During spring 2020, the COVID-19 pandemic caused massive reductions in emissions from industry, ground and airborne transportation. To explore the resulting atmospheric composition changes, we conducted the BLUESKY campaign with two research aircraft and measured trace gases, aerosols, and cloud properties from the boundary layer to the lower stratosphere. From 16 May to 9 June 2020, we performed 20 flights in the early COVID-19 lockdown phase over Europe and the Atlantic Ocean. We found up to 50% reductions in boundary layer nitrogen dioxide concentrations in urban areas from GOME-2B satellite data, along with carbon monoxide reductions in the pollution hot spots. We measured 20 to 70% reductions in total reactive nitrogen, carbon monoxide and fine mode aerosol concentration in profiles over German cities compared to a 10-year data set from passenger aircraft. The total aerosol mass was significantly reduced below 5 km altitude, and the organic aerosol fraction also aloft, indicative of decreased organic precursor gas emissions. The reduced aerosol optical thickness caused a perceptible shift in sky color towards the blue part of the spectrum (hence BLUESKY) and increased shortwave radiation at the surface. We find that the 80% decline in air traffic led to substantial reductions in nitrogen oxides at cruise altitudes, in contrail cover, and in resulting radiative forcing. The light extinction and depolarization by cirrus were also reduced in regions with substantially decreased air traffic. General circulation-chemistry model simulations indicate good agreement with the measurements when applying a reduced emission scenario. The comprehensive BLUESKY dataset documents the major impact of anthropogenic emissions on the atmospheric composition.
Abstract. Aerosols influence the Earth’s energy balance through direct radiative effects and indirectly by altering the cloud micro-physics. Anthropogenic aerosol emissions dropped considerably when the global COVID–19 pandemic resulted in severe restraints on mobility, production, and public life in spring 2020. Here we assess the effects of these reduced emissions on direct and indirect aerosol radiative forcing over Europe, excluding contributions from contrails. We simulate the atmospheric com- position with the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model in a baseline (business as usual) and a reduced emission scenario. The model results are compared to aircraft observations from the BLUESKY aircraft campaign performed in May/June 2020 over Europe. The model agrees well with most of the observations, except for sulfur dioxide, particulate sulfate and nitrate in the upper troposphere, likely due to a somewhat biased representation of stratospheric aerosol chemistry and missing information about volcanic eruptions which could have influenced the campaign. The comparison with a business as usual scenario shows that the largest relative differences for tracers and aerosols are found in the upper troposphere, around the aircraft cruise altitude, due to the reduced aircraft emissions, while the largest absolute changes are present at the surface. We also find an increase in shortwave radiation of 0.327 ± 0.105 Wm−2 at the surface in Europe for May 2020, solely attributable to the direct aerosol effect, which is dominated by decreased aerosol scattering of sunlight, followed by reduced aerosol absorption, caused by lower concentrations of inorganic and black carbon aerosols in the troposphere. A further in- crease in shortwave radiation from aerosol indirect effects was found to be much smaller than its variability. Impacts on ice crystal- and cloud droplet number concentrations and effective crystal radii are found to be negligible.
