Performance check of beta gauge method under high PM2.5 mass loading and varying meteorological conditions in an urban atmosphere

In available literature, performance of the beta gauge method has been checked and reported under real ambient conditions and fine particle concentration up to -100 mu g m- 3, although its measurement capability is claimed in different documents for more than 1000 mu g m- 3. Here, for the first time, we report performance of the beta gauge method at varying meteorology and high particle mass loading, i.e. up to 500 mu g m- 3 under urban ambient conditions. Year-round sampling in 2019 was performed at CSIR-NPL, New Delhi to cover a wider range of variability in meteorological parameters, i.e. temperature (8-38 degrees C), relative humidity (30-97%), ventilation coefficient (217-6761 m2 s-1) and boundary layer height (160-1386 m), and mass loading, i.e. PM2.5 mass concentration from 17 to 670 mu g m- 3 (annual mean 142 +/- 122 mu g m- 3). During high concentration period, i.e. in winter (December-February) and post-monsoon (October-November), the mean concentration and associated deviation were high (i.e. in winter 236 +/- 106 mu g m- 3 (+/- 45%) and post-monsoon 227 +/- 154 mu g m- 3 (+/- 68%)) which may be due to large variation and high particle mass loading caused by lower boundary layer height (winter 300 +/- 86 m and post-monsoon 420 +/- 87 m) with low ventilation coefficient (winter 731 +/- 401 m2 s-1 and post-monsoon 899 +/- 402 m2 s-1), and most importantly, high RH (winter 84% +/- 7% and post-monsoon 72% +/- 6%). Beta gauge method while comparing with collocated gravimetric method, demonstrated an excellent correlation (r2 > 0.90) and the observed bias between the methods was within 10%, when RH was <60% and PM2.5 was <130 mu g m- 3. More than 30% overestimation was observed especially when RH reaches over 60%. These performance results suggest that overestimation of PM mass concentration by beta gauge method occurs especially at high RH and increases by a factor up to -5 when particle mass loading also becomes high. To overcome this problem, two onsite correction factor approaches are suggested and applied to the beta gauge measurements. After correction, the mean normalized biases were decreased from 46.48 +/- 26.78% (original PM2.5_BG) to 1.72 +/- 18.60% (constant proportion correction) and to 0.41 +/- 6.11% (RH correction) which resulted in an acceptable level of accuracy thereby enhancing the field performance of the beta gauge monitor under variable meteorological conditions.