Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios — which differ in fossil fuel and microbial emissions — to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain

The Simons Observatory (SO) is a new cosmic microwave background experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. We describe the scientific goals of the experiment, motivate the design, and forecast its performance. SO will measure the temperature and polarization anisotropy of the cosmic microwave background in six frequency bands centered at: 27, 39, 93, 145, 225 and 280 GHz. The initial configuration of SO will have three small-aperture 0.5-m telescopes and one large-aperture 6-m telescope, with a total of 60,000 cryogenic bolometers. Our key science goals are to characterize the primordial perturbations, measure the number of relativistic species and the mass of neutrinos, test for deviations from a cosmological constant, improve our understanding of galaxy evolution, and constrain the duration of reionization. The small aperture telescopes will target the largest angular scales observable from Chile, mapping ≈ 10% of the sky to a white noise level of 2 µK-arcmin in combined 93 and 145 GHz bands, to measure the primordial tensor-to-scalar ratio, r, at a target level of σ(r) = 0.003. The large aperture telescope will map ≈ 40% of the sky at arcminute angular resolution to an expected white noise level of 6 µK-arcmin in combined 93 and 145 GHz bands, overlapping with the majority of the Large Synoptic Survey Telescope sky region and partially with the Dark Energy Spectroscopic Instrument. With up to an order of magnitude lower polarization noise than maps from the Planck satellite, the high-resolution sky maps will constrain cosmological parameters derived from the damping tail, gravitational lensing of the microwave background, the primordial bispectrum, and the thermal and kinematic Sunyaev-Zel'dovich effects, and will aid in delensing the large-angle polarization signal to measure the tensorto-scalar ratio. The survey will also provide a legacy catalog of 16,000 galaxy clusters and more than 20,000 extragalactic sources a .

A detailed and extended chemical mechanism describing tropospheric aqueous phase chemistry (147 species and 438 reactions) is presented here as Chemical Aqueous Phase Radical Mechanism (CAPRAM) 2.4 (MODAC mechanism). The mechanism based on the former version 2.3 [Herrmann et al., 2000] contains extended organic and transition metal chemistry and is formulated more explicitly based on a critical review of the literature. The aqueous chemistry has been coupled to the gas phase mechanism Regional Atmospheric Chemistry Modeling (RACM) [Stockwell et al., 1997], and phase exchange accounted for using the resistance model of Schwartz [1986]. A method for estimating mass accommodation coefficients (α) is described, which accounts for functional groups contained in a particular compound. A condensed version has also been developed to allow the use of CAPRAM 2.4 (MODAC mechanism) in higher‐scale models. Here the reproducibility of the concentration levels of selected target species (i.e., NOx, S(IV), H2O2, NO3, OH, O3, and H+) within the limits of ± 5% was used as a goal for eliminating insignificant reactions from the complete CAPRAM 2.4 (MODAC mechanism). This has been done using a range of initial conditions chosen to represent different atmospheric scenarios, and this produces a robust and concise set of reactions. The most interesting results are obtained using atmospheric conditions typical for an urban scenario, and the effects introduced by updating the aqueous phase chemistry are highlighted, in particular, with regard to radicals, redox cycling of transition metal ions and organic compounds. Finally, the reduced scheme has been incorporated into a one‐dimensional (1‐D) marine cloud model to demonstrate the applicability of this mechanism.

Rapid population growth and industrialization have driven substantial increases in Asian ozone precursor emissions over the past decade 1 , with highly uncertain impacts on regional and global tropospheric ozone levels. According to ozonesonde measurements 2,3 , tropospheric ozone concentrations at two Asian sites have increased by 1 to 3% per year since 2000, an increase thought to contribute to positive trends in the ozone levels observed at North America's West Coast 4,5 . However, model estimates of the Asian contribution to North American ozone levels are not well-constrained by observations 6,7 . Here we interpret Aura satellite measurements of tropospheric concentrations of ozone and its precursor NO 2 , along with its largest natural source, stratospheric ozone, using the TM5 global chemistry-transport model. We show that tropospheric ozone concentrations over China have increased by about 7% between 2005 and 2010 in response to two factors: a rise in Chinese emissions by about 21% and increased downward transport of stratospheric ozone. Furthermore, we find that transport from China of ozone and its precursors has o set about 43% of the 0.42 DU reduction in free-tropospheric ozone over the western United States that was expected between 2005 and 2010 as a result of emissions reductions associated with federal, state and local air quality policies. We conclude that global e orts may be required to address regional air quality and climate change.Elevated tropospheric ozone (O 3 ) concentrations have a direct adverse impact on human 8 and ecosystem health 9 at the surface, whereas in the free troposphere O 3 acts as a greenhouse gas 8,10 and drives the production of the hydroxyl radical, which controls the chemical lifetime of many atmospheric pollutants and reactive greenhouse gases. Previous studies have pointed not only to changing emissions of O 3 precursors 6,7,11-14 , but also to changes in both the net inflow of O 3 from the stratosphere 14-16 and largescale transport patterns 7,13-16 as contributing to observed trends and variability in tropospheric O 3 . For many regions outside Asia, in situ measurements suggest that tropospheric O 3 has remained relatively constant in the 2000s following substantial increases in the 1980s-1990s, at least partially due to changes in the emissions of O 3 precursors, including nitrogen oxides (NO X ) and hydrocarbons 17 . However, chemistry-transport models used for attribution of trends in Europe and North America tend to greatly underestimate the observed O 3 changes over the past several decades 18 . Furthermore, trends are generally derived from sparse surface and balloon measurements that are not necessarily representative of larger regions. This is particularly true for China, where measurements from only two ozonesonde stations are reported in the literature 2,3 .Over the past decade, satellite instruments have provided valuable information on the global distribution of tropospheric O 3 and NO X concentrations and their evolution in time. Here we use O 3 retr...