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The well\u2010known and much\u2010studied weekend ozone effect has been attributed to weekend reductions in nitrogen oxide (NO<jats:sub>x<\/jats:sub> = NO + NO<jats:sub>2<\/jats:sub>) emissions, which affect ozone levels via two processes: (1) reduced ozone loss by titration and (2) enhanced photochemical production of ozone due to an increased ratio of non\u2010methane volatile organic compounds (VOCs) to NO<jats:sub>x<\/jats:sub>. In accord with previous assessments, the 2010 airborne and ground\u2010based data show an average decrease in NO<jats:sub>x<\/jats:sub> of 46 \u00b1 11% and 34 \u00b1 4%, respectively, and an average increase in VOC\/NO<jats:sub>x<\/jats:sub>ratio of 48 \u00b1 8% and 43 \u00b1 22%, respectively, on weekends. This work extends current understanding of the weekend ozone effect in the SoCAB by identifying its major causes and quantifying their relative importance from the available CalNex data. Increased weekend production of a VOC\u2010NO<jats:sub>x<\/jats:sub>oxidation product, peroxyacetyl nitrate, compared to a radical termination product, nitric acid, indicates a significant contribution from increased photochemical production on weekends. Weekday\u2010to\u2010weekend differences in the products of NO<jats:sub>x<\/jats:sub> oxidation show 45 \u00b1 13% and 42 \u00b1 12% more extensive photochemical processing and, when compared with odd oxygen (O<jats:sub>x<\/jats:sub> = O<jats:sub>3<\/jats:sub> + NO<jats:sub>2<\/jats:sub>), 51 \u00b1 14% and 22 \u00b1 17% greater ozone production efficiency on weekends in the airborne and ground\u2010based data, respectively, indicating that both contribute to higher weekend ozone levels in the SoCAB.<\/jats:p>","DOI":"10.1029\/2011jd016772","type":"journal-article","created":{"date-parts":[[2011,12,6]],"date-time":"2011-12-06T10:00:20Z","timestamp":1323165620000},"source":"Crossref","is-referenced-by-count":99,"title":["Airborne and ground\u2010based observations of a weekend effect in ozone, precursors, and oxidation products in the California South Coast Air Basin"],"prefix":"10.1029","volume":"117","author":[{"given":"I. 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