Abstract. Chemical ionisation mass spectrometry (CIMS) using I− (the iodide anion), hereafter I-CIMS, as a primary reactant ion has previously been used to measure NO3 and N2O5 both in laboratory and field experiments. We show that reports of large daytime mixing ratios of NO3 and N2O5 (both usually present in detectable amounts only at night) are likely to be heavily biased by the ubiquitous presence of HNO3 in the troposphere and lower stratosphere. We demonstrate in a series of laboratory experiments that the CIMS detection of HNO3 at m/z 62 using I− ions is efficient in the presence of peroxy acetyl nitric anhydride (PAN) or peroxyacetic acid (PAA) and especially O3. We have characterised the dependence of the sensitivity to HNO3 detection on the presence of acetate anions (CH3CO2-, m/z 59, from either PAN or PAA). The loss of CH3CO2- via conversion to NO3- in the presence of HNO3 may represent a significant bias in I-CIMS measurements of PAN and PAA in which continuous calibration (e.g. via addition of isotopically labelled PAN) is not carried out. The greatest sensitivity to HNO3 at m/z 62 is achieved in the presence of ambient levels of O3 whereby the thermodynamically disfavoured, direct reaction of I− with HNO3 to form NO3- is bypassed by the formation of IOx-, which reacts with HNO3 to form, for example, iodic acid and NO3-. The ozone and humidity dependence of the detection of HNO3 at m/z 62 was characterised in laboratory experiments and applied to daytime, airborne measurements in which good agreement with measurements of the I−(HNO3) cluster ion (specific for HNO3 detection) was obtained. At high ozone mixing ratios, we show that the concentration of I− ions in our ion–molecule reactor (IMR) is significantly depleted. This is not reflected by changes in the measured I− signal at m/z 127 as the IOx- formed does not survive passage through the instrument but is likely detected after fragmentation to I−. This may result in a bias in measurements of trace gases using I-CIMS in stratospheric air masses unless a calibration gas is continuously added or the impact of O3 on sensitivity is characterised.
Abstract. Aerosols influence the Earth's energy balance directly by modifying the radiation transfer and indirectly by altering the cloud microphysics. Anthropogenic aerosol emissions dropped considerably when the global COVID-19 pandemic resulted in severe restraints on mobility, production, and public life in spring 2020. We assess the effects of these reduced emissions on direct and indirect aerosol radiative forcing over Europe, excluding contributions from contrails. We simulate the atmospheric composition with the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model in a baseline (business-as-usual) and a reduced emission scenario. The model results are compared to aircraft observations from the BLUESKY aircraft campaign performed in May–June 2020 over Europe. The model agrees well with most of the observations, except for sulfur dioxide, particulate sulfate, and nitrate in the upper troposphere, likely due to a biased representation of stratospheric aerosol chemistry and missing information about volcanic eruptions. The comparison with a baseline scenario shows that the largest relative differences for tracers and aerosols are found in the upper troposphere, around the aircraft cruise altitude, due to the reduced aircraft emissions, while the largest absolute changes are present at the surface. We also find an increase in all-sky shortwave radiation of 0.21 ± 0.05 W m−2 at the surface in Europe for May 2020, solely attributable to the direct aerosol effect, which is dominated by decreased aerosol scattering of sunlight, followed by reduced aerosol absorption caused by lower concentrations of inorganic and black carbon aerosols in the troposphere. A further increase in shortwave radiation from aerosol indirect effects was found to be much smaller than its variability. Impacts on ice crystal concentrations, cloud droplet number concentrations, and effective crystal radii are found to be negligible.
Abstract. Chemical Ionisation Mass Spectrometry (CIMS) using I− (the iodide anion) as primary chemi-ion has previously been used to measure NO3 and N2O5 both in laboratory and field experiments. We show that reports of the large daytime mixing ratios of NO3 and N2O5 (usually only present in detectable amounts at night-time) are likely to be heavily biased by the ubiquitous presence of HNO3 in the troposphere and lower stratosphere. We demonstrate in a series of laboratory experiments that the CIMS detection of HNO3 at m/z 62 using I− ions is efficient in the presence of PAN or peracetic acid (PAA) and especially O3. We have characterised the dependence of the sensitivity to HNO3 detection on the presence of acetate anions (CH3CO2−, m/z 59, from either PAN or PAA). The loss of CH3CO2− via conversion to NO3− in the presence of HNO3 may represent a significant bias in I-CIMS measurements of PAN and CH3C(O)OOH. The largest sensitivity to HNO3 at m/z 62 is achieved in the presence of ambient levels of O3 whereby the thermodynamically disfavoured, direct reaction of I− with HNO3 to form NO3− is bypassed by the formation of IOX− which react with HNO3 to form e.g. iodic acid and NO3−. The ozone and humidity dependence of the detection of HNO3 at m/z 62 was characterised in laboratory experiments and applied to daytime, airborne measurements in which very good agreement with measurements of the I−(HNO3) cluster-ion (specific for HNO3 detection) was obtained.
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