Measurements of CH&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in the upper troposphere

B A Nault,C Garland,P J Wooldridge,R C Cohen,S E Pusede,S R Hall,K Ullmann

doi:10.5194/amt-8-987-2015

Abstract

Abstract. 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.

Highlights

Methyl peroxy nitrate (CH3O2NO2) is predicted to be abundant at temperatures below 240 K (Fig. 1), where it serves as a temporary reservoir of NOx (NOx = NO + NO2)
We observe that CH3O2NO2 composed 7 % and HO2NO2 composed 5 % of the median NOy budget between 220 and 230 K during DC-3, where NOy is defined as the sum of NOx, PNs, ANs, and HNO3
During DC-3, these weakly bound species comprised a larger portion of the median NOy budget than HNO3 (4 %), which is a terminal sink for NOx

Summary

Introduction

The emphasis on low temperatures results from non-acyl peroxy nitrates, such as CH3O2NO2 and pernitric acid (HO2NO2), having lower thermal stability and shorter lifetimes than acyl peroxy nitrates, such as peroxy acetyl nitrate (PAN) and peroxy propionyl nitrate (PPN) (∼ 100 kJ mol−1 for non-acyl peroxy nitrates vs ∼ 120 kJ mol−1 for acyl peroxy nitrates). If the air sample is brought into a warm aircraft prior to detection, this very short lifetime results in substantial decomposition for residence times longer than even 0.1 s (Fig. 2). As a result of the decomposition of CH3O2NO2, there is a positive interference to NO2 measurements with residence times at warm temperatures longer than 0.1 s. In the absence of such a correction, it is necessary to interpret NO2 measurements as XNO2, where XNO2 is the sum of ambient NO2 and some or all of non-acyl peroxy nitrates (Browne et al, 2011)

Methods

Findings

Discussion

Conclusion