• Energy Research

Abstract. Biomass burning (BB) is a large source of primary and secondary organic aerosols (POA and SOA). This study addresses the physical and chemical evolution of BB organic aerosols. Firstly, the evolution and lifetime of BB POA and SOA signatures observed with the Aerodyne Aerosol Mass Spectrometer are investigated, focusing on measurements at high-latitudes acquired during the 2008 NASA ARCTAS mission, in comparison to data from other field studies and from laboratory aging experiments. The parameter f60, the ratio of the integrated signal at m/z 60 to the total signal in the organic component mass spectrum, is used as a marker to study the rate of oxidation and fate of the BB POA. A background level of f60~0.3% ± 0.06% for SOA-dominated ambient OA is shown to be an appropriate background level for this tracer. Using also f44 as a tracer for SOA and aged POA and a surrogate of organic O:C, a novel graphical method is presented to characterise the aging of BB plumes. Similar trends of decreasing f60 and increasing f44 with aging are observed in most field and lab studies. At least some very aged BB plumes retain a clear f60 signature. A statistically significant difference in f60 between highly-oxygenated OA of BB and non-BB origin is observed using this tracer, consistent with a substantial contribution of BBOA to the springtime Arctic aerosol burden in 2008. Secondly, a summary is presented of results on the net enhancement of OA with aging of BB plumes, which shows large variability. The estimates of net OA gain range from ΔOA/ΔCO(mass) = −0.01 to ~0.05, with a mean ΔOA/POA ~19%. With these ratios and global inventories of BB CO and POA a global net OA source due to aging of BB plumes of ~8 ± 7 Tg OA yr−1 is estimated, of the order of 5 % of recent total OA source estimates. Further field data following BB plume advection should be a focus of future research in order to better constrain this potentially important contribution to the OA burden.

SignificanceUnderstanding the sources of tropospheric ozone is important for effective air quality management and accurate radiative forcing attribution. Biomass burning emits large quantities of ozone precursors to the lower atmosphere. This source can drive regional-scale ozone production, but its impact on global tropospheric ozone is poorly constrained. Here, we present unique global in situ aircraft observations of ozone and continental pollution tracers. Ozone enhancements attributable to biomass burning equal or exceed those from urban emissions, a result that is not predicted by current chemical transport models. These findings represent a potentially major shift in the understanding of the sources of ozone in the lower atmosphere and indicate the need for model developments to improve the representation of global tropospheric ozone.

We used an ensemble of aircraft measurements with the GEOS-Chem chemical transport model to constrain present-day North American ethanol sources, and gauge potential long-range impacts of increased ethanol fuel use. We find that current ethanol emissions are underestimated by 50% in Western North America, and overestimated by a factor of 2 in the east. Our best estimate for year-2005 North American ethanol emissions is 670 GgC/y, with 440 GgC/y from the continental U.S. We apply these optimized source estimates to investigate two scenarios for increased ethanol fuel use in the U.S.: one that assumes a complete transition from gasoline to E85 fuel, and one tied to the biofuel requirements of the U.S. Energy Indepence and Security Act (EISA). For both scenarios, increased ethanol emissions lead to higher atmospheric acetaldehyde concentrations (by up to 14% during winter for the All-E85 scenario and 2% for the EISA scenario) and an associated shift in reactive nitrogen partitioning reflected by an increase in the peroxyacetyl nitrate (PAN) to NO(y) ratio. The largest relative impacts occur during fall, winter, and spring because of large natural emissions of ethanol and other organic compounds during summer. Projected changes in atmospheric PAN reflect a balance between an increased supply of peroxyacetyl radicals from acetaldehyde oxidation, and the lower NO(x) emissions for E85 relative to gasoline vehicles. The net effect is a general PAN increase in fall through spring, and a weak decrease over the U.S. Southeast and the Atlantic Ocean during summer. Predicted NO(x) concentrations decrease in surface air over North America (by as much 5% in the All-E85 scenario). Downwind of North America this effect is counteracted by higher NO(x) export efficiency driven by increased PAN production and transport. From the point of view of NO(x) export from North America, the increased PAN formation associated with E85 fuel use thus acts to offset the associated lower NO(x) emissions.

