in dense fog and 0.74, 0.87 when ambient RH crossed a threshold of 95%, much higher than non-foggy periods (with confidence level of ≥95%) pointing to enhanced formation of secondary aerosol in fog.

Characterization of atmospheric trace gases and water soluble inorganic chemical ions of PM1 and PM2.5 at Indira Gandhi International Airport, New Delhi during 2017-18 winter.

Water soluble inorganic chemical ions of PM1 and PM2.5 and atmospheric trace gases were monitored simultaneously on hourly resolution at Indira Gandhi International Airport (IGIA), Delhi during 8 December 2017-10 February 2018. Monitoring was made by MARGA (Monitoring AeRosol and Gases in ambient Air) under winter fog experiment (WIFEX) program of the Ministry of Earth Sciences (MoES), Government of India. The result based on the analysis of the data so generated reveals that Cl-, NH4+, NO3- and SO42- were dominant ions in order which collectively constituted 96.8 and 97.3% of the of the total measured ionic mass in PM1 and PM2.5 respectively. Their overall average concentrations in PM1 were 19.5 ± 19.7, 18.4 ± 10.5, 16.6 ± 8.7 and 10.3 ± 5.7 µg/m3 and in PM2.5 were 36.0 ± 33.9, 32.7 ± 17.2, 28.5 ± 13.6 and 19.9 ± 13.9 µg/m3. Average concentrations of HCl, HNO3, HNO2, SO2 and NH3 trace gases were 0.7 ± 0.3, 2.7 ± 1.1, 6.6 ± 4.7, 22.0 ± 12.3 and 25.7 ± 9.1 µg/m3 respectively. Weather parameters along with low mixing height played significant role in the occurrence of high concentration of these chemical species. NH4+ was the prime neutralizer of the acidic components and mostly occurred in (NH4)2SO4/NH4HSO4, NH4NO3 and NH4Cl molecular forms. Major sources of these chemical species were fossil fuel combustion in aviation activity and transportation, coal burning in thermal power plants, industrial processes and emissions from biomass burning and agro-based activity. The quality of air with respect to PM2.5 always remained deteriorated. It became alarming during low visibility period mainly due to high concentration of Cl-, NO3-, SO42- and NH4+. Both meteorological and chemical processes interactively fed each other which occasionally resulted in fog development and visibility degradation. The knowledge gained by this study will help in simulation of atmospheric processes which lead to fog development and dispersal in the Delhi region.

Characterization and source identification of PM2.5 and its chemical and carbonaceous constituents during Winter Fog Experiment 2015-16 at Indira Gandhi International Airport, Delhi.

Data on mass concentration of PM2.5 and its carbonaceous and water soluble inorganic chemical ions were compiled through sampling of PM2.5 at Indira Gandhi International Airport, Delhi during Dec. 16, 2015-Feb. 15, 2016 under Winter Fog Experiment (WIFEX) program of the Ministry of Earth Sciences (MoES) and analysing the samples. The data so generated were interpreted in terms of their variation on different time scales and apportioning their sources. It is found that mass concentration of PM2.5 averaged over the whole period of observation was 198.6±55.6. The concentration of organic carbon (OC) and elemental carbon (EC) was 24.7±9.4 and 11.7±4.7µg/m3 respectively with no any trend of increase or decrease over the observational period. SO42-, Cl- and NO3- dominated over other anions with their overall average concentration 34.0±23.1, 32.7±16.1 and 13.3±8.7µg/m3 respectively. Among cations, NH4+ showed highest concentration with an average value of 21.0±10.6µg/m3. Variation of daily average mass concentration of these parameters over the period of observation matched well with the variation of PM2.5 mass concentration indicating thereby to be the major contributors to the PM2.5 mass. NH4+ mostly occurred as NH4Cl and NH4NO3 and poorly as (NH4)2SO4 or NH4HSO4. H+ ion mostly occurred as H2SO4 and occasionally as HNO3. Carbonaceous aerosols and NO3- were mainly generated from fossil-fuel combustion. NH4+ and anthropogenic Cl- were mostly generated by biomass burning. The source of SO42- was found to be industries and thermal power plants. Continental Ca2+ and Mg2+ originated from thermal power plants and soil dust.

Surface ozone characterization at Larsemann Hills and Maitri, Antarctica.

