Evidence for a nitrous acid (HONO) reservoir at the ground surface in Bakersfield, CA, during CalNex 2010

Measurements of HONO(g) and particulate nitrite (NO2−(p)) were made with a modified Ambient Ion Monitor–Ion Chromatography (AIM‐IC) instrument during California at the Nexus of Air Quality and Climate 2010 in Bakersfield, CA (CalNex‐San Joaquin Valley (SJV)). Observations of gas and particulate matter (PM2.5) water‐soluble composition showed accumulation of both species at night, followed by loss the next day. Intercomparison with a Stripping Coil‐UV/Vis Absorption Photometer (SC‐AP) demonstrated excellent agreement with the AIM‐IC HONO(g) measurement (slope = 0.957, R2 = 0.86), and the particulate nitrite observations were validated to be free of known interferences for wet chemical instrumentation. The accumulation of nitrite into particulate matter was found to be enhanced when gaseous mixing ratios of HONO(g) were highest. Reactive uptake of HONO(g) on to lofted dust and the ground surface, forming a reservoir, is a potential mechanism to explain these observations. The AIM‐IC HONO(g) measurements were parameterized in a chemical model to calculate the ground surface daytime HONO(g) source strength at 4.5 m above the surface, found to be on the order of 1.27 ppb h−1, to determine the relative importance of a surface reservoir. If all deposited nighttime HONO(g) is reemitted the following day, up to 30% of the daytime HONO(g) source at CalNex‐SJV may be accounted for. The observations of HONO(g) and NO2−(p) in Bakersfield, during CalNex, suggest a surface sink and source of HONO(g). Extension of currently accepted unknown daytime HONO(g) source reactions to include a potential surface HONO(g) reservoir should therefore be sound, but quantitation of the relative contributions of each surface source toward daytime HONO(g) production remains to be resolved. Key Points AIM‐IC nitrous acid verified by intercomparison with SC‐APParticulate nitrite observed does not arise from equilibrium partitioningNocturnally deposited HONO could be a significant daytime source