Quantifying Nitrous Acid Formation Mechanisms Using Measured Vertical Profiles During the CalNex 2010 Campaign and 1D Column Modeling

Nitrous acid (HONO) is an important radical precursor that can impact secondary pollutant levels, especially in urban environments. Due to uncertainties in its heterogeneous formation mechanisms, models often under predict HONO concentrations. A number of heterogeneous sources at the ground have been proposed but there is no consensus about which play a significant role in the urban boundary layer. We present a new one-dimensional chemistry and transport model which performs surface chemistry based on molecular collisions and chemical conversion, allowing us to add detailed HONO formation chemistry at the ground. We conducted model runs for the 2010 CalNex campaign, finding good agreement with observations for key species such as O-3, NOx, and HOx. With the ground sources implemented, the model captures the diurnal and vertical profile of the HONO observations. Primary HOx production from HONO photolysis is 2-3 times more important than O-3 or HCHO photolysis at mid-day, below 10 m. The HONO concentration, and its contribution to HOx, decreases quickly with altitude. Heterogeneous chemistry at the ground provided a HONO source of 2.5 x 10(11) molecules cm(-2) s(-1) during the day and 5 x 10(10) molecules cm(-2) s(-1) at night. The night time source was dominated by NO2 hydrolysis. During the day, photolysis of surface HNO3/nitrate contributed 45%-60% and photo-enhanced conversion of NO2 contributed 20%-45%. Sensitivity studies addressing the uncertainties in both photolytic mechanisms show that, while the relative contribution of either source can vary, HNO3/nitrate is required to produce a surface HONO source that is strong enough to explain observations. Key Points Measured vertical profiles of HONO near Los Angeles in May 2010 suggest a heterogeneous HONO source at the ground A new 1D chemistry and transport model that includes HONO formation on the ground reproduces the HONO concentration profiles The main daytime HONO source is adsorbed nitric acid/nitrate photolysis, followed by photo-enhanced NO2 conversion

Automated summary

Nitrous acid (HONO) is an important radical precursor that can impact secondary pollutant levels in urban environments. This study developed a new one-dimensional chemistry and transport model to better simulate the formation processes and ground sources of HONO, improving predictions of its diurnal variation and vertical distribution. The results showed that photolysis of surface HNO3/nitrate and photo-enhanced NO2 conversion are the main daytime sources of HONO, while at night, the main source is NO2 hydrolysis.