Abstract
Abstract. Toxic nitrogen oxides produced by high temperature combustion are prevalent in urban environments, contributing to a significant health burden. Nitrogen oxides such as NO2 and HONO in pollution are important for hydroxyl radical (OH) production and overall oxidative capacity in urban environments; however, current mechanisms cannot explain high daytime levels of HONO observed in many urban and rural locations around the world. Here we present HONO, NO2 and aerosol extinction vertical distributions retrieved from multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements in suburban Melbourne, which are the first MAX-DOAS results from the Australian continent. Using the optimal estimation algorithm HEIPRO we show that vertical profiles for NO2 and HONO can be calculated with a low dependence on the retrieval forward model and a priori parameters, despite a lack of independent co-located aerosol or trace gas measurements. Between December 2016 and April 2017 average peak NO2 values of 8±2 ppb indicated moderate traffic pollution levels, and high daytime peak values of HONO were frequently detected, averaging 220±30 ppt in the middle of the day. HONO levels measured in Melbourne were typically lower than those recorded in the morning in other places around the world, indicating minimal overnight accumulation, but peaked in the middle of the day to be commensurate with midday concentrations in locations with much higher NO2 pollution. Regular midday peaks in the diurnal cycle of HONO surface concentrations have only previously been reported in rural locations. The HONO measured implies a daytime source term 1 ppb h−1 above the predicted photostationary state (PSS) concentration and represents an OH radical source up to 4 times stronger than from ozone photolysis alone in the lowest 500 m of the troposphere. The dependence of the high midday HONO levels on soil moisture, combined with the observed diurnal and vertical profiles, provides evidence for a strong photoactivated and ground-based daytime HONO source.
Highlights
The World Health Organization indicates that ambient air pollution exposure represents the largest environmental risk to human health, with as many as one in nine deaths attributable to poor air quality (WHO, 2016)
Results were screened for the likely presence of cloud by applying a filter based on the colour index (CI), defined as the ratio between spectral intensities at 330 and 390 nm (Wagner et al, 2016)
We report on the first MAX-DOAS measurements from Melbourne, Australia (37.7◦ S, 144.9◦ E), from December 2016 to November 2017
Summary
The World Health Organization indicates that ambient air pollution exposure represents the largest environmental risk to human health, with as many as one in nine deaths attributable to poor air quality (WHO, 2016). Considerable efforts have been made to determine the mechanism of the missing source(s) and the importance of HONO as a tropospheric radical source through vertical gradient measurements using long-path DOAS (Stutz et al, 2010; Wong et al, 2012; Young et al, 2012), aeroplanes (Neuman et al, 2016), Zeppelins (Li et al, 2014) and towers (Kleffmann et al, 2003; VandenBoer et al, 2013) Compared to these expensive, short-term campaign platforms, MAX-DOAS measurements of HONO vertical profiles as demonstrated in Hendrick et al (2014) and GarciaNieto et al (2018) have the significant advantages of simple autonomous instrumentation, being cheap to run and able to be deployed in any environment for long-term monitoring programmes. The results presented in this paper demonstrate the ability of MAX-DOAS measurements to provide important air quality data in Melbourne and contribute to an improved understanding of how HONO impacts the budget of tropospheric oxidants
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