Data are analyzed in terms of daily average ozone, its diurnal variation and its relation with meteorological parameters like dry bulb temperature (T), wet bulb temperature (Tw), atmospheric pressure and wind speed based on measurement of these parameters at two Indian Antarctic stations (Larsemann Hills, and Maitri) during 28th Indian Scientific Expedition of Antarctica (ISEA) organized during Antarctic summer of the year 2008-09. The work has been carried out to investigate summer time ozone level and its day-to-day and diurnal variability at these coastal locations and to highlight possible mechanism of ozone production and destruction. The result of the analysis indicates that daily average ozone concentration at Larsemann Hills varied from ~13 and ~20ppb with overall average value of ~16ppb and at Maitri, it varied from ~16 and ~21ppb with overall average value of ~18ppb. Photochemistry is found to partially contribute occasionally to the surface layer ozone at both the stations. Lower concentration of ozone at Maitri during beginning of the observational days may be due to destruction of ozone through activated halogens, whereas higher ozone on latter days may be due to photochemistry and advective transport from east to south-east areas. Ozone concentration during blizzard episodes at both the stations is reduced due to slow photochemical production of ozone, its photochemical removal and removal through deposition of ozone molecules on precipitation particles. Diurnal variation of ozone at Larsemann Hills and Maitri has been found to be absent.

Large inter annual variation in air quality during the annual festival 'Diwali' in an Indian megacity.

A network of air quality and weather monitoring stations was established under the System of Air Quality Forecasting and Research (SAFAR) project in Delhi. We report observations of ozone (O3), nitrogen oxides (NOx), carbon monoxide (CO) and particulate matter (PM2.5 and PM10) before, during and after the Diwali in two consecutive years, i.e., November 2010 and October 2011. The Diwali days are characterised by large firework displays throughout India. The observations show that the background concentrations of particulate matter are between 5 and 10 times the permissible limits in Europe and the United States. During the Diwali-2010, the highest observed PM10 and PM2.5 mass concentration is as high as 2070µg/m3 and 1620µg/m(3), respectively (24hr mean), which was about 20 and 27 times to National Ambient Air Quality Standards (NAAQS). For Diwali-2011, the increase in PM10 and PM2.5 mass concentrations was much less with their peaks of 600 and of 390µg/m(3) respectively, as compared to the background concentrations. Contrary to previous reports, firework display was not found to strongly influence the NOx, and O3 mixing ratios, with the increase within the observed variability in the background. CO mixing ratios showed an increase. We show that the large difference in 2010 and 2011 pollutant concentrations is controlled by weather parameters.

Impact of meteorological parameters on the development of fine and coarse particles over Delhi.

Measurements of ambient particulate matters (viz., PM10 and PM2.5) were made with an hourly sampling frequency at Indian Institute of Tropical Meteorology (IITM), New Delhi Branch (a residential area) during a period from December 2010 to November 2011. The data so generated were analyzed to understand frequency distribution of their concentrations and the impact of meteorological parameters on the distribution of particulate matters on different time scales. It is found that the particulate matters with cut off aerodynamic diameter of 10 µm (PM10) preferentially occurred in the concentration range of 301-350 µg/m(3) during winter and post-monsoon, 251-300 µg/m(3) during summer and 51-100 µg/m(3) during monsoon season. The particulate matters with cut off aerodynamic diameter of 2.5 µm (PM2.5) preferentially occurred in the concentration range of 201-250 µg/m(3) during winter and 51-100 µg/m(3) during the remaining seasons. The concentration of particulate matters (PM10 and PM2.5) remained always above the National Ambient Air Quality Standards (NAAQS) except during monsoon season. Annual distribution of the concentration of particulate matters showed seasonality with maximum in winter and minimum in monsoon season. Diurnal variation of PM10 and PM2.5 showed bimodal distribution with one maximum in the forenoon and the other at around mid-night. The observed seasonality and diurnal variability in the distribution are attributed mainly to the meteorology.

Seasonal factors influencing in chemical composition of total suspended particles at Pune, India.

A study on the chemical characterization of boundary layer aerosols is made based on the collection of TSP and size separated aerosol mass samples at Pune during March 2007-February 2008. This study will be helpful in simulating atmospheric processes responsible for aerosol development over Pune region and understanding its environmental implications related to radiation budget and climate. It is found that major fraction of Ca(2+) is locally generated by suspension of soil dust during all the seasons. During pre-monsoon season, coarse Mg(2+) is originated from the soil and the sea salt, whereas fine Mg(2+) is generated from the local biomass burning. Sizeable amount of SO(4)(2-) is emitted from local industrial and brick kiln's activities. Neutralization of NO(3)(-), generated both from biogenic and anthropogenic sources, is made by NH(3) gas generated mainly from anthropogenic sources. The data are further examined in terms of the factors specific to the individual seasons influencing physical and chemical characteristics of the boundary layer aerosols. The specific factors are: (a) Intense local convection during pre-monsoon season; (b) southwesterly wind flow and rainfall activity during monsoon season; and (c) Day time convection and occurrence of low level inversion during post-monsoon and winter seasons.