Abstract
Abstract. Observations of peroxynitric acid (HO2NO2) obtained simultaneously with those of NO and NO2 provide a sensitive measure of the ozone photochemical production rate. We illustrate this technique for constraining the ozone production rate with observations obtained from the NCAR C-130 aircraft platform during the Megacity Initiative: Local and Global Research Observations (MILAGRO) intensive in Mexico during the spring of 2006. Sensitive and selective measurements of HO2NO2 were made in situ using chemical ionization mass spectrometry (CIMS). Observations were compared to modeled HO2NO2 concentrations obtained from the NASA Langley highly-constrained photochemical time-dependent box model. The median observed-to-calculated ratio of HO2NO2 is 1.18. At NOx levels greater than 15 ppbv, the photochemical box model underpredicts observations with an observed-to-calculated ratio of HO2NO2 of 1.57. As a result, we find that at high NOx, the ozone production rate calculated using measured HO2NO2 is faster than predicted using accepted photochemistry. Inclusion of an additional HOx source from the reaction of excited state NO2 with H2O or reduction in the rate constant of the reaction of OH with NO2 improves the agreement.
Highlights
Peroxynitric acid, HO2NO2, is an important reservoir of both HOx (HOx=OH+HO2) and NOx (NOx=NO+NO2) radicals
Non-methane hydrocarbons (NMHC) were measured using whole air sampling (WAS) and subsequent analysis was conducted by gas chromatography/mass spectrometry/flame ionization detection/electron capture detection (Colman et al, 2001; Blake et al, 2003)
Due to the short lifetime of HO2NO2 with respect to thermal decomposition for the conditions experienced during the Megacity Initiative: Local and Global Research Observations (MILAGRO) flights, photochemical steady state of HO2NO2 was assumed
Summary
Peroxynitric acid, HO2NO2, is an important reservoir of both HOx (HOx=OH+HO2) and NOx (NOx=NO+NO2) radicals. These radicals are of primary atmospheric importance as they influence the production and degradation of tropospheric ozone and numerous volatile organic compounds (Murphy et al, 2004). The formation of HO2NO2 followed by its loss via reaction with OH forms a NOx-catalyzed sink of HOx radicals (Roehl et al, 2002). We present in situ measurements of HO2NO2 in and around Mexico City. These measurements were made in March of 2006 from the NCAR C-130 aircraft platform during the MILAGRO field experiment. A simplified method of estimating the rate of ozone production using HO2NO2 is compared to full NASA LaRC box model calculations of the rate of ozone production
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