The Indo-Gangetic Plain (IGP) is one of the dominant sources of air pollution worldwide. During winter, the variations in planetary boundary layer (PBL) height, driven by a strong radiative thermal inversion, affect the regional air pollution dispersion. To date, measurements of aerosol-water vapour interactions, especially cloud condensation nuclei (CCN) activity, are limited in the Indian sub-continent, causing large uncertainties in the radiative forcing estimates of aerosol-cloud interactions. We present the results of a one-month field campaign (February-March 2018) in the megacity, Delhi, a significant polluter in the IGP. We measured the composition of fine particulate matter (PM1) and size-resolved CCN properties over a wide range of water vapour supersaturations. The analysis includes PBL modelling, backward trajectories, and fire spots to elucidate the influence of PBL and air mass origins on the aerosols. The aerosol properties depended strongly on the PBL height, and a simple power-law fit could parameterize the observed correlations of PM1 mass, aerosol particle number, and CCN number with PBL height, indicating PBL induced changes in aerosol accumulation. The low inorganic mass fractions, low aerosol hygroscopicity and high externally mixed weakly CCN-active particles under low PBL height (<100 m) indicated the influence of the PBL on aerosol aging processes. In contrast, aerosol properties did not depend strongly on air mass origins or wind direction, implying that the observed aerosol and CCN are from local emissions. An error function could parameterize the relationship between CCN number and supersaturation throughout the campaign.

Our abilities to predict the extent and impacts of atmospheric black carbon depend on the accuracy of inventories, which are known to be highly uncertain in the developing world. This is because of less regulation of industry and vehicles, the private use of lower-quality fuels and appliances and a lack of data on activity. In order to provide better constraint on emissions from a developing megacity, the fluxes of refractory black carbon were measured using a Single Particle Soot Photometer (SP2) and the eddy covariance method, which is a relatively new technique. These were made on top of a purpose-built tower alongside a suite of other aerosol and gas flux measurements as part of the NERC/Newton Fund ‘DelhiFlux’ project, part of the Air Pollution and Human Health (APHH) Delhi programme. The location was the campus of the Indira Gandhi Delhi Technical University for Women (IGDTUW) in Old Delhi, where emissions were deemed to be representative of the less economically developed areas of the city. Statistically significant rBC mass fluxes of around 10-30 ng m-2 s-1 were measured and these were strongest in the morning. The rBC particles observed could be categorised into distinct types according to their coating thicknesses according to the SP2 Leading Edge Only (LEO) method, however unlike previously published observations in London and Beijing, no clear sources could be attributed to the different coating types. Through comparisons with other measurements such as NOx and AMS factorisation, it appears that the main sector responsible for rBC emissions in the area is transport, which is consistent with the SAFAR inventory, although cooking also seemed to contribute. However, the magnitude and diurnal profile of the measured emissions differed significantly from the inventory, with the measurements being lower by a factor of 50-60 and peaking earlier in the day.

Covid lockdown presented an important opportunity to study relatively cleaner conditions in India. The complex factors of power production, industry, and transportation could be more carefully dissected because of the extreme reduction in the influence of the latter two emission sources. Measurements of cloud condensation nuclei (CCN) activity and other chemical properties of atmospheric aerosols showed that newly formed aerosol particles were produced in the SO2 plume from a large coal-fired power plant, contrary to normal conditions of heavy pollution. The sulfate-rich particles had high CCN activity and number concentration, indicating high cloud-forming potential. Examining the sensitivity of CCN properties under relatively clean conditions over India provides important new constraints on the perturbations of past and future climate forcing by anthropogenic emissions. Because most sensitive regime of aerosol climate forcing on cloud development is the midpoint of relatively clean conditions afforded by the Covid lockdown between background and polluted conditions.