Reactive nitrogen in the global upper troposphere from NASA DC8 and MOZAIC aircraft campaigns

<p>Reactive nitrogen in the upper troposphere (~8-12 km) impacts global climate, air quality and the oxidizing capacity of the whole troposphere. Here we use aircraft observations from instruments onboard the NASA DC8 aircraft for campaigns from 1997 (SONEX) to the recent ATom campaign (2016-2018) and the MOZAIC commercial aircraft campaign (2003-2005) to address uncertainties in the dynamics of reactive nitrogen (NO<sub>y</sub> = NO<sub>x</sub> + NO<sub>x</sub> reservoir compounds) in the global upper troposphere (UT). Our initial analysis of the DC8 aircraft observations is consistent with previous work in that PAN is the dominant NO<sub>y</sub> component (average: 43%; range: 40-60%), followed by NO<sub>x </sub>(on average, 21%), with smaller contributions (on average, 3.5-12.5%) from pernitric acid (HNO<sub>4</sub>), organonitrate (RONO<sub>2</sub>) and nitric acid (HNO<sub>3</sub>). We go on to compare multiyear mean NO<sub>y</sub> from MOZAIC to the combination of all NASA DC8 campaigns to determine whether we can build a near-global climatology of NO<sub>y</sub> and its components to compare to GEOS-Chem to assess our understanding of these very important atmospheric components.</p>

Measurements of CH₃O₂NO₂ in the upper troposphere

Methyl peroxy nitrate (CH3O2NO2) is a non-acyl peroxy nitrate that is important for photochemistry at low temperatures characteristic of the upper troposphere. We report the first measurements of CH3O2NO2, which we achieved through a new aircraft inlet configuration, combined with thermal-dissociation laser-induced fluorescence (TD-LIF) detection of NO2, and describe the accuracy, specificity, and interferences to CH3O2NO2 measurements. CH3O2NO2 is predicted to be a ubiquitous interference to upper-tropospheric NO2 measurements. We describe an experimental strategy for obtaining NO2 observations free of the CH3O2NO2 interference. Using these new methods, we made observations during two recent aircraft campaigns: the Deep Convective Clouds and Chemistry (DC-3) and the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) experiments. The CH3O2NO2 measurements we report have a detection limit (S/N = 2) of 15 pptv at 1 min averaging on a background of 200 pptv NO2 and an accuracy of ±40%. Observations are used to constrain the interference of pernitric acid (HO2NO2) to the CH3O2NO2 measurements, as HO2NO2 partially decomposes (~11%) along with CH3O2NO2 in the heated CH3O2NO2 channel used to detect CH3O2NO2.

Atmospheric nitrogen oxides (NO and NO₂) at Dome C, East Antarctica, during the OPALE campaign

Mixing ratios of the atmospheric nitrogen oxides NO and NO2 were measured as part of the OPALE (Oxidant Production in Antarctic Lands & Export) campaign at Dome C, East Antarctica (75.1° S, 123.3° E, 3233 m), during December 2011 to January 2012. Profiles of NOx mixing ratios of the lower 100 m of the atmosphere confirm that, in contrast to South Pole, air chemistry at Dome C is dominated by strong diurnal cycles in solar irradiance and atmospheric stability. Depth profiles of mixing ratios in firn air suggest that the upper snowpack at Dome C holds a significant reservoir of photolytically produced NO2 and is a sink of gas phase ozone (O3). First-time observations of BrO at Dome C suggest 2–3 pptv near the ground, with higher levels in the free troposphere. Assuming steady-state, observed mixing ratios of BrO and RO2 radicals are too low to explain the large NO2 : NO ratios found in ambient air. A previously not considered interference with pernitric acid (HNO4) may explain part of this inconsistency. During 2011–2012 NOx mixing ratios and flux were larger than in 2009–2010 consistent with also larger surface O3 mixing ratios resulting from increased net O3 production. Large NOx mixing ratios arose from a combination of changes in mixing height and NOx snow emission flux FNOx. During 23 December 2011–12 January 2012 median FNOx was twice that during the same period in 2009–2010 due to significantly larger atmospheric turbulence and a slightly stronger snowpack source. A tripling of FNOx in December 2011 was largely due to changes in snow pack source strength caused primarily by changes in NO3− concentrations in the snow skin layer, and only to a secondary order by decrease of total column O3 and associated increase in NO3− photolysis rates. Systematic changes in the quantum yield of NO3− photolysis over time may contribute to the observed FNOx variability.

Measurements of CH₃O₂NO₂ in the upper troposphere

The non-acyl peroxy nitrates, HO2NO2 and CH3O2NO2, are predicted to be important for photochemistry at low temperatures characteristic of the upper troposphere. We report the first measurements of methyl peroxy nitrate (CH3O2NO2). During the Deep Convective Clouds and Chemistry (DC-3) and the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) experiments, different inlet configurations for the UC Berkeley Thermal Dissociation-Laser Induced Instrument were tested to optimize measurements of CH3O2NO2 from the NASA DC-8. In addition, the inlet modifications were optimized for measurements of NO2 without CH3O2NO2 interferences. The CH3O2NO2 measurements we report have a detection limit (S/N = 2) of 15 pptv (parts per trillion by volume) at 1 min averaging on a background of 200 pptv NO2 and an accuracy of ±40%. Both observations and theoretical calculations were used to constrain the interference of pernitric acid (HO2NO2), which partially decomposes (~ 11%) along with CH3O2NO2 in our heated CH3O2NO2 channel. Evaluation of the accuracy of the CH3O2NO2 measurements is presented.

Near-IR photodissociation of peroxy acetyl nitrate

Measurements of the C-H overtone transition strengths combined with estimates of the photodissociation cross sections for these transitions suggest that near-IR photodissociation of peroxy acetyl nitrate (PAN) is less significant (Jnear-IR≈3x10-8s-1 at noon) in the lower atmosphere than competing sinks resulting from unimolecular decomposition and ultraviolet photolysis. This is in contrast to the photochemical behavior of a related peroxy nitrate, pernitric acid (PNA), that undergoes rapid near-IR photolysis in the atmosphere with Jnear-IR≈10-5s-1 at noon (Roehl et al., 2002). This difference is attributed to the larger binding energy and larger number of vibrational degrees of freedom in PAN, which make 4νCH the lowest overtone excitation with a high photodissociation yield (as opposed to 2νOH in PNA).

Near-IR photodissociation of peroxy acetyl nitrate

Measurements of the C-H overtone transition strengths combined with estimates of the photodissociation cross sections for these transitions suggest that near-IR photodissociation of peroxy acetyl nitrate (PAN) is less significant (Jnear-IR≈3×10−8s−1 at noon) in the lower atmosphere than competing sinks resulting from unimolecular decomposition and ultraviolet photolysis. This is in contrast to the photochemical behavior of a related peroxy nitrate, pernitric acid (PNA), that undergoes rapid near-IR photolysis in the atmosphere with Jnear-IR≈10−5s−1 at noon (Roehl et al., 2002). This difference is attributed to the larger binding energy and larger number of vibrational degrees of freedom in PAN, which make 4νCH the lowest overtone excitation with a high photodissociation yield (as opposed to 2νOH in PNA).