A Monte Carlo study of upper tropospheric reactive nitrogen during the Pacific Exploratory Mission in the Western Pacific Ocean (PEM-West B)

A subset of Pacific Exploratory Mission in the Western Pacific Ocean (PEM-West B) data (northwestern Pacific, March 1994), selected to represent upper troposphere (UT) midlatitude conditions, is analyzed to answer the following questions: (1) Is there a shortfall in total reactive nitrogen (NO_y) as measured during PEM-West B in this region? (2) If so, what are the likely contributions of interfering constituents like HCN or of other reactive nitrogen species? For question 1 our analyses show that 87% of total reactive nitrogen measured in the UT is accounted for by NO_x + HNO₃ + PAN (similar to Kondo et al. [this issue (a, b)], Talbot et al. [this issue], and Singh et al. [1997a, b]). For question 2 we find that less than 20 pptv (<5% of mean NO_y) is possibly HCN. A one-dimensional model that simulates mean mixing ratios of this PEM-West B data is used with a Monte Carlo approach to explore other candidates for unmeasured nitrogen species and NO_y partitioning. Using standard gas-phase reactions with varying rate coefficients [Thompson and Stewart, 1991; Stewart and Thompson, 1996], it is found, on average, that NO_x + HNO₃ + PAN = 399 pptv (observed mean equal to 432 ± 97 pptv). A more complete inventory for total reactive nitrogen (Σ NO_i = NO_x + HNO₃ + PAN + HNO₄ + CH₃O₂NO₂ + alkyl nitrates + C₂H₅O₂NO₂) is 494 ± 91 pptv, with 19% consisting of HNO₄ + CH₃O₂NO₂ + alkyl nitrates. Thus, whether unmeasured forms of reactive nitrogen are present or not, total reactive nitrogen as measured at midlatitude UT during PEM-West B is accounted for within the measurement uncertainty. The greatest kinetics uncertainties are in thermolytic losses for HNO₄, PAN, and CH₃O₂NO₂ (120-150% by the method of Stewart and Thompson [1996]). Nonetheless, comparison with PEM-West B data shows that panel-recommended kinetics expressions [Demore et al., 1994] can explain reactive nitrogen observations without invoking extreme rates or heterogeneous processes. In summary, large concentrations of unmeasured reactive nitrogen species were not prevalent during midlatitude UT PEM-West B sampling although the observed shortfall (13%) can be explained by HNO₄ + alkyl nitrates + CH₃O₂NO₂. Agreement between theory and observations may also reflect improved instrument capabilities for measuring reactive nitrogen.