Spatiotemporal distribution of air pollutants during a heat wave-induced forest fire event in Uttarakhand

Prevailing dry conditions and rainfall deficit during the spring season in North India led to heat wave conditions which resulted in widespread and intense forest fire events in the Himalayan state of Uttarakhand during April 16-30, 2022. A total of 7589 active fires were detected by VIIRS during the second half of April 2022 compared to 1558 during the first half. The TROPOMI observed total column values of CO and NO2 increased by 4.4% and 11.7%, respectively during April 16-30, 2022 with respect to April 1-15, 2022. A noticeable increase in surface level concentration of trace gases was also observed at Dehradun. In situ measurements of CO, NOx, and O3 during April 16-30, 2022 show an increase of 133, 35, and 6% compared to previous year observations during the same period. Weather Research and Forecasting model with chemistry (WRF-Chem) is utilized to quantitatively estimate the contribution of this event on the distribution of air pollutants over this state. The model results were evaluated against ERA5 reanalysis, upper air soundings, and TROPOMI-retrieved total column density (TCD) of CO, NO2, and O3. Two simulations with (Fire) and without (NoFire) biomass burning emissions input were performed to quantify the contribution of forest fires to the concentration of trace gases and particulates. The CO, NO2, and O3 emitted/produced from forest fire over Uttarakhand during April 2022 contributed approximately 39.95, 35.73, and 9.97% to the surface concentration of respective gas. In the case of aerosols, it was around 71.20, 71.44, and 33.62% for PM2.5, PM10, and BC respectively. The vertical profile analysis of pollutants revealed that extreme forest fire events can perturb the distribution of air pollutants from the surface up to 450 hPa.

Bounding Global Aerosol Radiative Forcing of Climate Change

Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol-radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of -1.6 to -0.6 W m-2, or -2.0 to -0.4 W m-2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial-era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds.

Atmospheric aerosol radiative forcing over a semi-continental location Tripura in North-East India: Model results and ground observations

Northeast India (NEI) is located within the boundary of the great Himalayas in the north and the Bay of Bengal (BoB) in the southwest, experiences the mixed influence of the westerly dust advection from the Indian desert, anthropogenic aerosols from the highly polluted Indo-Gangetic Plains (IGP) and marine aerosols from BoB. The present study deals with the estimation and characterization of aerosol radiative forcing over a semi-continental site Tripura, which is a strategic location in the western part of NEI having close proximity to the outflow of the IGP. Continuous long term measurements of aerosol black carbon (BC) mass concentrations and columnar aerosol optical depth (AOD) are used for the estimation of aerosol radiative forcing in each monthly time scale. The study revealed that the surface forcing due to aerosols was higher during both winter and pre-monsoon seasons, having comparable values of 32W/m² and 33.45W/m² respectively. The atmospheric forcing was also higher during these months due to increased columnar aerosol loadings (higher AOD ~0.71) shared by abundant BC concentrations (SSA ~0.7); while atmospheric forcing decreased in monsoon due to reduced magnitude of BC (SSA ~0.94 in July) as well as columnar AOD. The top of the atmosphere (TOA) forcing is positive in pre-monsoon and monsoon months with the highest positive value of 3.78W/m² in June 2012. The results are discussed in light of seasonal source impact and transport pathways from adjacent regions.

Method for integrating the absorption cross sections of spheres over wavelength or diameter

The absorption cross sections of spherical particles and droplets must be integrated over frequency or droplet size or both for various applications. Morphology-dependent resonances (MDRs) of the spheres can make evaluation of such integrals difficult because the MDRs can contribute significantly to the integrals even when their linewidths are extremely narrow, especially when the absorption is weak. A method of evaluating these integrals by use of Lorentzian approximations near MDRs is described. Calculated frequency-integrated absorption cross sections illustrate how the method obtains accurate cross sections with far fewer integration points than a method that uses equally spaced points. The method reported here suggests a way to integrate over frequency in more-complicated scattering and emission problems and should also be useful for integrating scattering and absorption by other shapes, e.g., spheroids and cylinders, for which the MDR positions and linewidths can be calculated.