Abstract. Nitrous oxide (N2O) is both a greenhouse gas in the troposphere and an ozone depleting substance in the stratosphere and is rapidly increasing in the atmosphere. The spatial distribution of N2O emissions and the sources leading to rising concentrations in the global atmosphere are highly uncertain. We measured the global distribution of tropospheric N2O mixing ratios during the airborne Atmospheric Tomography (ATom) mission. ATom measured mixing ratios of ~300 gas species and aerosol properties in 647 vertical profiles spanning the Pacific, Atlantic, Arctic, and much of the Southern Ocean basins, from nearly Pole to Pole, over four seasons (2016–2018). We measured N2O mixing ratios at 1 Hz using a Quantum Cascade Laser Spectrometer and a new spectral retrieval method to account for the pressure and temperature sensitivity of the instrument when deployed on aircraft. This retrieval strategy improved the precision of our N2O measurements by a factor of 3, enabling us to recover the precision to that of previous missions. Most of the variance of N2O mixing ratios in the troposphere is driven by the influence of N2O-depleted stratospheric air, especially at mid and high latitudes. We observe the downward propagation of lower N2O mixing ratios (compared to surface stations) that tracks the influence of stratosphere-troposphere exchange through the tropospheric column down to the surface, resulting in a seasonal minimum at the surface 2–3 months after the peak stratosphere-to-troposphere exchange in spring. The highest N2O mixing ratios occur close to the equator, extending through the boundary layer and free troposphere. We observed influences from a complex and diverse mixture of N2O sources, with emission source types identified using the rich suite of chemical species measured on ATom and with the geographical origin calculated using an atmospheric transport model. Although ATom flights were mostly over the oceans, the most prominent N2O enhancements were associated with anthropogenic emissions, including industry, oil and gas, urban and biomass burning, especially in the tropical Atlantic outflow from Africa. Enhanced N2O mixing ratios are mostly associated with pollution-related tracers arriving from the coastal area of Nigeria. Peaks of N2O are often well-correlated with indicators of photochemical processing, suggesting possible unexpected source processes. The difficulty of separating the mixture of different sources in the atmosphere contributes to uncertainties in the N2O global budget. The extensive data set from ATom will help improve the understanding of N2O emission processes and their representation in global models.

AbstractCH4is the most abundant reactive greenhouse gas and a complete understanding of its atmospheric fate is needed to formulate mitigation policies. Current chemistry-climate models tend to underestimate the lifetime of CH4, suggesting uncertainties in its sources and sinks. Reactive halogens substantially perturb the budget of tropospheric OH, the main CH4loss. However, such an effect of atmospheric halogens is not considered in existing climate projections of CH4burden and radiative forcing. Here, we demonstrate that reactive halogen chemistry increases the global CH4lifetime by 6–9% during the 21st century. This effect arises from significant halogen-mediated decrease, mainly by iodine and bromine, in OH-driven CH4loss that surpasses the direct Cl-induced CH4sink. This increase in CH4lifetime helps to reduce the gap between models and observations and results in a greater burden and radiative forcing during this century. The increase in CH4burden due to halogens (up to 700 Tg or 8% by 2100) is equivalent to the observed atmospheric CH4growth during the last three to four decades. Notably, the halogen-driven enhancement in CH4radiative forcing is 0.05 W/m2at present and is projected to increase in the future (0.06 W/m2by 2100); such enhancement equals ~10% of present-day CH4radiative forcing and one-third of N2O radiative forcing, the third-largest well-mixed greenhouse gas. Both direct (Cl-driven) and indirect (via OH) impacts of halogens should be included in future CH4projections.