Research

Atmospheric Chemistry

The chemistry that controls what we breathe and how the atmosphere cleans itself: from basic principles to the themes of my research, from Indian megacities to the Amazon and the tropical upper troposphere.

The big picture

Gases and particles are constantly emitted into the atmosphere by vegetation, oceans, fires, traffic, industry and homes. Sunlight and reactive molecules, above all the hydroxyl radical (OH), then transform them. These reactions produce ozone and aerosol, remove pollutants, and influence climate. Atmospheric chemistry asks which emissions matter, how fast they react, and what they turn into.

Emissions VOCs (plants, traffic, fuels) NOx (combustion, lightning) CO, methane Ocean gases (DMS) Smoke and dust Chemistry OH sunlight + radicals oxidation, photolysis transport and mixing Outcomes Ground-level ozone Secondary aerosol (PM) New particles, clouds Removal of pollutants Health and climate effects
Figure A. From emissions to outcomes: a simplified view of atmospheric chemistry.
Chemistry basics (click to open)

OH, the atmosphere's "detergent"

OH forms when sunlight splits ozone in the presence of water vapour, and it reacts with almost every trace gas, usually within seconds. It sets how long methane, VOCs and many pollutants survive, so it is often called the atmosphere's oxidising capacity. OH and HO2 together are called HOx, and they are rapidly interconverted.

VOC oxidation

Volatile organic compounds (VOCs) react with OH to form organic peroxy radicals (RO2). What RO2 does next decides the outcome: with NO it produces NO2, carbonyl compounds and HO2, which leads to ozone; with HO2 it often forms peroxides; and some also undergo internal rearrangements.

Ozone formation

Near the ground, NO2 splits in sunlight and the oxygen atom forms ozone. VOCs speed up the conversion of NO to NO2 without consuming ozone, so ozone builds up. Whether VOCs or NOx limit the production changes from place to place (the "NOx-limited" and "VOC-limited" regimes), which matters for pollution control.

From gas to particle

Oxidation makes VOC products less volatile, so some condense onto existing particles (secondary organic aerosol) or help create new ones (new particle formation). Particles affect health through PM2.5 and influence clouds and climate.

Why the tropics and the upper troposphere matter

The tropics have strong sunshine, abundant water vapour and large natural emissions, so a large part of global oxidation happens there. Deep convection carries gases quickly aloft, where chemistry differs from near the ground and small changes can affect ozone and particle formation.

Research themes

My work spans several environments. Below are the main themes with representative published papers; the full list is on the Publications page.

Urban air quality in India

VOC sourcesOzoneSecondary organic aerosolDelhi

Real-time measurements in Delhi, Lucknow and western India: what emits VOCs in a megacity, how they turn into ozone and secondary aerosol, and how meteorology and events such as COVID-19 lockdowns change the picture.

  • Seasonal variability and source apportionment of non-methane VOCs using PTR-TOF-MS measurements in Delhi, India
  • Insights into the Regional Transport and Local Formation of Secondary Organic Aerosol in Delhi, India
  • Non-methane volatile organic compounds emitted from domestic fuels in Delhi: emission factors and total city-wide emissions
  • Impact of COVID-19 pandemic lockdown in ambient concentrations of aromatic volatile organic compounds in a metropolitan city of western India

Biogenic VOCs and forests

IsopreneMonoterpenesAmazonWestern Ghats

Plants emit large amounts of reactive gases such as isoprene and monoterpenes. This theme studies their emissions, their dependence on weather, and how land-use change such as forest clearing alters them.

  • Impacts of convection, chemistry, and forest clearing on biogenic volatile organic compounds over the Amazon
  • Ambient air characteristics of biogenic volatile organic compounds at a tropical evergreen forest site in Central Western Ghats of India
  • Emissions and atmospheric concentrations of α-pinene at an urban site of India: role of changes in meteorology

Marine atmosphere

DMSMarine isopreneArabian SeaAerosol formation

Gases from the ocean, including dimethyl sulfide (DMS) and isoprene from phytoplankton, and how they relate to the formation of aerosol over the Arabian Sea and northern Indian Ocean.

  • Processes Controlling DMS Variability in Marine Boundary Layer of the Arabian Sea During Post-Monsoon Season of 2021
  • High levels of isoprene in the marine boundary layer of the Arabian Sea during spring inter-monsoon: role of phytoplankton blooms
  • Sources and Distribution of Light NMHCs in the Marine Boundary Layer of the Northern Indian Ocean During Winter

Convection and the upper troposphere

Deep convectionHOxNOxNew particle formation

Deep convection lifts surface gases to the upper troposphere within minutes. There, low temperatures and different chemistry control radicals, ozone and new particle formation.

  • Isoprene nitrates drive new particle formation in Amazon’s upper troposphere
  • Impacts of convection, chemistry, and forest clearing on biogenic volatile organic compounds over the Amazon

Aerosols and haze

PM2.5Biomass burningSOA

How gases become particles: secondary organic aerosol, biomass-burning haze and the composition of fine particulate matter in polluted cities.

  • Effect of Biomass Burning on PM2.5 Composition and Secondary Aerosol Formation During Post-Monsoon and Winter Haze Episodes in Delhi
  • Real-time quantification and source apportionment of fine particulate matter including organics and elements in Delhi during summertime
  • Air Pollution in New Delhi during Late Winter: An Overview of a Group of Campaign Studies

Meteorology and trace gases

Carbon monoxideMeteorologyCyclone

Local weather, convection and cyclones shape the distribution of pollutants such as carbon monoxide and particulates.

  • The influence of local meteorology and convection on carbon monoxide distribution over Chennai
  • The role of local meteorology on ambient particulate and gaseous species at an urban site of western India

Tools

Observations and models complement each other.

VOC measurementsPTR-ToF-MS, GC-MS/FID and field campaigns. Volatile organic compoundsSources, oxidation and effects. CAABA/MECCA box modelProcess-level chemistry of an air parcel. VOC & BVOC hubInteractive charts: lifetimes, NOx fate, Amazon study. EMAC global modelChemistry and transport on the whole globe.

This page is an overview. The paper currently under review is listed on the Publications page but its results are not summarised here.

All research topics