Sally E. Pusede, Trevor C. VandenBoer, Jennifer G. Murphy, Milos Z. Markovic, Cora J. Young,

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1 SUPPLEMENTAL INFORMATION for An Atmospheric Constraint on the NO 2 Dependence of Daytime Near- Surface Nitrous Acid (HONO) Sally E. Pusede, Trevor C. VandenBoer, Jennifer G. Murphy, Milos Z. Markovic, Cora J. Young, Patrick R. Veres, James M. Roberts, Rebecca A. Washenfelder, Steven S. Brown, Xinrong Ren, Catalina Tsai, Jochen Stutz, William H. Brune, Eleanor C. Browne, Paul J. Wooldridge, Ashley R. Graham, Robin Weber, Allen H. Goldstein, Sebastien Dusanter, Stephen M. Griffith, Philip S. Stevens, Barry L. Lefer, and Ronald C. Cohen Supplementary information contains pages S1 S12 and Figures S1 S8. S1

2 NO 2 Weekday-Weekend Patterns In Bakersfield, higher NO 2 was observed at night before midnight on weekends. This is driven by higher NO x on Sunday nights and appears to be a consistent feature of the NO x time series. We hypothesize this pattern is due to emissions from the heavy-duty diesel truck fleet, which, having been parked for the weekend, begins the workweek on Sunday evening. In Pasadena, where a large portion of the local NO x is emitted in upwind Los Angeles, integrated weekend daytime (8 am 4 pm LT) NO 2 is lower on weekends, but there is a shift in timing of peak daytime concentrations. This is consistent with previous accounts of diurnal profiles of mobile NO x emissions by day of week. 1 In Figs. S4 S8, we show the weekday-weekend variability in the measured atmospheric conditions, light (photosynthetically active radiation in Bakersfield and solar radiation in Pasadena), relative humidity (RH), and temperature. Each of the five figures resamples the individual environmental parameter datasets to only include those observations when both NO 2 and HONO, for the different analytical techniques, exist. This was done in order to represent the atmospheric conditions of Figs. 1 and S1 S3, as opposed to average atmospheric conditions during the CalNex experiment. Calculating P(HONO) inferred P(HONO) inferred (Eq. 1) was calculated using measurements of HONO, NO, and OH in both cities. Conditions during CalNex were generally sunny but cloudy sky conditions did occur and these data were not removed. The rates, k HONO+OH and k NO+OH, are taken from Sander et al. 2 and j HONO is described below. In Bakersfield, OH was measured by laser-induced fluorescence 3 and S2

3 NO by a home-built chemiluminescence instrument. 4 In Pasadena, OH was measured by fluorescent assay by gas expansion 5 and NO by a chemiluminescence Thermo 42i-TL detector. (1) P(HONO) inferred = j HONO [HONO] + k HONO+OH [HONO][OH] k NO+OH [NO][OH] In Bakersfield, j HONO was calculated hourly using the TUV calculator ( 6 for a clear sky day (7 June ). This j HONO was then used to scale the measured photosynthetically active radiation for the entire CalNex time period at 1-minute time resolution. Inputs to the TUV calculator were as follows: O 3 optical depth equal to 3 Dobson units, as measured by OMI on 7 June ( 7 measurement altitude equal to.14 km (asl); and surface albedo equal to.8, determined using the MODIS 16 day 5-meter product in band 3 ( nm) for 18 June reprojected at a fine resolution (.1 degree ~ m). In Pasadena, j HONO was measured and these data used. Any missing j HONO (8 days) were filled by computing j HONO hourly using the TUV calculator for clear sky day 5 June, scaling the TUV values by the ratio of j NO2 measured at 1-minute time resolution to the calculated j NO2 for entire study period, and multiplying modeled j HONO by the slope of the correlation of measured versus TUV j HONO for the entire CalNex time period. S3

4 a) b) 8 NO 2 (ppb) 15 5 HONO (ppt) Figure S1. Panel a: Hourly medians of 1-minute LIF NO 2 as a function of time of day on weekdays (black closed circles) and weekends (blue open diamonds) in Bakersfield. Data are only included when HONO SC-AP measurements also exist. Error bars show 1σ variability. Panel b: Hourly medians of corresponding 2-minute HONO on weekdays and weekends, with HONO data only included when NO 2 data also exist. Excluding Saturdays from this data, the longest dataset, had no effect on the observed HONO NO 2 dependence. S4

