Equilibrium of particle nitrite with gas phase HONO: Tropospheric measurements in the high Arctic during polar sunrise

Gas phase HONO(g) and nitrite in particles of <5‐μm size were measured in the troposphere during the Polar Sunrise Experiment at Alert, Northwest Territories, Canada, during January 19 to April 20, 1992, using denuder‐filter pack sampling and IC‐UV detection. The measurements indicated that HONO(g) existed at concentrations of up to 70 ppt before polar sunrise but gradually decreased to 5–10 ppt after sunrise. The calculated OH formation rate from HONO(g) photolysis was greater than from the photolysis of both O 3 and CH 2 O by more than one order of magnitude during the sunlit period and led to moderately high levels of OH, e.g., 3×10 5 molecules cm −3 OH at noontime on April 5. Particle nitrite measurements showed a gradual increase in concentrations with increasing solar insolation, but the concentrations were generally less than 10 ppt. The p H and the sulfate molar concentrations of the particles and the water vapor mixing ratio indicate that the particles were highly acidic being approximately 70% (W/W) H 2 SO 4 solution. In such highly concentrated H 2 SO 4 solution, most particle nitrite should exist as hydrated nitrosonium ion H 2 ONO + . Taking this into consideration, the particle nitrite was in an approximate equilibrium with the measured HONO(g). This equilibrium, with HONO(g) rapidly photolyzed, was a good indication that the particles were effective sources of HONO(g) and implied rapid production of particle N(+III) during this period. Two possible pathways leading to the formation of particle N(+III) species are suggested, i.e., reduction of HNO 3 (aq) by SO 2 (g) and reduction of NO 3 − ; (aq) by Br − (aq). However, N 2 O 5 reaction with NaBr cannot be ruled out as the alternative HONO(g) formation mechanism which bypasses the equilibrium.