Global variability in atmospheric new particle formation mechanisms.

A key challenge in aerosol pollution studies and climate change assessment is to understand how atmospheric aerosol particles are initially formed1,2. Although new particle formation (NPF) mechanisms have been described at specific sites3-6, in most regions, such mechanisms remain uncertain to a large extent because of the limited ability of atmospheric models to simulate critical NPF processes1,7. Here we synthesize molecular-level experiments to develop comprehensive representations of 11 NPF mechanisms and the complex chemical transformation of precursor gases in a fully coupled global climate model. Combined simulations and observations show that the dominant NPF mechanisms are distinct worldwide and vary with region and altitude. Previously neglected or underrepresented mechanisms involving organics, amines, iodine oxoacids and HNO3 probably dominate NPF in most regions with high concentrations of aerosols or large aerosol radiative forcing; such regions include oceanic and human-polluted continental boundary layers, as well as the upper troposphere over rainforests and Asian monsoon regions. These underrepresented mechanisms also play notable roles in other areas, such as the upper troposphere of the Pacific and Atlantic oceans. Accordingly, NPF accounts for different fractions (10-80%) of the nuclei on which cloud forms at 0.5% supersaturation over various regions in the lower troposphere. The comprehensive simulation of global NPF mechanisms can help improve estimation and source attribution of the climate effects of aerosols.

Mechanism of SOA formation determines magnitude of radiative effects.

Secondary organic aerosol (SOA) nearly always exists as an internal mixture, and the distribution of this mixture depends on the formation mechanism of SOA. A model is developed to examine the influence of using an internal mixing state based on the mechanism of formation and to estimate the radiative forcing of SOA in the future. For the present day, 66% of SOA is internally mixed with sulfate, while 34% is internally mixed with primary soot. Compared with using an external mixture, the direct effect of SOA is decreased due to the decrease in total aerosol surface area and the increase of absorption efficiency. Aerosol number concentrations are sharply reduced, and this is responsible for a large decrease in the cloud albedo effect. Internal mixing decreases the radiative effect of SOA by a factor of >4 compared with treating SOA as an external mixture. The future SOA burden increases by 24% due to CO2 increases and climate change, leading to a total (direct plus cloud albedo) radiative forcing of -0.05 W m-2 When the combined effects of changes in climate, anthropogenic emissions, and land use are included, the SOA forcing is -0.07 W m-2, even though the SOA burden only increases by 6.8%. This is caused by the substantial increase of SOA associated with sulfate in the Aitken mode. The Aitken mode increase contributes to the enhancement of first indirect radiative forcing, which dominates the total radiative forcing.

Radiative forcing associated with particulate carbon emissions resulting from the use of mercury control technology.

Injection of powdered activated carbon (PAC) adsorbents into the flue gas of coal fired power plants with electrostatic precipitators (ESPs) is the most mature technology to control mercury emissions for coal combustion. However, the PAC itself can penetrate ESPs to emit into the atmosphere. These emitted PACs have similar size and optical properties to submicron black carbon (BC) and thus could increase BC radiative forcing unintentionally. The present paper estimates, for the first time, the potential emission of PAC together with their climate forcing. The global average maximum potential emissions of PAC is 98.4 Gg/yr for the year 2030, arising from the assumed adoption of the maximum potential PAC injection technology, the minimum collection efficiency, and the maximum PAC injection rate. These emissions cause a global warming of 2.10 mW m(-2) at the top of atmosphere and a cooling of -2.96 mW m(-2) at the surface. This warming represents about 2% of the warming that is caused by BC from direct fossil fuel burning and 0.86% of the warming associated with CO2 emissions from coal burning in power plants. Its warming is 8 times more efficient than the emitted CO2 as measured by the 20-year-integrated radiative forcing per unit of carbon input (the 20-year Global Warming Potential).

Projections of mortality risk attributable to short-term exposure to landscape fire smoke in China, 2021-2100: a health impact assessment study.

BACKGROUND: Landscape fire smoke, including smoke from all vegetation burning in natural and cultural landscapes, remains a threat to the health of the population. However, the future health impacts of landscape fire smoke in China have not been sufficiently investigated. We aimed to estimate the mortality risk attributable to landscape fire-related PM2·5 under different scenarios. METHODS: In this health impact assessment study, we used the projected population and landscape fire-related PM2·5 concentration to calculate deaths attributable to short-term exposure to landscape fire smoke PM2·5 during 2021-2100. We did the analysis in three defined future periods: 2021-40 (near term), 2051-70 (medium term), and 2081-2100 (long term), with 1986-2005 as the historical period. We used fire-specific short-term epidemiological functions with the regional parameters specific to China. We assessed the mortality risks of landscape fire-related smoke and further identified their spatiotemporal distribution under two shared socioeconomic pathway (SSP) scenarios: SSP1-2·6, an optimistic scenario with strict control of carbon emissions, and SSP2-4·5, an intermediate scenario with weaker control of carbon emissions. FINDINGS: The national mortality rate attributable to short-term exposure (ie, a few days) to landscape fire-related PM2·5 is projected to increase compared with historical values. The national deaths attributable to landscape fire smoke PM2·5 could peak in 2021-40, with increases of 28·10% (95% CI 14·08-53·11) under the SSP1-2·6 scenario and 37·38% (14·08-53·11) under the SSP2-4·5 scenario. Deaths would then decrease slightly during 2051-70 and 2081-2100. The provinces with the highest projected number of deaths attributable to landscape fire-related PM2·5 are located in east and south-central China, and those with the largest percentage increase in projected deaths are located in northwest and southwest China. INTERPRETATION: Our results suggest that global warming could increase the contribution of landscape fire smoke to the total PM2·5 concentration, leading to an increase in the mortality rate in China. Our findings could help policy makers implement effective interventions in hotspot areas during different periods to reduce the impact of landscape fire smoke on human health. FUNDING: The National Natural Science Foundation of China, National Key Research and Development Program of China, and the Wellcome Trust.

Radiative forcing by light-absorbing aerosols of pyrogenetic iron oxides.

Iron (Fe) oxides in aerosols are known to absorb sun light and heat the atmosphere. However, the radiative forcing (RF) of light-absorbing aerosols of pyrogenetic Fe oxides is ignored in climate models. For the first time, we use a global chemical transport model and a radiative transfer model to estimate the RF by light-absorbing aerosols of pyrogenetic Fe oxides. The model results suggest that strongly absorbing Fe oxides (magnetite) contribute a RF that is about 10% of the RF due to black carbon (BC) over East Asia. The seasonal average of the RF due to dark Fe-rich mineral particles over East Asia (0.4-1.0 W m-2) is comparable to that over major biomass burning regions. This additional warming effect is amplified over polluted regions where the iron and steel industries have been recently developed. These findings may have important implications for the projection of the climate change, due to the rapid growth in energy consumption of the heavy industry in newly developing countries.