Airborne quantification of net methane and carbon dioxide fluxes from European Arctic wetlands in Summer 2019.

Arctic wetlands and surrounding ecosystems are both a significant source of methane (CH4) and a sink of carbon dioxide (CO2) during summer months. However, precise quantification of this regional CH4 source and CO2 sink remains poorly characterized. A research flight using the UK Facility for Airborne Atmospheric Measurement was conducted in July 2019 over an area (approx. 78 000 km2) of mixed peatland and forest in northern Sweden and Finland. Area-averaged fluxes of CH4 and carbon dioxide were calculated using an aircraft mass balance approach. Net CH4 fluxes normalized to wetland area ranged between 5.93 ± 1.87 mg m-2 h-1 and 4.44 ± 0.64 mg m-2 h-1 (largest to smallest) over the region with a meridional gradient across three discrete areas enclosed by the flight survey. From largest to smallest, net CO2 sinks ranged between -513 ± 74 mg m-2 h-1 and -284 ± 89 mg m-2 h-1 and result from net uptake of CO2 by vegetation and soils in the biosphere. A clear gradient of decreasing bulk and area-averaged CH4 flux was identified from north to south across the study region, correlated with decreasing peat bog land area from north to south identified from CORINE land cover classifications. While N2O mole fraction was measured, no discernible gradient was measured over the flight track, but a minimum flux threshold using this mass balance method was calculated. Bulk (total area) CH4 fluxes determined via mass balance were compared with area-weighted upscaled chamber fluxes from the same study area and were found to agree well within measurement uncertainty. The mass balance CH4 fluxes were found to be significantly higher than the CH4 fluxes reported by many land-surface process models compiled as part of the Global Carbon Project. There was high variability in both flux distribution and magnitude between the individual models. This further supports previous studies that suggest that land-surface models are currently ill-equipped to accurately capture carbon fluxes inthe region. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

Isotopic signatures of methane emissions from tropical fires, agriculture and wetlands: the MOYA and ZWAMPS flights.

We report methane isotopologue data from aircraft and ground measurements in Africa and South America. Aircraft campaigns sampled strong methane fluxes over tropical papyrus wetlands in the Nile, Congo and Zambezi basins, herbaceous wetlands in Bolivian southern Amazonia, and over fires in African woodland, cropland and savannah grassland. Measured methane δ13CCH4 isotopic signatures were in the range -55 to -49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely -60 ± 1‰ from Upper Congo and Zambezi wetlands. Very similar δ13CCH4 signatures were measured over the Amazonian wetlands of NE Bolivia (around -59‰) and the overall δ13CCH4 signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was -59 ± 2‰. These results were more negative than expected. For African cattle, δ13CCH4 values were around -60 to -50‰. Isotopic ratios in methane emitted by tropical fires depended on the C3 : C4 ratio of the biomass fuel. In smoke from tropical C3 dry forest fires in Senegal, δ13CCH4 values were around -28‰. By contrast, African C4 tropical grass fire δ13CCH4 values were -16 to -12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had δ13CCH4 around -37 to -36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric δ13CCH4 values predicted by global atmospheric models are highly sensitive to the δ13CCH4 isotopic signatures applied to tropical wetland emissions. Field and aircraft campaigns also observed widespread regional smoke pollution over Africa, in both the wet and dry seasons, and large urban pollution plumes. The work highlights the need to understand tropical greenhouse gas emissions in order to meet the goals of the UNFCCC Paris Agreement, and to help reduce air pollution over wide regions of Africa. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

Characterizing the aging of biomass burning organic aerosol by use of mixing ratios: a meta-analysis of four regions.

Characteristic organic aerosol (OA) emission ratios (ERs) and normalized excess mixing ratios (NEMRs) for biomass burning (BB) events have been calculated from ambient measurements recorded during four field campaigns. Normalized OA mass concentrations measured using Aerodyne Research Inc. quadrupole aerosol mass spectrometers (Q-AMS) reveal a systematic variation in average values between different geographical regions. For each region, a consistent, characteristic ratio is seemingly established when measurements are collated from plumes of all ages and origins. However, there is evidence of strong regional and local-scale variability between separate measurement periods throughout the tropical, subtropical, and boreal environments studied. ERs close to source typically exceed NEMRs in the far-field, despite apparent compositional change and increasing oxidation with age. The absence of any significant downwind mass enhancement suggests no regional net source of secondary organic aerosol (SOA) from atmospheric aging of BB sources, in contrast with the substantial levels of net SOA formation associated with urban sources. A consistent trend of moderately reduced ΔOA/ΔCO ratios with aging indicates a small net loss of OA, likely as a result of the evaporation of organic material from initial fire emissions. Variability in ERs close to source is shown to substantially exceed the magnitude of any changes between fresh and aged OA, emphasizing the importance of fuel and combustion conditions in determining OA loadings from biomass burning.

Assessment of the photochemistry of OH and NO3 on Jeju Island during the Asian-dust-storm period in the spring of 2001.

In this study, we examined the influence of the long-range transport of dust particles and air pollutants on the photochemistry of OH and NO3 on Jeju Island, Korea (33.17 degrees N, 126.10 degrees E) during the Asian-dust-storm (ADS) period of April 2001. Three ADS events were observed during the periods of April 10-12, 13-14, and 25-26. Average concentration levels of daytime OH and nighttime NO3 on Jeju Island during the ADS period were estimated to be about 1x10(6) and 2x10(8) moleculescm(-3) ( approximately 9 pptv), respectively. OH levels during the ADS period were lower than those during the non-Asian-dust-storm (NADS) period by a factor of 1.5. This was likely to result from higher CO levels and the significant loading of dust particles, reducing the photolysis frequencies of ozone. Decreases in NO3 levels during the ADS period was likely to be determined mainly by the enhancement of the N2O5 heterogeneous reaction on dust aerosol surfaces. Averaged over 24 h, the reaction between HO2 and NO was the most important source of OH during the study period, followed by ozone photolysis, which contributed more than 95% of the total source. The reactions with CO, NO2, and non-methane hydrocarbons (NMHCs) during the study period were major sinks for OH. The reaction of N2O5 on aerosol surfaces was a more important sink for nighttime NO3 during the ADS due to the significant loading of dust particles. The reaction of NO3 with NMHCs and the gas-phase reaction of N2O5 with water vapor were both significant loss mechanisms during the study period, especially during the NADS. However, dry deposition of these oxidized nitrogen species and a heterogeneous reaction of NO3 were of no importance.