Nocturnal Measurements of HONO, NO2 and NO3 by Differential Optical Absorption Spectroscopy in Polluted Marine and Urban Atmospheres

Nitrogen oxides are ubiquitous throughout the lower atmosphere and significantly affect the chemistry of the atmosphere, air quality, and climate. A data-set obtained using differential optical absorption spectroscopy (DOAS) was analyzed in order to quantify the NO3, HONO and NO2 concentrations at Saturna Island, and concentrations of N2O5 were calculated. Nocturnal measurements of NO3, NO2 and HONO were also performed using active-DOAS at York University. A method for calculating the lifetimes of NO3 without assuming a steady-state approximation was determined and non steady-state lifetimes of NO3 were calculated for both studies. The direct (via NO3) and indirect (via N2O5) rate loss constants of NO3 from the combined nocturnal reservoir (NO3+N2O5) were determined as a function of time of night. Measurements of HONO over the polluted open ocean were performed for the first time. Rapidly established steady-states of HONO were observed, persisting throughout the night until sunrise. During the steady-state period (d[HONO]/dt≈0), HONO was independent of the air mass source and NO2, leading to a zero-order HONO formation with respect to NO2, contrary to expectations. Potential reservoirs of HONO were explored and a conceptual model for HONO formation over aqueous surfaces was hypothesized. Subsequently, nocturnal measurements of HONO in the urban area were made at York University for a total of 242 nights. This urban data-set showed two types of HONO behavior. Firstly, a steady-state behavior was clearly observed for a subset of the data-set, similar to that observed in the aqueous environment at Saturna. Secondly, HONO concentrations were observed to highly correlate with NO2 for another subset of the data-set (d([HONO]/[NO2])/dt≈0), showing evidence of first-order behavior as expected for the accepted heterogeneous NO2 hydrolysis mechanism of HONO formation (2NO2 + H2O → HONO + HNO3). Steady-states of HONO were observed during atmospherically unstable nights, while HONO was strongly correlated with NO2 during stable nights. It was discovered that the main parameters distinguishing these two types of behavior were atmospheric stability and NO2 concentration.