Nocturnal loss of NO_x during the 2010 CalNex‐LA study in the Los Angeles Basin

Abstract

The chemical removal of NO x at night in urban areas remains poorly constrained due to uncertainties in the contribution of various loss pathways and the impact of the suppressed nocturnal vertical mixing. Here we present long-path differential optical absorption spectroscopy observations of nocturnal vertical concentration profiles of O 3 , NO 2 , and NO 3 in the lower atmosphere (33-556 m above ground level) measured during the CalNex-LA 2010 study. Positive nocturnal vertical gradients of O 3 and NO 3 and negative gradients of NO 2 were observed during the night. Relatively short lifetime of nocturnal NO 3 (less than 1000 s) and high nighttime steady state N 2 O 5 mixing ratios (up to 2 ppb) indicated active nocturnal chemistry during CalNex.Comparison of modeled and observed altitude-resolved NO 3 loss frequencies shows that hydrolysis of N 2 O 5 on aerosols was the dominant loss pathway of NO 3 and NO x . Based on this argument, the nocturnal loss rates of NO x , L(NO x ), at different altitudes and averaged over the lowest 550 m of the atmosphere were calculated. The nocturnally averaged L(NO x ) ranged between 0.8 and 1.3 ppb h À1 for the lower atmosphere with the L(NO x ) for the first 8 days at about 1 ppb h À1 . This number is close to the one previously determined in Houston in 2009 of~0.9 ppb h À1 . Comparisons between daytime NO x loss due to the OH + NO 2 reaction and nighttime L(NO x ) show that during CalNex, nocturnal chemistry contributed an average of 60% to the removal of NO x in a 24 h period in the lower atmosphere.

Nitrous acid formation in a snow-free wintertime polluted rural area

“…It is thus necessary to convert these vertically averaged trace gas observations to vertical concentrations gradients and profiles. This is typically achieved using a geometric approach, where the ground is considered flat (Stutz et al, 2004;Wang et al, 2006;Wong et al, 2012Wong et al, , 2013Tsai et al, 2014). While the setup of the LP-DOAS system is similar to that of previous studies, the terrain under the light paths during UBWOS exhibited significant topographical variations compared to the height interval covered by the light paths ( Fig.…”

“…A variety of measurements were performed at the site, including observations of basic meteorological and radiometric parameters, as well as measurements of different trace gas concentrations. Here we will focus on the observations of concentrations and concentration gradients and fluxes made by UCLA's Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument (Stutz and Platt, 1997;Wong et al, 2011Wong et al, , 2012Tsai et al, 2014). …”

“…LP-DOAS instruments have been used to retrieve deposition velocities, vertical gradients, and fluxes of SO 2 (Platt and Perner, 1979) and reactive nitrogen species (Stutz et al, 2004;Wang et al, 2006;Wong et al, 2012Wong et al, , 2013Tsai et al, 2014). As illustrated in Fig.…”

“…RCAT8.2 is a highly resolved one-dimensional chemistry and transport model that has been used to investigate the impact of nocturnal processes and daytime chemistry of HONO and NO 3 (e.g., Wong and Stutz, 2010;Wong et al, 2013;Tsai et al, 2014). It is based on the gas-phase Regional Atmospheric Chemistry Mechanism (RACM), which contains 77 species, 237 reactions, and aggregates atmospheric VOCs into 23 classes: four alkanes, four alkenes, three biogenic, three aromatics, and nine carbonyls (Stockwell et al, 1997).…”

Nitrous acid formation in a snow-free wintertime polluted rural area

“…The measurement main site, also known as Horse-pool site (40 Optical Absorption Spectroscopy (LP-DOAS) instrument (Stutz and Platt, 1997;Wong et al, 2011Wong et al, , 2012Tsai et al, 2014).…”