We present an assessment of the impacts on atmospheric composition and radiative forcing of short-lived pollutants following a worldwide decrease in anthropogenic activity and emissions comparable to what has occurred in response to the COVID-19 pandemic, using the global composition-climate model United Kingdom Chemistry and Aerosols Model (UKCA). Emission changes reduce tropospheric hydroxyl radical and ozone burdens, increasing methane lifetime. Reduced SO2 emissions and oxidizing capacity lead to a decrease in sulfate aerosol and increase in aerosol size, with accompanying reductions to cloud droplet concentration. However, large reductions in black carbon emissions increase aerosol albedo. Overall, the changes in ozone and aerosol direct effects (neglecting aerosol-cloud interactions which were statistically insignificant but whose response warrants future investigation) yield a radiative forcing of -33 to -78 mWm(-2). Upon cessation of emission reductions, the short-lived climate forcers rapidly return to pre-COVID levels; meaning, these changes are unlikely to have lasting impacts on climate assuming emissions return to pre-intervention levels.

Volatile organic compounds (VOCs) play an essential role in climate change and air pollution by modulating tropospheric oxidation capacity and providing precursors for ozone and aerosol formation. Arctic permafrost buries large quantities of frozen soil carbon, which could be released as VOCs with permafrost thawing or collapsing as a consequence of global warming. However, due to the lack of reported studies in this field and the limited capability of the conventional measurement techniques, it is poorly understood how much VOCs could be emitted from thawing permafrost and the chemical speciation of the released VOCs. Here we apply a Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) in laboratory incubations for the first time to examine the release of VOCs from thawing permafrost peatland soils sampled from Finnish Lapland. The warming-induced rapid VOC emissions from the thawing soils were mainly attributed to the direct release of old, trapped gases from the permafrost. The average VOC fluxes from thawing permafrost were four times as high as those from the active layer (the top layer of soil in permafrost terrain). The emissions of less volatile compounds, i.e. sesquiterpenes and diterpenes, increased substantially with rising temperatures. Results in this study demonstrate the potential for substantive VOC releases from thawing permafrost. We anticipate that future global warming could stimulate VOC emissions from the Arctic permafrost, which may significantly influence the Arctic atmospheric chemistry and climate change.

Aerosols generated from aqueous samples of readily obtainable humic material standards are often used as proxies for organic particulates found in the atmosphere in various investigations, such as consideration of radiative forcing effects. Here, we present results for the retrieved complex index of refraction, m = n + ik, at a wavelength of 403 nm for aerosols prepared from six humic material standards using a calibrated cavity ring-down spectrometer: a humic acid sodium salt, Pahokee peat humic and fulvic acids, Elliott soil humic and fulvic acids, and Suwannee river fulvic acid. In addition, we have conducted UV-vis spectrometric studies to measure the mass absorption coefficients, molar absorptivities, and absorption Angstrom exponents of bulk aqueous solutions of the humic materials. We find clear differences between the humic acid (HA) and fulvic acid (FA) samples with the HA having larger values for the imaginary part of the refractive index, k. The mean value for the HA samples is k = 0.170 while the mean is k = 0.037 for the FA materials. We have examined correlations between the retrieved refractive index and humic material characteristics obtained from spectroscopic and elemental analysis, including aromatic content and the oxygen-to-carbon atomic ratio, where the molar absorption coefficient yields the strongest correlation. Finally, we compare the humic material optical properties to those of authentic and laboratory generated organic carbon samples in order to assess the usefulness of these humic standards as proxies for light absorbing aerosol.

This paper provides an overview and summary of the current state of knowledge regarding critical atmospheric processes that affect the distribution and concentrations of greenhouse gases and aerosols emitted from wildland fires or produced through subsequent chemical reactions in the atmosphere. These critical atmospheric processes include the dynamics of plume rise, chemical reactions involving smoke plume constituents, the long-range transport of smoke plumes, and the potential transport of gases and aerosols from wildland fires into the stratosphere. In the area of plume-rise dynamics, synthesis information is provided on (I) the relevance of plume height for assessing impacts of gases and aerosol from wildland fires on the climate system, (2) recent scientific advances in understanding the role of multiple updraft cores in plume behavior, and (3) some of the current modeling tools and remote sensing monitoring techniques available for predicting and measuring smoke plume heights. In the area of atmospheric chemistry associated with wildland fire emissions, synthesis information is provided on what is currently known about the atmospheric fate of wildland fire smoke-plume constituents and the relationship of their atmospheric chemistry to radiative forcing. Synthesis information related to long-range atmospheric transport of wildland fire emissions is presented and summarizes many of the recent published observational and modeling studies that provide clear evidence of intercontinental, continental, and regional transport of North American fire emissions, including black carbon, to locations far-removed from the fire-event locations. Recent studies are also highlighted that examined the significance of troposphere-stratosphere exchange processes, which can result in the transport of greenhouse gases and aerosols from North American wildland fires into the stratosphere where they can remain for very long periods of time and alter the radiative balance and typical chemical reactions that occur there. Finally, specific research gaps and needs related to plume dynamics, atmospheric transport and deposition processes, and the atmospheric chemistry of wildland fire emissions are identified and discussed. Published by Elsevier B.V.