Semi-quantitative understanding of source contribution to nitrous acid (HONO) based on 1 year of continuous observation at the SORPES station in eastern China

Nitrous acid (HONO), an important precursor of the hydroxyl radical (OH), has long been recognized as of significance to atmospheric chemistry, but its sources are still debated. In this study, we conducted continuous measurement of HONO from November 2017 to November 2018 at the SORPES station in Nanjing of eastern China. The yearly average mixing ratio of observed HONO was 0.69±0.58 ppb, showing a larger contribution to OH relative to ozone with a mean OH production rate of 1.16 ppb h−1. To estimate the effect of combustion emissions of HONO, the emitted ratios of HONO to NOx were derived from 55 fresh plumes (NO/NOx > 0.85), with a mean value of 0.79 %. During the nighttime, the chemistry of HONO was found to depend on RH, and heterogeneous reaction of NO2 on an aerosol surface was presumably responsible for HONO production. The average nighttime NO2-to-HONO conversion frequency (CHONO) was determined to be 0.0055± 0.0032 h−1 from 137 HONO formation cases. The missing source of HONO around noontime seemed to be photo-induced, with an average Punknown of 1.04 ppb h−1, based on a semi-quantitative HONO budget analysis. An over-determined system of equations was applied to obtain the monthly variations in nocturnal HONO sources. Besides the burning-emitted HONO (accounting for about 23 % of the total concentration), the contribution of HONO formed heterogeneously on ground surfaces to measured HONO was an approximately constant proportion of 36 % throughout the year. The soil emission revealed clear seasonal variation and contributed up to 40 % of observed HONO in July and August. A higher propensity for generating HONO on aerosol surfaces occurred in severe hazes (accounting for 40 % of the total concentration in January). Our results highlight ever-changing contributions of HONO sources and encourage more long-term observations to evaluate the contributions from varied sources.


Introduction
. 76 Notably this reaction rate is drastically reduced after the first few seconds due to     (1)  (Table 2). Fig.1 shows the seasonal pattern The HONO to NOx ratio or the HONO to NO2 ratio has been used extensively in affecting the highest achievable ratio at nighttime will be discussed in section 3.3.

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What's interesting here is the peak of the HONO/NOx ratio in the midday sun in 259 spring, summer and autumn, and even in winter, the ratio doesn't decline but remains The elevated mixing ratio of HONO presents an efficient source of OH radicals 269 during daytime in Nanjing. We calculate the net OH production rate from HONO, i.e.   Within the one-year dataset, we select 55 freshly emitted plumes satisfying the criteria 323 above (Table S1)

RH dependence of HONO chemistry
376 377 It appears that NO2 hydrolysis on humid surfaces (R4), having a first order  formation of HONO and the extremely impactful loss of HONO will result in a 417 dramatic decline of the HONOcorr/NO2 ratio ( Fig. 6(a) and Fig. 6(b)).

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Notably, the constant HONOcorr/NO2 value at RH between 75-95% under the 420 condition of high PM2.5 mass loading ( Fig. 6(b)), compared to the downward trend of 421 HONOcorr/NO2 within the same humidity range in low PM2.5 mass concentration (Fig.   422 6(a)), implies a contribution of aerosol surfaces to the NO2-HONO conversion. Since 423 both HONOcorr/NO2 in Fig. 6(a) and Fig. 6(b) are affected by the ground surfaces, we 424 can use the difference of HONOcorr/NO2 between the two figures to represent the 425 influence of aerosol. As the area of shadow showed in Fig. 6(b), the aerosol-affected 426 HONOcorr/NO2 is positively related to RH before RH reaches 95%. With the increase 427 of RH, the hygroscopic growth of aerosol particles should provide larger surface area.

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When RH is higher than 75%, which has exceeded the mutual deliquescence relative

Impact of aerosols on HONO formation 438 439
To further understand the heterogeneous formation of HONO on aerosol, we carry out  (Fig. 7b). As shown by larger triangles with gray borders in Fig. 7( After discussing the nocturnal formation mechanism of HONO, we now focus on 501 the chemistry of daytime HONO whose concentrations are still about 0.25-0.6 ppb at 502 noon with a lifetime of only 10-20 min (Fig. 2). We are not certain if the observed 503 HONO can be provided by known mechanisms (gas phase reaction (R4) and    Hence, we perform a correlation analysis to explore the potential unknown daytime 552 mechanisms of HONO (Table 3). Punknown is better correlated with NO2*UVB than 553 with NO2 or UVB alone in spring and autumn (p=0.05), perhaps associated with the Our study suggest that the missing source of HONO should be considered in the air 570 quality forecasting or regional models to characterize atmospheric oxidizing capacity 571 better, especially in warm seasons (spring and summer). Based on the measurement 572 (Fig. S3), the light-induced heterogeneous conversion of NO2 to HONO on aerosol