Measurements of the HONO photodissociation constant

Abstract

Measurements of the photodissociation constant for nitrous acid (jHONO) were made at an urban site in Toronto, Canada, during the months of May-July 2005, using an optically thin actinometer. Operating details of the jHONO monitor are reported, along with laboratory tests. Measurements of jHONO were obtained for solar zenith angles ranging from 20-75 0 , under clear and cloudy skies. Maximum error estimates on jHONO under clear skies range from 11 % at sunrise, to 4% at solar noon, with a minimum detection limit of 5.7 x 10-4 /sec for our actinometer. Measured clear-sky values of jHONO were compared with values calculated by a four-stream discrete ordinate radiative transfer (RT) model (ACD TUV version 4.1), and were found to be within better than 10% agreement for solar zenith angles <65 0 • For conditions of scattered cloud, enhancement and suppression of the jHONO values occurred by as much as 16%-70%, and 59%-80%, respectively. The integrated band area of the mr* transition for gas-phase nitrous acid yields an oscillator strength, f = (1.06 ± 0.044) x 10-3 (based on clear-sky data), 19.1 % higher than the value reported by Bongartz et al. (1991).

Oxidative capacity of the Mexico City atmosphere – Part 1: A radical source perspective

“…The highest daytime HONO concentrations are expected during such "cold-surge" events, as HONO concentrations have been shown to correlate with relative humidity (Stutz et al, 2004). For each of the three model scenarios, the sensitivity of the model output was tested by varying the OH concentration within measurement uncertainty (offset of 0.01 pptv was subtracted), varying the dilution case, and varying J HONO within twice the experimental uncertainty (multiplication of J-value based on (Stutz et al, 2000) by 1.43 reproduces our observations, and is in general agreement with recent direct measurements of J HONO by Wall et al (2006) that indicate scaling by a factor of 1.28). Any photochemical sources are evaluated as the difference to model predicted and "observed" daytime HONO.…”

“…The actinic flux calibration of the spectroradiometer was performed before and after the campaign at the Forschungszentrum Juelich in Germany using certified irradiance standards as described in Kraus et al (2000). Unless otherwise noted, the ab- sorption cross-sections used to calculate photolysis frequencies were the same as described above; for O 3 , the crosssection at 295 K was employed; for HONO the current crosssection recommendation (Sander et al, 2006) was multiplied by 1.43 in order to match recent absolute measurements of J HONO (Wall et al, 2006); for CH 3 CHO and CH 3 COCH 3 the spectrum and quantum yield data for radical producing channels were taken from Sander et al (2006). Resultant J-values were multiplied by 1.08 (surface albedo) to account for the upwelling portion of the photon actinic flux.…”

“…The bottom two graphs show that model predicted HONO is systematically higher than the observations during mid-morning; an additional HONO sink is needed. Use of a higher value of J HONO according to Wall et al (2006) improves the agreement between measured and modeled HONO concentrations significantly.…”

Isoprene photooxidation: new insights into the production of acids and organic nitrates

“…σ HONO is scaled using the oscillator strength recently reported by Wall et al (2006). This gives J HOOH = 3.1×10 −6 s −1 .…”

Measurements of HONO during BAQS-Met

“…HONO photolysis frequencies (J HONO ) were estimated from measurements using an Eppley total UV radiometer on the roof of the trailer. The estimation was based on a previously quantified relationship between the UV Radiometer and a HONO Chemical Actinometer (Wall et al, 2006). In that study a Heraeus Amersil Electrically Fused quartz cell was used as the photolysis cell on the roof of a building in Toronto, Ontario.…”