Abstract
Steady state approximation for interpreting NO3 and N2O5 has large uncertainty under complicated ambient conditions and could even produces incorrect results unconsciously. To provide an assessment and solution to the dilemma, we formulate data sets based on in-situ observations to reassess the applicability of the method. In most of steady state cases, we find a prominent discrepancy between Keq (equilibrium coefficient for reversible reactions of NO3 and N2O5) and correspondingly simulated [N2O5]/([NO2]×[NO3]), especially in wintertime high aerosol conditions. This gap reveals the accuracy of Keq has a critical impact on the steady state analysis in polluted region. In addition, the accuracy of γ(N2O5) derived by steady state fit depends closely on the reactivity of NO3 (kNO3) and N2O5 (kN2O5). Based on a complete set of simulations, air mass of kNO3 less than 0.01 s−1 with high aerosol and temperature higher than 10 °C is suggested to be the best suited for steady state analysis of NO3–N2O5 chemistry. Instead of confirming the validity of steady state by numerical modeling for every case, this work directly provides concentration ranges appropriate for accurate steady state approximation, with implications for choosing suited methods to interpret nighttime chemistry in high aerosol air mass.
Highlights
Nitrate radical (NO3), an extremely reactive species prone to build up at night, is an ideal candidate for steady state analysis in combine with dinitrogen pentoxide (N2O5) due to fast equilibrium reactions between them (R1).NO2 + NO3 + M → N2O5 + M (R1a)N2O5+M → NO2 + NO3+M (R1b)Numerous works have taken the advantage of the steady state calculation to quantify the total first-order loss rate for NO3 or N2O5 such that they drew conclusions about the oxidation capacity and reactive nitrogen budgets contributed by this chemical system (Allan et al., 1999;Allan et al, 2000;Carslaw et al, 1997;Platt et al, 1984;Vrekoussis et al, 2007;Wang et al, 2013)
We presume that if any data point outputted from this model is still invalid for steady state, the sink rate constant of air mass represented by this data point should be too weak for steady state analysis in ambient conditions within a reasonable timescale
The rates of NO3-N2O5 reversible reactions are expected to be equal for the steady state case, so that the equilibrium coefficient Keq can be determined from either the rate constant ratio of
Summary
Nitrate radical (NO3), an extremely reactive species prone to build up at night, is an ideal candidate for steady state analysis in combine with dinitrogen pentoxide (N2O5) due to fast equilibrium reactions between them (R1). These situations are prevalent in nocturnal boundary layer (Phillips et al., 2016;Stutz et al, 2004;Wang et al, 2017a;Wang et al, 2017c) and increase the difficulty of applying steady state directly on NO3-N2O5 observation data, whereas few studies have systematically characterized the error source and application conditions of this method (Brown et al, 2009). We illustrate the reasons for Keq values distinct from parameterization in ambient conditions, the possible uncertainties of linear fit resulted from different Keq, and the influence of other atmospheric variables on γ(N2O5) derivation via steady state method. A series of ambient condition tests specify the exact ranges suited for steady state analysis according to the validity of steady state and Keq values, which optimizes the validity check by numerical modeling in previous research (Brown et al, 2009;Brown et al, 2003) and develops complete standard for data filtering
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