5 a) b) 3 15 NO 2 (ppb) HONO (ppt) Figure S2. Panel a: Hourly daily medians of 1-minute CRDS NO 2 on weekdays (black closed circles) and weekends (blue open diamonds) in Pasadena. Data are only included when HONO IBBCEAS measurements also exist and the y-axis is truncated to highlight daytime differences. Error bars show 1σ variability. Panel b: Hourly medians of corresponding -minute HONO on weekdays and weekends. HONO data are only included in the averaging when NO 2 data also exist. S5

6 NO 2 (ppb) HONO (ppt) Low Mid High Highest Figure S3. Top row, left to right: 9-minute medians of NO 2 measured by LP-DOAS at the low (78 m), middle (121 m), high (255 m), and highest (556 m) light paths on weekdays (black closed circles) and weekends (blue open diamonds) in the Los Angeles area. Error bars are 1σ variability. Bottom row, left to right: 9-minute medians of LP-DOAS HONO at the same heights. The y-axis is truncated to highlight daytime differences S6

7 light (µmol m 2 s 1 ) a) b) c) relative humitidy (%) temperature ( o C) Figure S4. Panel a: Hourly medians of photosynthetically active radiation (µmol m 2 s 1 ) (panel a), relative humidity (%) (panel b), and temperature ( o C) (panel c) on weekdays (black closed circles) and weekends (blue open diamonds) in Bakersfield when AIM-IC HONO observations also exist. S7

8 light (W m 1 ) 8 a) b) c) relative humitidy (%) temperature ( o C) Figure S5. Hourly daily medians of solar radiation (W m 2 ) (panel a), relative humidity (%) (panel b), and temperature ( o C) (panel c) on weekdays (black closed circles) and weekends (blue open diamonds) in Pasadena when NI-PT-CIMS HONO observations also exist. S8

9 light (µmol m 2 s 1 ) a) b) c) relative humitidy (%) temperature ( o C) Figure S6. Hourly daily medians of photosynthetically active radiation (µmol m 2 s 1 ) (panel a), relative humidity (%) (panel b), and temperature ( o C) (panel c) on weekdays (black closed circles) and weekends (blue open diamonds) in Bakersfield when SC-AP HONO observations also exist. S9

10 light (W m 1 ) 8 a) b) c) relative humitidy (%) temperature ( o C) Figure S7. Hourly daily medians of solar radiation (W m 2 ) (panel a), relative humidity (%) (panel b), and temperature ( o C) (panel c) on weekdays (black closed circles) and weekends (blue open diamonds) in Pasadena when IBBCEAS HONO observations also exist. S

11 light (W m 1 ) a) b) c) relative humitidy (%) temperature ( o C) Figure S8. Diurnal 9-minute medians of solar radiation at the surface in Pasadena (W m 2 ) (panel a), relative humidity (%) (panel b), and temperature ( o C) (panel c) on weekdays (black closed circles) and weekends (blue open diamonds) in Los Angeles when LP-DOAS HONO observations also exist. S11

12 References 1. Marr, L. C.; Harley, R. A., Environ. Sci. Technol. 2, 36 (19), Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K.; Orkin, V. L.; Wine, P. H. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation No. 17; Pasadena: Jet Propulsion Laboratory, Stevens, P. S.; Mather, J. H.; Brune, W. H., J. Geophys. Res.-Atmos. 1994, 99 (D2), Min, K. E.; Pusede, S. E.; Browne, E. C.; LaFranchi, B. W.; Cohen, R. C., Atmos. Chem. Phys. 14, 14 (11), Dusanter, S.; Vimal, D.; Stevens, P. S.; Volkamer, R.; Molina, L. T., Atmos. Chem. Phys. 9, 9 (5), Madronich, S., J. Geophys. Res.-Atmos. 1987, 92 (D8), Veefkind, J. P.; de Haan, J. F.; Brinksma, E. J.; Kroon, M.; Levelt, P. F., IEEE Trans. Geo. Rem. Sens. 6, 44, S12

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