UCLA

Nitrous acid (HONO) is an important radical precursor that can influence secondary pollutant levels, especially in areas impacted by urban emissions and wildfire smoke. Due to uncertainties in its emissions and heterogeneous formation mechanisms, models often under predict HONO concentrations. A number of heterogeneous sources at the ground and on aerosols have been proposed but there is no consensus about which play a significant role in these polluted environments. In this thesis, a new one-dimensional chemistry and transport model (PACT-1D) is used to interpret field measurements and analyze the HONO budget. PACT-1D performs surface chemistry based on molecular collisions and chemical conversion, allowing the addition of detailed HONO formation chemistry at the ground and on particles.

Model runs were conducted for the 2010 CalNex campaign, finding good agreement with observations of key species such as O3, 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 O3 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.

Two wildfire smoke plumes from the 2019 FIREX-AQ experiment were also successfully simulated. Both plumes show high levels of HONO, with concentrations exceeding 20 ppb in young smoke. PACT-1D captures the rapid decay of HONO with smoke age, due to a mix of chemical loss and dilution. Vertical profiles show highest levels in the plume center that quickly decrease towards the plume top and bottom. The decrease occurs more rapidly than less-reactive species, creating a narrower vertical distribution of HONO. The large quantities of HONO emitted from fires provide the dominant source and result in HONO providing 80% of the total HOx formation in young smoke. Sensitivity studies show that emissions also drive O3 production, with O3 levels reduced up to 10 ppb when HONO emissions are omitted. Low photolysis rates and minimal vertical mixing in the plumes allow these emissions to be transported hours downwind, continuing to provide a source of OH. HONO's contribution to the HOx budget decreases as emissions are lost and by 6 hours downwind, HCHO photolysis becomes the dominant HOx source. Secondary sources of HONO are minor compared to emissions but pNO3 photolysis becomes significant in older smoke and helps HONO continue to contribute at least 5% to HOx production.