Delhi, India, suffers from periods of very poor air quality,
but
little is known about the chemical production of secondary pollutants
in this highly polluted environment. During the postmonsoon period
in 2018, extremely high nighttime concentrations of NOx(NO and NO2) and volatile organic compounds (VOCs) were
observed, with median NOxmixing ratios of ∼200
ppbV (maximum of ∼700 ppbV). A detailed chemical box model
constrained to a comprehensive suite of speciated VOC and NOxmeasurements revealed very low nighttime concentrations of oxidants,
NO3, O3, and OH, driven by high nighttime NO
concentrations. This results in an atypical NO3diel profile,
not previously reported in other highly polluted urban environments,
significantly perturbing nighttime radical oxidation chemistry. Low
concentrations of oxidants and high nocturnal primary emissions coupled
with a shallow boundary layer led to enhanced early morning photo-oxidation
chemistry. This results in a temporal shift in peak O3concentrations
when compared to the premonsoon period (12:00 and 15:00 local time,
respectively). This shift will likely have important implications
on local air quality, and effective urban air quality management should
consider the impacts of nighttime emission sources during the postmonsoon
period.