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

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 −1 for the lower atmosphere with the L (NO x ) for the first 8 days at about 1 ppb h −1 . This number is close to the one previously determined in Houston in 2009 of ~0.9 ppb h −1 . 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.